=head1 NAME
-perlfaq4 - Data Manipulation ($Revision: 1.25 $, $Date: 1998/07/16 22:49:55 $)
+perlfaq4 - Data Manipulation ($Revision: 1.61 $, $Date: 2005/03/11 16:27:53 $)
=head1 DESCRIPTION
-The section of the FAQ answers question related to the manipulation
-of data as numbers, dates, strings, arrays, hashes, and miscellaneous
-data issues.
+This section of the FAQ answers questions related to manipulating
+numbers, dates, strings, arrays, hashes, and miscellaneous data issues.
=head1 Data: Numbers
=head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
-The infinite set that a mathematician thinks of as the real numbers can
-only be approximate on a computer, since the computer only has a finite
-number of bits to store an infinite number of, um, numbers.
-
-Internally, your computer represents floating-point numbers in binary.
-Floating-point numbers read in from a file or appearing as literals
-in your program are converted from their decimal floating-point
-representation (eg, 19.95) to the internal binary representation.
-
-However, 19.95 can't be precisely represented as a binary
-floating-point number, just like 1/3 can't be exactly represented as a
-decimal floating-point number. The computer's binary representation
-of 19.95, therefore, isn't exactly 19.95.
-
-When a floating-point number gets printed, the binary floating-point
-representation is converted back to decimal. These decimal numbers
-are displayed in either the format you specify with printf(), or the
-current output format for numbers (see L<perlvar/"$#"> if you use
-print. C<$#> has a different default value in Perl5 than it did in
-Perl4. Changing C<$#> yourself is deprecated.
-
-This affects B<all> computer languages that represent decimal
-floating-point numbers in binary, not just Perl. Perl provides
-arbitrary-precision decimal numbers with the Math::BigFloat module
-(part of the standard Perl distribution), but mathematical operations
-are consequently slower.
-
-To get rid of the superfluous digits, just use a format (eg,
-C<printf("%.2f", 19.95)>) to get the required precision.
-See L<perlop/"Floating-point Arithmetic">.
+Internally, your computer represents floating-point numbers
+in binary. Digital (as in powers of two) computers cannot
+store all numbers exactly. Some real numbers lose precision
+in the process. This is a problem with how computers store
+numbers and affects all computer languages, not just Perl.
+
+L<perlnumber> show the gory details of number
+representations and conversions.
+
+To limit the number of decimal places in your numbers, you
+can use the printf or sprintf function. See the
+L<"Floating Point Arithmetic"|perlop> for more details.
+
+ printf "%.2f", 10/3;
+
+ my $number = sprintf "%.2f", 10/3;
+
+=head2 Why is int() broken?
+
+Your int() is most probably working just fine. It's the numbers that
+aren't quite what you think.
+
+First, see the above item "Why am I getting long decimals
+(eg, 19.9499999999999) instead of the numbers I should be getting
+(eg, 19.95)?".
+
+For example, this
+
+ print int(0.6/0.2-2), "\n";
+
+will in most computers print 0, not 1, because even such simple
+numbers as 0.6 and 0.2 cannot be presented exactly by floating-point
+numbers. What you think in the above as 'three' is really more like
+2.9999999999999995559.
=head2 Why isn't my octal data interpreted correctly?
-Perl only understands octal and hex numbers as such when they occur
-as literals in your program. If they are read in from somewhere and
-assigned, no automatic conversion takes place. You must explicitly
-use oct() or hex() if you want the values converted. oct() interprets
-both hex ("0x350") numbers and octal ones ("0350" or even without the
-leading "0", like "377"), while hex() only converts hexadecimal ones,
-with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
+Perl only understands octal and hex numbers as such when they occur as
+literals in your program. Octal literals in perl must start with a
+leading "0" and hexadecimal literals must start with a leading "0x".
+If they are read in from somewhere and assigned, no automatic
+conversion takes place. You must explicitly use oct() or hex() if you
+want the values converted to decimal. oct() interprets hex ("0x350"),
+octal ("0350" or even without the leading "0", like "377") and binary
+("0b1010") numbers, while hex() only converts hexadecimal ones, with
+or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".
+The inverse mapping from decimal to octal can be done with either the
+"%o" or "%O" sprintf() formats.
This problem shows up most often when people try using chmod(), mkdir(),
-umask(), or sysopen(), which all want permissions in octal.
+umask(), or sysopen(), which by widespread tradition typically take
+permissions in octal.
- chmod(644, $file); # WRONG -- perl -w catches this
+ chmod(644, $file); # WRONG
chmod(0644, $file); # right
-=head2 Does perl have a round function? What about ceil() and floor()? Trig functions?
+Note the mistake in the first line was specifying the decimal literal
+644, rather than the intended octal literal 0644. The problem can
+be seen with:
+
+ printf("%#o",644); # prints 01204
+
+Surely you had not intended C<chmod(01204, $file);> - did you? If you
+want to use numeric literals as arguments to chmod() et al. then please
+try to express them as octal constants, that is with a leading zero and
+with the following digits restricted to the set 0..7.
+
+=head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
Remember that int() merely truncates toward 0. For rounding to a
certain number of digits, sprintf() or printf() is usually the easiest
printf("%.3f", 3.1415926535); # prints 3.142
-The POSIX module (part of the standard perl distribution) implements
+The POSIX module (part of the standard Perl distribution) implements
ceil(), floor(), and a number of other mathematical and trigonometric
functions.
$ceil = ceil(3.5); # 4
$floor = floor(3.5); # 3
-In 5.000 to 5.003 Perls, trigonometry was done in the Math::Complex
-module. With 5.004, the Math::Trig module (part of the standard perl
+In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex
+module. With 5.004, the Math::Trig module (part of the standard Perl
distribution) implements the trigonometric functions. Internally it
uses the Math::Complex module and some functions can break out from
the real axis into the complex plane, for example the inverse sine of
being used by Perl, but to instead implement the rounding function you
need yourself.
-=head2 How do I convert bits into ints?
+To see why, notice how you'll still have an issue on half-way-point
+alternation:
+
+ for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i}
+
+ 0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7
+ 0.8 0.8 0.9 0.9 1.0 1.0
+
+Don't blame Perl. It's the same as in C. IEEE says we have to do this.
+Perl numbers whose absolute values are integers under 2**31 (on 32 bit
+machines) will work pretty much like mathematical integers. Other numbers
+are not guaranteed.
+
+=head2 How do I convert between numeric representations/bases/radixes?
+
+As always with Perl there is more than one way to do it. Below
+are a few examples of approaches to making common conversions
+between number representations. This is intended to be representational
+rather than exhaustive.
+
+Some of the examples below use the Bit::Vector module from CPAN.
+The reason you might choose Bit::Vector over the perl built in
+functions is that it works with numbers of ANY size, that it is
+optimized for speed on some operations, and for at least some
+programmers the notation might be familiar.
+
+=over 4
+
+=item How do I convert hexadecimal into decimal
+
+Using perl's built in conversion of 0x notation:
+
+ $dec = 0xDEADBEEF;
+
+Using the hex function:
+
+ $dec = hex("DEADBEEF");
+
+Using pack:
+
+ $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
+
+Using the CPAN module Bit::Vector:
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new_Hex(32, "DEADBEEF");
+ $dec = $vec->to_Dec();
+
+=item How do I convert from decimal to hexadecimal
+
+Using sprintf:
+
+ $hex = sprintf("%X", 3735928559); # upper case A-F
+ $hex = sprintf("%x", 3735928559); # lower case a-f
+
+Using unpack:
+
+ $hex = unpack("H*", pack("N", 3735928559));
+
+Using Bit::Vector:
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new_Dec(32, -559038737);
+ $hex = $vec->to_Hex();
+
+And Bit::Vector supports odd bit counts:
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new_Dec(33, 3735928559);
+ $vec->Resize(32); # suppress leading 0 if unwanted
+ $hex = $vec->to_Hex();
+
+=item How do I convert from octal to decimal
+
+Using Perl's built in conversion of numbers with leading zeros:
+
+ $dec = 033653337357; # note the leading 0!
+
+Using the oct function:
+
+ $dec = oct("33653337357");
+
+Using Bit::Vector:
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new(32);
+ $vec->Chunk_List_Store(3, split(//, reverse "33653337357"));
+ $dec = $vec->to_Dec();
+
+=item How do I convert from decimal to octal
+
+Using sprintf:
+
+ $oct = sprintf("%o", 3735928559);
+
+Using Bit::Vector:
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new_Dec(32, -559038737);
+ $oct = reverse join('', $vec->Chunk_List_Read(3));
+
+=item How do I convert from binary to decimal
+
+Perl 5.6 lets you write binary numbers directly with
+the 0b notation:
+
+ $number = 0b10110110;
+
+Using oct:
+
+ my $input = "10110110";
+ $decimal = oct( "0b$input" );
+
+Using pack and ord:
+
+ $decimal = ord(pack('B8', '10110110'));
+
+Using pack and unpack for larger strings:
+
+ $int = unpack("N", pack("B32",
+ substr("0" x 32 . "11110101011011011111011101111", -32)));
+ $dec = sprintf("%d", $int);
+
+ # substr() is used to left pad a 32 character string with zeros.
+
+Using Bit::Vector:
-To turn a string of 1s and 0s like C<10110110> into a scalar containing
-its binary value, use the pack() function (documented in
-L<perlfunc/"pack">):
+ $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111");
+ $dec = $vec->to_Dec();
- $decimal = pack('B8', '10110110');
+=item How do I convert from decimal to binary
-Here's an example of going the other way:
+Using sprintf (perl 5.6+):
- $binary_string = join('', unpack('B*', "\x29"));
+ $bin = sprintf("%b", 3735928559);
+
+Using unpack:
+
+ $bin = unpack("B*", pack("N", 3735928559));
+
+Using Bit::Vector:
+
+ use Bit::Vector;
+ $vec = Bit::Vector->new_Dec(32, -559038737);
+ $bin = $vec->to_Bin();
+
+The remaining transformations (e.g. hex -> oct, bin -> hex, etc.)
+are left as an exercise to the inclined reader.
+
+=back
+
+=head2 Why doesn't & work the way I want it to?
+
+The behavior of binary arithmetic operators depends on whether they're
+used on numbers or strings. The operators treat a string as a series
+of bits and work with that (the string C<"3"> is the bit pattern
+C<00110011>). The operators work with the binary form of a number
+(the number C<3> is treated as the bit pattern C<00000011>).
+
+So, saying C<11 & 3> performs the "and" operation on numbers (yielding
+C<3>). Saying C<"11" & "3"> performs the "and" operation on strings
+(yielding C<"1">).
+
+Most problems with C<&> and C<|> arise because the programmer thinks
+they have a number but really it's a string. The rest arise because
+the programmer says:
+
+ if ("\020\020" & "\101\101") {
+ # ...
+ }
+
+but a string consisting of two null bytes (the result of C<"\020\020"
+& "\101\101">) is not a false value in Perl. You need:
+
+ if ( ("\020\020" & "\101\101") !~ /[^\000]/) {
+ # ...
+ }
=head2 How do I multiply matrices?
results:
foreach $iterator (@array) {
- &my_func($iterator);
+ some_func($iterator);
}
To call a function on each integer in a (small) range, you B<can> use:
- @results = map { &my_func($_) } (5 .. 25);
+ @results = map { some_func($_) } (5 .. 25);
but you should be aware that the C<..> operator creates an array of
all integers in the range. This can take a lot of memory for large
@results = ();
for ($i=5; $i < 500_005; $i++) {
- push(@results, &my_func($i));
+ push(@results, some_func($i));
}
+This situation has been fixed in Perl5.005. Use of C<..> in a C<for>
+loop will iterate over the range, without creating the entire range.
+
+ for my $i (5 .. 500_005) {
+ push(@results, some_func($i));
+ }
+
+will not create a list of 500,000 integers.
+
=head2 How can I output Roman numerals?
-Get the http://www.perl.com/CPAN/modules/by-module/Roman module.
+Get the http://www.cpan.org/modules/by-module/Roman module.
=head2 Why aren't my random numbers random?
-The short explanation is that you're getting pseudorandom numbers, not
-random ones, because computers are good at being predictable and bad
-at being random (despite appearances caused by bugs in your programs
-:-). A longer explanation is available on
-http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of Tom
-Phoenix. John von Neumann said, ``Anyone who attempts to generate
-random numbers by deterministic means is, of course, living in a state
-of sin.''
-
-You should also check out the Math::TrulyRandom module from CPAN. It
-uses the imperfections in your system's timer to generate random
-numbers, but this takes quite a while. If you want a better
+If you're using a version of Perl before 5.004, you must call C<srand>
+once at the start of your program to seed the random number generator.
+
+ BEGIN { srand() if $] < 5.004 }
+
+5.004 and later automatically call C<srand> at the beginning. Don't
+call C<srand> more than once---you make your numbers less random, rather
+than more.
+
+Computers are good at being predictable and bad at being random
+(despite appearances caused by bugs in your programs :-). see the
+F<random> article in the "Far More Than You Ever Wanted To Know"
+collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of
+Tom Phoenix, talks more about this. John von Neumann said, ``Anyone
+who attempts to generate random numbers by deterministic means is, of
+course, living in a state of sin.''
+
+If you want numbers that are more random than C<rand> with C<srand>
+provides, you should also check out the Math::TrulyRandom module from
+CPAN. It uses the imperfections in your system's timer to generate
+random numbers, but this takes quite a while. If you want a better
pseudorandom generator than comes with your operating system, look at
-``Numerical Recipes in C'' at http://nr.harvard.edu/nr/bookc.html .
+``Numerical Recipes in C'' at http://www.nr.com/ .
+
+=head2 How do I get a random number between X and Y?
+
+C<rand($x)> returns a number such that
+C<< 0 <= rand($x) < $x >>. Thus what you want to have perl
+figure out is a random number in the range from 0 to the
+difference between your I<X> and I<Y>.
+
+That is, to get a number between 10 and 15, inclusive, you
+want a random number between 0 and 5 that you can then add
+to 10.
+
+ my $number = 10 + int rand( 15-10+1 );
+
+Hence you derive the following simple function to abstract
+that. It selects a random integer between the two given
+integers (inclusive), For example: C<random_int_in(50,120)>.
+
+ sub random_int_in ($$) {
+ my($min, $max) = @_;
+ # Assumes that the two arguments are integers themselves!
+ return $min if $min == $max;
+ ($min, $max) = ($max, $min) if $min > $max;
+ return $min + int rand(1 + $max - $min);
+ }
=head1 Data: Dates
-=head2 How do I find the week-of-the-year/day-of-the-year?
+=head2 How do I find the day or week of the year?
+
+The localtime function returns the day of the year. Without an
+argument localtime uses the current time.
-The day of the year is in the array returned by localtime() (see
-L<perlfunc/"localtime">):
+ $day_of_year = (localtime)[7];
- $day_of_year = (localtime(time()))[7];
+The POSIX module can also format a date as the day of the year or
+week of the year.
-or more legibly (in 5.004 or higher):
+ use POSIX qw/strftime/;
+ my $day_of_year = strftime "%j", localtime;
+ my $week_of_year = strftime "%W", localtime;
- use Time::localtime;
- $day_of_year = localtime(time())->yday;
+To get the day of year for any date, use the Time::Local module to get
+a time in epoch seconds for the argument to localtime.
-You can find the week of the year by dividing this by 7:
+ use POSIX qw/strftime/;
+ use Time::Local;
+ my $week_of_year = strftime "%W",
+ localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );
- $week_of_year = int($day_of_year / 7);
+The Date::Calc module provides two functions for to calculate these.
-Of course, this believes that weeks start at zero. The Date::Calc
-module from CPAN has a lot of date calculation functions, including
-day of the year, week of the year, and so on.
+ use Date::Calc;
+ my $day_of_year = Day_of_Year( 1987, 12, 18 );
+ my $week_of_year = Week_of_Year( 1987, 12, 18 );
+
+=head2 How do I find the current century or millennium?
+
+Use the following simple functions:
+
+ sub get_century {
+ return int((((localtime(shift || time))[5] + 1999))/100);
+ }
+
+ sub get_millennium {
+ return 1+int((((localtime(shift || time))[5] + 1899))/1000);
+ }
+
+On some systems, the POSIX module's strftime() function has
+been extended in a non-standard way to use a C<%C> format,
+which they sometimes claim is the "century". It isn't,
+because on most such systems, this is only the first two
+digits of the four-digit year, and thus cannot be used to
+reliably determine the current century or millennium.
=head2 How can I compare two dates and find the difference?
If you're storing your dates as epoch seconds then simply subtract one
from the other. If you've got a structured date (distinct year, day,
-month, hour, minute, seconds values) then use one of the Date::Manip
-and Date::Calc modules from CPAN.
+month, hour, minute, seconds values), then for reasons of accessibility,
+simplicity, and efficiency, merely use either timelocal or timegm (from
+the Time::Local module in the standard distribution) to reduce structured
+dates to epoch seconds. However, if you don't know the precise format of
+your dates, then you should probably use either of the Date::Manip and
+Date::Calc modules from CPAN before you go hacking up your own parsing
+routine to handle arbitrary date formats.
=head2 How can I take a string and turn it into epoch seconds?
=head2 How can I find the Julian Day?
-Neither Date::Manip nor Date::Calc deal with Julian days. Instead,
-there is an example of Julian date calculation that should help you in
-http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz
-.
+(contributed by brian d foy and Dave Cross)
+
+You can use the Time::JulianDay module available on CPAN. Ensure that
+you really want to find a Julian day, though, as many people have
+different ideas about Julian days. See
+http://www.hermetic.ch/cal_stud/jdn.htm for instance.
+
+You can also try the DateTime module, which can convert a date/time
+to a Julian Day.
+
+ $ perl -MDateTime -le'print DateTime->today->jd'
+ 2453401.5
+
+Or the modified Julian Day
+
+ $ perl -MDateTime -le'print DateTime->today->mjd'
+ 53401
+
+Or even the day of the year (which is what some people think of as a
+Julian day)
+
+ $ perl -MDateTime -le'print DateTime->today->doy'
+ 31
+
+=head2 How do I find yesterday's date?
+
+If you only need to find the date (and not the same time), you
+can use the Date::Calc module.
-=head2 Does Perl have a year 2000 problem? Is Perl Y2K compliant?
+ use Date::Calc qw(Today Add_Delta_Days);
-Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl
-is Y2K compliant.
+ my @date = Add_Delta_Days( Today(), -1 );
-Long answer: Perl is just as Y2K compliant as your pencil--no more,
-and no less. The date and time functions supplied with perl (gmtime
-and localtime) supply adequate information to determine the year well
-beyond 2000 (2038 is when trouble strikes for 32-bit machines). The
-year returned by these functions when used in an array context is the
-year minus 1900. For years between 1910 and 1999 this I<happens> to
-be a 2-digit decimal number. To avoid the year 2000 problem simply do
-not treat the year as a 2-digit number. It isn't.
+ print "@date\n";
+
+Most people try to use the time rather than the calendar to
+figure out dates, but that assumes that your days are
+twenty-four hours each. For most people, there are two days
+a year when they aren't: the switch to and from summer time
+throws this off. Russ Allbery offers this solution.
+
+ sub yesterday {
+ my $now = defined $_[0] ? $_[0] : time;
+ my $then = $now - 60 * 60 * 24;
+ my $ndst = (localtime $now)[8] > 0;
+ my $tdst = (localtime $then)[8] > 0;
+ $then - ($tdst - $ndst) * 60 * 60;
+ }
+
+Should give you "this time yesterday" in seconds since epoch relative to
+the first argument or the current time if no argument is given and
+suitable for passing to localtime or whatever else you need to do with
+it. $ndst is whether we're currently in daylight savings time; $tdst is
+whether the point 24 hours ago was in daylight savings time. If $tdst
+and $ndst are the same, a boundary wasn't crossed, and the correction
+will subtract 0. If $tdst is 1 and $ndst is 0, subtract an hour more
+from yesterday's time since we gained an extra hour while going off
+daylight savings time. If $tdst is 0 and $ndst is 1, subtract a
+negative hour (add an hour) to yesterday's time since we lost an hour.
+
+All of this is because during those days when one switches off or onto
+DST, a "day" isn't 24 hours long; it's either 23 or 25.
+
+The explicit settings of $ndst and $tdst are necessary because localtime
+only says it returns the system tm struct, and the system tm struct at
+least on Solaris doesn't guarantee any particular positive value (like,
+say, 1) for isdst, just a positive value. And that value can
+potentially be negative, if DST information isn't available (this sub
+just treats those cases like no DST).
+
+Note that between 2am and 3am on the day after the time zone switches
+off daylight savings time, the exact hour of "yesterday" corresponding
+to the current hour is not clearly defined. Note also that if used
+between 2am and 3am the day after the change to daylight savings time,
+the result will be between 3am and 4am of the previous day; it's
+arguable whether this is correct.
+
+This sub does not attempt to deal with leap seconds (most things don't).
+
+
+
+=head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
+
+Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
+Y2K compliant (whatever that means). The programmers you've hired to
+use it, however, probably are not.
+
+Long answer: The question belies a true understanding of the issue.
+Perl is just as Y2K compliant as your pencil--no more, and no less.
+Can you use your pencil to write a non-Y2K-compliant memo? Of course
+you can. Is that the pencil's fault? Of course it isn't.
+
+The date and time functions supplied with Perl (gmtime and localtime)
+supply adequate information to determine the year well beyond 2000
+(2038 is when trouble strikes for 32-bit machines). The year returned
+by these functions when used in a list context is the year minus 1900.
+For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
+number. To avoid the year 2000 problem simply do not treat the year as
+a 2-digit number. It isn't.
When gmtime() and localtime() are used in scalar context they return
a timestamp string that contains a fully-expanded year. For example,
That doesn't mean that Perl can't be used to create non-Y2K compliant
programs. It can. But so can your pencil. It's the fault of the user,
not the language. At the risk of inflaming the NRA: ``Perl doesn't
-break Y2K, people do.'' See http://language.perl.com/news/y2k.html for
+break Y2K, people do.'' See http://www.perl.org/about/y2k.html for
a longer exposition.
=head1 Data: Strings
It depends just what you mean by ``escape''. URL escapes are dealt
with in L<perlfaq9>. Shell escapes with the backslash (C<\>)
-character are removed with:
+character are removed with
s/\\(.)/$1/g;
To turn C<"abbcccd"> into C<"abccd">:
- s/(.)\1/$1/g;
+ s/(.)\1/$1/g; # add /s to include newlines
+
+Here's a solution that turns "abbcccd" to "abcd":
+
+ y///cs; # y == tr, but shorter :-)
=head2 How do I expand function calls in a string?
print "My sub returned @{[mysub(1,2,3)]} that time.\n";
-If you prefer scalar context, similar chicanery is also useful for
-arbitrary expressions:
-
- print "That yields ${\($n + 5)} widgets\n";
-
-Version 5.004 of Perl had a bug that gave list context to the
-expression in C<${...}>, but this is fixed in version 5.005.
-
-See also ``How can I expand variables in text strings?'' in this
-section of the FAQ.
-
=head2 How do I find matching/nesting anything?
This isn't something that can be done in one regular expression, no
characters, a pattern like C</x([^x]*)x/> will get the intervening
bits in $1. For multiple ones, then something more like
C</alpha(.*?)omega/> would be needed. But none of these deals with
-nested patterns, nor can they. For that you'll have to write a
-parser.
+nested patterns. For balanced expressions using C<(>, C<{>, C<[>
+or C<< < >> as delimiters, use the CPAN module Regexp::Common, or see
+L<perlre/(??{ code })>. For other cases, you'll have to write a parser.
If you are serious about writing a parser, there are a number of
-modules or oddities that will make your life a lot easier. There is
-the CPAN module Parse::RecDescent, the standard module Text::Balanced,
-the byacc program, and Mark-Jason Dominus's excellent I<py> tool at
-http://www.plover.com/~mjd/perl/py/ .
+modules or oddities that will make your life a lot easier. There are
+the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced;
+and the byacc program. Starting from perl 5.8 the Text::Balanced
+is part of the standard distribution.
One simple destructive, inside-out approach that you might try is to
pull out the smallest nesting parts one at a time:
- while (s//BEGIN((?:(?!BEGIN)(?!END).)*)END/gs) {
+ while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) {
# do something with $1
- }
+ }
+
+A more complicated and sneaky approach is to make Perl's regular
+expression engine do it for you. This is courtesy Dean Inada, and
+rather has the nature of an Obfuscated Perl Contest entry, but it
+really does work:
+
+ # $_ contains the string to parse
+ # BEGIN and END are the opening and closing markers for the
+ # nested text.
+
+ @( = ('(','');
+ @) = (')','');
+ ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs;
+ @$ = (eval{/$re/},$@!~/unmatched/i);
+ print join("\n",@$[0..$#$]) if( $$[-1] );
=head2 How do I reverse a string?
1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
-Or you can just use the Text::Tabs module (part of the standard perl
+Or you can just use the Text::Tabs module (part of the standard Perl
distribution).
use Text::Tabs;
=head2 How do I reformat a paragraph?
-Use Text::Wrap (part of the standard perl distribution):
+Use Text::Wrap (part of the standard Perl distribution):
use Text::Wrap;
print wrap("\t", ' ', @paragraphs);
The paragraphs you give to Text::Wrap should not contain embedded
newlines. Text::Wrap doesn't justify the lines (flush-right).
-=head2 How can I access/change the first N letters of a string?
+Or use the CPAN module Text::Autoformat. Formatting files can be easily
+done by making a shell alias, like so:
-There are many ways. If you just want to grab a copy, use
-substr():
+ alias fmt="perl -i -MText::Autoformat -n0777 \
+ -e 'print autoformat $_, {all=>1}' $*"
- $first_byte = substr($a, 0, 1);
+See the documentation for Text::Autoformat to appreciate its many
+capabilities.
-If you want to modify part of a string, the simplest way is often to
-use substr() as an lvalue:
+=head2 How can I access or change N characters of a string?
- substr($a, 0, 3) = "Tom";
+You can access the first characters of a string with substr().
+To get the first character, for example, start at position 0
+and grab the string of length 1.
-Although those with a pattern matching kind of thought process will
-likely prefer:
- $a =~ s/^.../Tom/;
+ $string = "Just another Perl Hacker";
+ $first_char = substr( $string, 0, 1 ); # 'J'
+
+To change part of a string, you can use the optional fourth
+argument which is the replacement string.
+
+ substr( $string, 13, 4, "Perl 5.8.0" );
+
+You can also use substr() as an lvalue.
+
+ substr( $string, 13, 4 ) = "Perl 5.8.0";
=head2 How do I change the Nth occurrence of something?
You have to keep track of N yourself. For example, let's say you want
to change the fifth occurrence of C<"whoever"> or C<"whomever"> into
-C<"whosoever"> or C<"whomsoever">, case insensitively.
+C<"whosoever"> or C<"whomsoever">, case insensitively. These
+all assume that $_ contains the string to be altered.
$count = 0;
s{((whom?)ever)}{
++$count == 5 # is it the 5th?
? "${2}soever" # yes, swap
: $1 # renege and leave it there
- }igex;
+ }ige;
In the more general case, you can use the C</g> modifier in a C<while>
loop, keeping count of matches.
$WANT = 3;
$count = 0;
+ $_ = "One fish two fish red fish blue fish";
while (/(\w+)\s+fish\b/gi) {
if (++$count == $WANT) {
print "The third fish is a $1 one.\n";
- # Warning: don't `last' out of this loop
}
}
=head2 How can I count the number of occurrences of a substring within a string?
-There are a number of ways, with varying efficiency: If you want a
+There are a number of ways, with varying efficiency. If you want a
count of a certain single character (X) within a string, you can use the
C<tr///> function like so:
- $string = "ThisXlineXhasXsomeXx'sXinXit":
+ $string = "ThisXlineXhasXsomeXx'sXinXit";
$count = ($string =~ tr/X//);
- print "There are $count X charcters in the string";
+ print "There are $count X characters in the string";
This is fine if you are just looking for a single character. However,
if you are trying to count multiple character substrings within a
while ($string =~ /-\d+/g) { $count++ }
print "There are $count negative numbers in the string";
+Another version uses a global match in list context, then assigns the
+result to a scalar, producing a count of the number of matches.
+
+ $count = () = $string =~ /-\d+/g;
+
=head2 How do I capitalize all the words on one line?
To make the first letter of each word upper case:
$line =~ s/\b(\w)/\U$1/g;
This has the strange effect of turning "C<don't do it>" into "C<Don'T
-Do It>". Sometimes you might want this, instead (Suggested by Brian
-Foy):
+Do It>". Sometimes you might want this. Other times you might need a
+more thorough solution (Suggested by brian d foy):
$string =~ s/ (
(^\w) #at the beginning of the line
characters by placing a C<use locale> pragma in your program.
See L<perllocale> for endless details on locales.
-=head2 How can I split a [character] delimited string except when inside
-[character]? (Comma-separated files)
+This is sometimes referred to as putting something into "title
+case", but that's not quite accurate. Consider the proper
+capitalization of the movie I<Dr. Strangelove or: How I Learned to
+Stop Worrying and Love the Bomb>, for example.
-Take the example case of trying to split a string that is comma-separated
-into its different fields. (We'll pretend you said comma-separated, not
-comma-delimited, which is different and almost never what you mean.) You
-can't use C<split(/,/)> because you shouldn't split if the comma is inside
-quotes. For example, take a data line like this:
+Damian Conway's L<Text::Autoformat> module provides some smart
+case transformations:
+
+ use Text::Autoformat;
+ my $x = "Dr. Strangelove or: How I Learned to Stop ".
+ "Worrying and Love the Bomb";
+
+ print $x, "\n";
+ for my $style (qw( sentence title highlight ))
+ {
+ print autoformat($x, { case => $style }), "\n";
+ }
+
+=head2 How can I split a [character] delimited string except when inside [character]?
+
+Several modules can handle this sort of pasing---Text::Balanced,
+Text::CSV, Text::CSV_XS, and Text::ParseWords, among others.
+
+Take the example case of trying to split a string that is
+comma-separated into its different fields. You can't use C<split(/,/)>
+because you shouldn't split if the comma is inside quotes. For
+example, take a data line like this:
SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
Due to the restriction of the quotes, this is a fairly complex
-problem. Thankfully, we have Jeffrey Friedl, author of a highly
-recommended book on regular expressions, to handle these for us. He
+problem. Thankfully, we have Jeffrey Friedl, author of
+I<Mastering Regular Expressions>, to handle these for us. He
suggests (assuming your string is contained in $text):
@new = ();
If you want to represent quotation marks inside a
quotation-mark-delimited field, escape them with backslashes (eg,
-C<"like \"this\"">. Unescaping them is a task addressed earlier in
-this section.
+C<"like \"this\"">.
-Alternatively, the Text::ParseWords module (part of the standard perl
+Alternatively, the Text::ParseWords module (part of the standard Perl
distribution) lets you say:
use Text::ParseWords;
@new = quotewords(",", 0, $text);
+There's also a Text::CSV (Comma-Separated Values) module on CPAN.
+
=head2 How do I strip blank space from the beginning/end of a string?
-Although the simplest approach would seem to be:
+Although the simplest approach would seem to be
$string =~ s/^\s*(.*?)\s*$/$1/;
-This is unneccesarily slow, destructive, and fails with embedded newlines.
-It is much better faster to do this in two steps:
+not only is this unnecessarily slow and destructive, it also fails with
+embedded newlines. It is much faster to do this operation in two steps:
$string =~ s/^\s+//;
$string =~ s/\s+$//;
s/\s+$//;
}
-This idiom takes advantage of the C<for(each)> loop's aliasing
+This idiom takes advantage of the C<foreach> loop's aliasing
behavior to factor out common code. You can do this
-on several strings at once, or arrays, or even the
-values of a hash if you use a slide:
+on several strings at once, or arrays, or even the
+values of a hash if you use a slice:
- # trim whitespace in the scalar, the array,
+ # trim whitespace in the scalar, the array,
# and all the values in the hash
foreach ($scalar, @array, @hash{keys %hash}) {
s/^\s+//;
s/\s+$//;
}
+=head2 How do I pad a string with blanks or pad a number with zeroes?
+
+In the following examples, C<$pad_len> is the length to which you wish
+to pad the string, C<$text> or C<$num> contains the string to be padded,
+and C<$pad_char> contains the padding character. You can use a single
+character string constant instead of the C<$pad_char> variable if you
+know what it is in advance. And in the same way you can use an integer in
+place of C<$pad_len> if you know the pad length in advance.
+
+The simplest method uses the C<sprintf> function. It can pad on the left
+or right with blanks and on the left with zeroes and it will not
+truncate the result. The C<pack> function can only pad strings on the
+right with blanks and it will truncate the result to a maximum length of
+C<$pad_len>.
+
+ # Left padding a string with blanks (no truncation):
+ $padded = sprintf("%${pad_len}s", $text);
+ $padded = sprintf("%*s", $pad_len, $text); # same thing
+
+ # Right padding a string with blanks (no truncation):
+ $padded = sprintf("%-${pad_len}s", $text);
+ $padded = sprintf("%-*s", $pad_len, $text); # same thing
+
+ # Left padding a number with 0 (no truncation):
+ $padded = sprintf("%0${pad_len}d", $num);
+ $padded = sprintf("%0*d", $pad_len, $num); # same thing
+
+ # Right padding a string with blanks using pack (will truncate):
+ $padded = pack("A$pad_len",$text);
+
+If you need to pad with a character other than blank or zero you can use
+one of the following methods. They all generate a pad string with the
+C<x> operator and combine that with C<$text>. These methods do
+not truncate C<$text>.
+
+Left and right padding with any character, creating a new string:
+
+ $padded = $pad_char x ( $pad_len - length( $text ) ) . $text;
+ $padded = $text . $pad_char x ( $pad_len - length( $text ) );
+
+Left and right padding with any character, modifying C<$text> directly:
+
+ substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) );
+ $text .= $pad_char x ( $pad_len - length( $text ) );
+
=head2 How do I extract selected columns from a string?
Use substr() or unpack(), both documented in L<perlfunc>.
-If you prefer thinking in terms of columns instead of widths,
+If you prefer thinking in terms of columns instead of widths,
you can use this kind of thing:
# determine the unpack format needed to split Linux ps output
# arguments are cut columns
my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);
- sub cut2fmt {
+ sub cut2fmt {
my(@positions) = @_;
my $template = '';
my $lastpos = 1;
for my $place (@positions) {
- $template .= "A" . ($place - $lastpos) . " ";
+ $template .= "A" . ($place - $lastpos) . " ";
$lastpos = $place;
}
$template .= "A*";
=head2 How do I find the soundex value of a string?
-Use the standard Text::Soundex module distributed with perl.
+(contributed by brian d foy)
+
+You can use the Text::Soundex module. If you want to do fuzzy or close
+matching, you might also try the String::Approx, and Text::Metaphone,
+and Text::DoubleMetaphone modules.
=head2 How can I expand variables in text strings?
-Let's assume that you have a string like:
+Let's assume that you have a string that contains placeholder
+variables.
$text = 'this has a $foo in it and a $bar';
-If those were both global variables, then this would
-suffice:
-
- $text =~ s/\$(\w+)/${$1}/g;
+You can use a substitution with a double evaluation. The
+first /e turns C<$1> into C<$foo>, and the second /e turns
+C<$foo> into its value. You may want to wrap this in an
+C<eval>: if you try to get the value of an undeclared variable
+while running under C<use strict>, you get a fatal error.
-But since they are probably lexicals, or at least, they could
-be, you'd have to do this:
-
- $text =~ s/(\$\w+)/$1/eeg;
- die if $@; # needed on /ee, not /e
+ eval { $text =~ s/(\$\w+)/$1/eeg };
+ die if $@;
It's probably better in the general case to treat those
variables as entries in some special hash. For example:
- %user_defs = (
+ %user_defs = (
foo => 23,
bar => 19,
);
$text =~ s/\$(\w+)/$user_defs{$1}/g;
-See also ``How do I expand function calls in a string?'' in this section
-of the FAQ.
-
=head2 What's wrong with always quoting "$vars"?
-The problem is that those double-quotes force stringification,
-coercing numbers and references into strings, even when you
-don't want them to be.
+The problem is that those double-quotes force stringification--
+coercing numbers and references into strings--even when you
+don't want them to be strings. Think of it this way: double-quote
+expansion is used to produce new strings. If you already
+have a string, why do you need more?
If you get used to writing odd things like these:
number, such as the magical C<++> autoincrement operator or the
syscall() function.
-Stringification also destroys arrays.
+Stringification also destroys arrays.
@lines = `command`;
print "@lines"; # WRONG - extra blanks
print @lines; # right
-=head2 Why don't my <<HERE documents work?
+=head2 Why don't my E<lt>E<lt>HERE documents work?
Check for these three things:
=over 4
-=item 1. There must be no space after the << part.
+=item There must be no space after the E<lt>E<lt> part.
-=item 2. There (probably) should be a semicolon at the end.
+=item There (probably) should be a semicolon at the end.
-=item 3. You can't (easily) have any space in front of the tag.
+=item You can't (easily) have any space in front of the tag.
=back
-If you want to indent the text in the here document, you
+If you want to indent the text in the here document, you
can do this:
# all in one
HERE_TARGET
But the HERE_TARGET must still be flush against the margin.
-If you want that indented also, you'll have to quote
+If you want that indented also, you'll have to quote
in the indentation.
($quote = <<' FINIS') =~ s/^\s+//gm;
would deliver us. You are a liar, Saruman, and a corrupter
of men's hearts. --Theoden in /usr/src/perl/taint.c
FINIS
- $quote =~ s/\s*--/\n--/;
+ $quote =~ s/\s+--/\n--/;
A nice general-purpose fixer-upper function for indented here documents
follows. It expects to be called with a here document as its argument.
It looks to see whether each line begins with a common substring, and
-if so, strips that off. Otherwise, it takes the amount of leading
-white space found on the first line and removes that much off each
+if so, strips that substring off. Otherwise, it takes the amount of leading
+whitespace found on the first line and removes that much off each
subsequent line.
sub fix {
local $_ = shift;
- my ($white, $leader); # common white space and common leading string
+ my ($white, $leader); # common whitespace and common leading string
if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
($white, $leader) = ($2, quotemeta($1));
} else {
@@@ runops() {
@@@ SAVEI32(runlevel);
@@@ runlevel++;
- @@@ while ( op = (*op->op_ppaddr)() ) ;
+ @@@ while ( op = (*op->op_ppaddr)() );
@@@ TAINT_NOT;
@@@ return 0;
@@@ }
MAIN_INTERPRETER_LOOP
-Or with a fixed amount of leading white space, with remaining
+Or with a fixed amount of leading whitespace, with remaining
indentation correctly preserved:
$poem = fix<<EVER_ON_AND_ON;
=head1 Data: Arrays
+=head2 What is the difference between a list and an array?
+
+An array has a changeable length. A list does not. An array is something
+you can push or pop, while a list is a set of values. Some people make
+the distinction that a list is a value while an array is a variable.
+Subroutines are passed and return lists, you put things into list
+context, you initialize arrays with lists, and you foreach() across
+a list. C<@> variables are arrays, anonymous arrays are arrays, arrays
+in scalar context behave like the number of elements in them, subroutines
+access their arguments through the array C<@_>, and push/pop/shift only work
+on arrays.
+
+As a side note, there's no such thing as a list in scalar context.
+When you say
+
+ $scalar = (2, 5, 7, 9);
+
+you're using the comma operator in scalar context, so it uses the scalar
+comma operator. There never was a list there at all! This causes the
+last value to be returned: 9.
+
=head2 What is the difference between $array[1] and @array[1]?
-The former is a scalar value, the latter an array slice, which makes
+The former is a scalar value; the latter an array slice, making
it a list with one (scalar) value. You should use $ when you want a
scalar value (most of the time) and @ when you want a list with one
scalar value in it (very, very rarely; nearly never, in fact).
@bad[0] = `same program that outputs several lines`;
-The B<-w> flag will warn you about these matters.
+The C<use warnings> pragma and the B<-w> flag will warn you about these
+matters.
-=head2 How can I extract just the unique elements of an array?
+=head2 How can I remove duplicate elements from a list or array?
There are several possible ways, depending on whether the array is
ordered and whether you wish to preserve the ordering.
=over 4
-=item a) If @in is sorted, and you want @out to be sorted:
+=item a)
+
+If @in is sorted, and you want @out to be sorted:
(this assumes all true values in the array)
- $prev = 'nonesuch';
- @out = grep($_ ne $prev && ($prev = $_), @in);
+ $prev = "not equal to $in[0]";
+ @out = grep($_ ne $prev && ($prev = $_, 1), @in);
This is nice in that it doesn't use much extra memory, simulating
-uniq(1)'s behavior of removing only adjacent duplicates. It's less
-nice in that it won't work with false values like undef, 0, or "";
-"0 but true" is ok, though.
+uniq(1)'s behavior of removing only adjacent duplicates. The ", 1"
+guarantees that the expression is true (so that grep picks it up)
+even if the $_ is 0, "", or undef.
-=item b) If you don't know whether @in is sorted:
+=item b)
+
+If you don't know whether @in is sorted:
undef %saw;
@out = grep(!$saw{$_}++, @in);
-=item c) Like (b), but @in contains only small integers:
+=item c)
+
+Like (b), but @in contains only small integers:
@out = grep(!$saw[$_]++, @in);
-=item d) A way to do (b) without any loops or greps:
+=item d)
+
+A way to do (b) without any loops or greps:
undef %saw;
@saw{@in} = ();
@out = sort keys %saw; # remove sort if undesired
-=item e) Like (d), but @in contains only small positive integers:
+=item e)
+
+Like (d), but @in contains only small positive integers:
undef @ary;
@ary[@in] = @in;
- @out = @ary;
+ @out = grep {defined} @ary;
=back
-=head2 How can I tell whether a list or array contains a certain element?
+But perhaps you should have been using a hash all along, eh?
+
+=head2 How can I tell whether a certain element is contained in a list or array?
Hearing the word "in" is an I<in>dication that you probably should have
used a hash, not a list or array, to store your data. Hashes are
That being said, there are several ways to approach this. If you
are going to make this query many times over arbitrary string values,
-the fastest way is probably to invert the original array and keep an
-associative array lying about whose keys are the first array's values.
+the fastest way is probably to invert the original array and maintain a
+hash whose keys are the first array's values.
@blues = qw/azure cerulean teal turquoise lapis-lazuli/;
- undef %is_blue;
+ %is_blue = ();
for (@blues) { $is_blue{$_} = 1 }
Now you can check whether $is_blue{$some_color}. It might have been a
array. This kind of an array will take up less space:
@primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31);
- undef @is_tiny_prime;
- for (@primes) { $is_tiny_prime[$_] = 1; }
+ @is_tiny_prime = ();
+ for (@primes) { $is_tiny_prime[$_] = 1 }
+ # or simply @istiny_prime[@primes] = (1) x @primes;
Now you check whether $is_tiny_prime[$some_number].
Please do not use
- $is_there = grep $_ eq $whatever, @array;
+ ($is_there) = grep $_ eq $whatever, @array;
or worse yet
- $is_there = grep /$whatever/, @array;
+ ($is_there) = grep /$whatever/, @array;
These are slow (checks every element even if the first matches),
inefficient (same reason), and potentially buggy (what if there are
-regexp characters in $whatever?).
+regex characters in $whatever?). If you're only testing once, then
+use:
+
+ $is_there = 0;
+ foreach $elt (@array) {
+ if ($elt eq $elt_to_find) {
+ $is_there = 1;
+ last;
+ }
+ }
+ if ($is_there) { ... }
=head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;
}
-=head2 How do I find the first array element for which a condition is true?
+Note that this is the I<symmetric difference>, that is, all elements in
+either A or in B but not in both. Think of it as an xor operation.
-You can use this if you care about the index:
+=head2 How do I test whether two arrays or hashes are equal?
- for ($i=0; $i < @array; $i++) {
- if ($array[$i] eq "Waldo") {
- $found_index = $i;
- last;
- }
+The following code works for single-level arrays. It uses a stringwise
+comparison, and does not distinguish defined versus undefined empty
+strings. Modify if you have other needs.
+
+ $are_equal = compare_arrays(\@frogs, \@toads);
+
+ sub compare_arrays {
+ my ($first, $second) = @_;
+ no warnings; # silence spurious -w undef complaints
+ return 0 unless @$first == @$second;
+ for (my $i = 0; $i < @$first; $i++) {
+ return 0 if $first->[$i] ne $second->[$i];
+ }
+ return 1;
}
-Now C<$found_index> has what you want.
+For multilevel structures, you may wish to use an approach more
+like this one. It uses the CPAN module FreezeThaw:
+
+ use FreezeThaw qw(cmpStr);
+ @a = @b = ( "this", "that", [ "more", "stuff" ] );
+
+ printf "a and b contain %s arrays\n",
+ cmpStr(\@a, \@b) == 0
+ ? "the same"
+ : "different";
+
+This approach also works for comparing hashes. Here
+we'll demonstrate two different answers:
+
+ use FreezeThaw qw(cmpStr cmpStrHard);
+
+ %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] );
+ $a{EXTRA} = \%b;
+ $b{EXTRA} = \%a;
+
+ printf "a and b contain %s hashes\n",
+ cmpStr(\%a, \%b) == 0 ? "the same" : "different";
+
+ printf "a and b contain %s hashes\n",
+ cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
+
+
+The first reports that both those the hashes contain the same data,
+while the second reports that they do not. Which you prefer is left as
+an exercise to the reader.
+
+=head2 How do I find the first array element for which a condition is true?
+
+To find the first array element which satisfies a condition, you can
+use the first() function in the List::Util module, which comes with
+Perl 5.8. This example finds the first element that contains "Perl".
+
+ use List::Util qw(first);
+
+ my $element = first { /Perl/ } @array;
+
+If you cannot use List::Util, you can make your own loop to do the
+same thing. Once you find the element, you stop the loop with last.
+
+ my $found;
+ foreach my $element ( @array )
+ {
+ if( /Perl/ ) { $found = $element; last }
+ }
+
+If you want the array index, you can iterate through the indices
+and check the array element at each index until you find one
+that satisfies the condition.
+
+ my( $found, $index ) = ( undef, -1 );
+ for( $i = 0; $i < @array; $i++ )
+ {
+ if( $array[$i] =~ /Perl/ )
+ {
+ $found = $array[$i];
+ $index = $i;
+ last;
+ }
+ }
=head2 How do I handle linked lists?
In general, you usually don't need a linked list in Perl, since with
regular arrays, you can push and pop or shift and unshift at either end,
or you can use splice to add and/or remove arbitrary number of elements at
-arbitrary points. Both pop and shift are both O(1) operations on perl's
+arbitrary points. Both pop and shift are both O(1) operations on Perl's
dynamic arrays. In the absence of shifts and pops, push in general
needs to reallocate on the order every log(N) times, and unshift will
need to copy pointers each time.
If you really, really wanted, you could use structures as described in
L<perldsc> or L<perltoot> and do just what the algorithm book tells you
-to do.
+to do. For example, imagine a list node like this:
+
+ $node = {
+ VALUE => 42,
+ LINK => undef,
+ };
+
+You could walk the list this way:
+
+ print "List: ";
+ for ($node = $head; $node; $node = $node->{LINK}) {
+ print $node->{VALUE}, " ";
+ }
+ print "\n";
+
+You could add to the list this way:
+
+ my ($head, $tail);
+ $tail = append($head, 1); # grow a new head
+ for $value ( 2 .. 10 ) {
+ $tail = append($tail, $value);
+ }
+
+ sub append {
+ my($list, $value) = @_;
+ my $node = { VALUE => $value };
+ if ($list) {
+ $node->{LINK} = $list->{LINK};
+ $list->{LINK} = $node;
+ } else {
+ $_[0] = $node; # replace caller's version
+ }
+ return $node;
+ }
+
+But again, Perl's built-in are virtually always good enough.
=head2 How do I handle circular lists?
=head2 How do I shuffle an array randomly?
-Use this:
+If you either have Perl 5.8.0 or later installed, or if you have
+Scalar-List-Utils 1.03 or later installed, you can say:
+
+ use List::Util 'shuffle';
+
+ @shuffled = shuffle(@list);
+
+If not, you can use a Fisher-Yates shuffle.
- # fisher_yates_shuffle( \@array ) :
- # generate a random permutation of @array in place
sub fisher_yates_shuffle {
- my $array = shift;
- my $i;
- for ($i = @$array; --$i; ) {
+ my $deck = shift; # $deck is a reference to an array
+ my $i = @$deck;
+ while ($i--) {
my $j = int rand ($i+1);
- next if $i == $j;
- @$array[$i,$j] = @$array[$j,$i];
+ @$deck[$i,$j] = @$deck[$j,$i];
}
}
- fisher_yates_shuffle( \@array ); # permutes @array in place
+ # shuffle my mpeg collection
+ #
+ my @mpeg = <audio/*/*.mp3>;
+ fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place
+ print @mpeg;
-You've probably seen shuffling algorithms that works using splice,
-randomly picking another element to swap the current element with:
+Note that the above implementation shuffles an array in place,
+unlike the List::Util::shuffle() which takes a list and returns
+a new shuffled list.
+
+You've probably seen shuffling algorithms that work using splice,
+randomly picking another element to swap the current element with
srand;
@new = ();
$_ *= (4/3) * 3.14159; # this will be constant folded
}
-If you want to do the same thing to modify the values of the hash,
-you may not use the C<values> function, oddly enough. You need a slice:
+which can also be done with map() which is made to transform
+one list into another:
+
+ @volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
+
+If you want to do the same thing to modify the values of the
+hash, you can use the C<values> function. As of Perl 5.6
+the values are not copied, so if you modify $orbit (in this
+case), you modify the value.
- for $orbit ( @orbits{keys %orbits} ) {
- ($orbit **= 3) *= (4/3) * 3.14159;
+ for $orbit ( values %orbits ) {
+ ($orbit **= 3) *= (4/3) * 3.14159;
}
+Prior to perl 5.6 C<values> returned copies of the values,
+so older perl code often contains constructions such as
+C<@orbits{keys %orbits}> instead of C<values %orbits> where
+the hash is to be modified.
+
=head2 How do I select a random element from an array?
Use the rand() function (see L<perlfunc/rand>):
- # at the top of the program:
- srand; # not needed for 5.004 and later
-
- # then later on
$index = rand @array;
$element = $array[$index];
-Make sure you I<only call srand once per program, if then>.
-If you are calling it more than once (such as before each
-call to rand), you're almost certainly doing something wrong.
+Or, simply:
+ my $element = $array[ rand @array ];
=head2 How do I permute N elements of a list?
-Here's a little program that generates all permutations
-of all the words on each line of input. The algorithm embodied
-in the permute() function should work on any list:
-
- #!/usr/bin/perl -n
- # tsc-permute: permute each word of input
- permute([split], []);
- sub permute {
- my @items = @{ $_[0] };
- my @perms = @{ $_[1] };
- unless (@items) {
- print "@perms\n";
- } else {
- my(@newitems,@newperms,$i);
- foreach $i (0 .. $#items) {
- @newitems = @items;
- @newperms = @perms;
- unshift(@newperms, splice(@newitems, $i, 1));
- permute([@newitems], [@newperms]);
- }
+Use the List::Permutor module on CPAN. If the list is
+actually an array, try the Algorithm::Permute module (also
+on CPAN). It's written in XS code and is very efficient.
+
+ use Algorithm::Permute;
+ my @array = 'a'..'d';
+ my $p_iterator = Algorithm::Permute->new ( \@array );
+ while (my @perm = $p_iterator->next) {
+ print "next permutation: (@perm)\n";
+ }
+
+For even faster execution, you could do:
+
+ use Algorithm::Permute;
+ my @array = 'a'..'d';
+ Algorithm::Permute::permute {
+ print "next permutation: (@array)\n";
+ } @array;
+
+Here's a little program that generates all permutations of
+all the words on each line of input. The algorithm embodied
+in the permute() function is discussed in Volume 4 (still
+unpublished) of Knuth's I<The Art of Computer Programming>
+and will work on any list:
+
+ #!/usr/bin/perl -n
+ # Fischer-Kause ordered permutation generator
+
+ sub permute (&@) {
+ my $code = shift;
+ my @idx = 0..$#_;
+ while ( $code->(@_[@idx]) ) {
+ my $p = $#idx;
+ --$p while $idx[$p-1] > $idx[$p];
+ my $q = $p or return;
+ push @idx, reverse splice @idx, $p;
+ ++$q while $idx[$p-1] > $idx[$q];
+ @idx[$p-1,$q]=@idx[$q,$p-1];
+ }
}
- }
+
+ permute {print"@_\n"} split;
=head2 How do I sort an array by (anything)?
@list = sort { $a <=> $b } @list;
The default sort function is cmp, string comparison, which would
-sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<E<lt>=E<gt>>, used above, is
+sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is
the numerical comparison operator.
If you have a complicated function needed to pull out the part you
}
@sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
-Which could also be written this way, using a trick
+which could also be written this way, using a trick
that's come to be known as the Schwartzian Transform:
@sorted = map { $_->[0] }
sort { $a->[1] cmp $b->[1] }
- map { [ $_, uc((/\d+\s*(\S+)/ )[0] ] } @data;
+ map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
If you need to sort on several fields, the following paradigm is useful.
This can be conveniently combined with precalculation of keys as given
above.
-See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about
-this approach.
+See the F<sort> article in the "Far More Than You Ever Wanted
+To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for
+more about this approach.
See also the question below on sorting hashes.
$vec = '';
foreach(@ints) { vec($vec,$_,1) = 1 }
-And here's how, given a vector in $vec, you can
+Here's how, given a vector in $vec, you can
get those bits into your @ints array:
sub bitvec_to_list {
This method gets faster the more sparse the bit vector is.
(Courtesy of Tim Bunce and Winfried Koenig.)
+You can make the while loop a lot shorter with this suggestion
+from Benjamin Goldberg:
+
+ while($vec =~ /[^\0]+/g ) {
+ push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8;
+ }
+
+Or use the CPAN module Bit::Vector:
+
+ $vector = Bit::Vector->new($num_of_bits);
+ $vector->Index_List_Store(@ints);
+ @ints = $vector->Index_List_Read();
+
+Bit::Vector provides efficient methods for bit vector, sets of small integers
+and "big int" math.
+
+Here's a more extensive illustration using vec():
+
+ # vec demo
+ $vector = "\xff\x0f\xef\xfe";
+ print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ",
+ unpack("N", $vector), "\n";
+ $is_set = vec($vector, 23, 1);
+ print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n";
+ pvec($vector);
+
+ set_vec(1,1,1);
+ set_vec(3,1,1);
+ set_vec(23,1,1);
+
+ set_vec(3,1,3);
+ set_vec(3,2,3);
+ set_vec(3,4,3);
+ set_vec(3,4,7);
+ set_vec(3,8,3);
+ set_vec(3,8,7);
+
+ set_vec(0,32,17);
+ set_vec(1,32,17);
+
+ sub set_vec {
+ my ($offset, $width, $value) = @_;
+ my $vector = '';
+ vec($vector, $offset, $width) = $value;
+ print "offset=$offset width=$width value=$value\n";
+ pvec($vector);
+ }
+
+ sub pvec {
+ my $vector = shift;
+ my $bits = unpack("b*", $vector);
+ my $i = 0;
+ my $BASE = 8;
+
+ print "vector length in bytes: ", length($vector), "\n";
+ @bytes = unpack("A8" x length($vector), $bits);
+ print "bits are: @bytes\n\n";
+ }
+
=head2 Why does defined() return true on empty arrays and hashes?
-See L<perlfunc/defined> in the 5.004 release or later of Perl.
+The short story is that you should probably only use defined on scalars or
+functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
+in the 5.004 release or later of Perl for more detail.
=head1 Data: Hashes (Associative Arrays)
=head2 What happens if I add or remove keys from a hash while iterating over it?
-Don't do that.
+(contributed by brian d foy)
+
+The easy answer is "Don't do that!"
+
+If you iterate through the hash with each(), you can delete the key
+most recently returned without worrying about it. If you delete or add
+other keys, the iterator may skip or double up on them since perl
+may rearrange the hash table. See the
+entry for C<each()> in L<perlfunc>.
=head2 How do I look up a hash element by value?
$by_value{$value} = $key;
}
-If your hash could have repeated values, the methods above will only
-find one of the associated keys. This may or may not worry you.
+If your hash could have repeated values, the methods above will only find
+one of the associated keys. This may or may not worry you. If it does
+worry you, you can always reverse the hash into a hash of arrays instead:
+
+ while (($key, $value) = each %by_key) {
+ push @{$key_list_by_value{$value}}, $key;
+ }
=head2 How can I know how many entries are in a hash?
If you mean how many keys, then all you have to do is
-take the scalar sense of the keys() function:
+use the keys() function in a scalar context:
- $num_keys = scalar keys %hash;
+ $num_keys = keys %hash;
-In void context it just resets the iterator, which is faster
-for tied hashes.
+The keys() function also resets the iterator, which means that you may
+see strange results if you use this between uses of other hash operators
+such as each().
=head2 How do I sort a hash (optionally by value instead of key)?
} keys %hash; # and by value
Here we'll do a reverse numeric sort by value, and if two keys are
-identical, sort by length of key, and if that fails, by straight ASCII
-comparison of the keys (well, possibly modified by your locale -- see
+identical, sort by length of key, or if that fails, by straight ASCII
+comparison of the keys (well, possibly modified by your locale--see
L<perllocale>).
@keys = sort {
=head2 What's the difference between "delete" and "undef" with hashes?
-Hashes are pairs of scalars: the first is the key, the second is the
-value. The key will be coerced to a string, although the value can be
-any kind of scalar: string, number, or reference. If a key C<$key> is
-present in the array, C<exists($key)> will return true. The value for
-a given key can be C<undef>, in which case C<$array{$key}> will be
-C<undef> while C<$exists{$key}> will return true. This corresponds to
-(C<$key>, C<undef>) being in the hash.
+Hashes contain pairs of scalars: the first is the key, the
+second is the value. The key will be coerced to a string,
+although the value can be any kind of scalar: string,
+number, or reference. If a key $key is present in
+%hash, C<exists($hash{$key})> will return true. The value
+for a given key can be C<undef>, in which case
+C<$hash{$key}> will be C<undef> while C<exists $hash{$key}>
+will return true. This corresponds to (C<$key>, C<undef>)
+being in the hash.
-Pictures help... here's the C<%ary> table:
+Pictures help... here's the %hash table:
keys values
+------+------+
And these conditions hold
- $ary{'a'} is true
- $ary{'d'} is false
- defined $ary{'d'} is true
- defined $ary{'a'} is true
- exists $ary{'a'} is true (perl5 only)
- grep ($_ eq 'a', keys %ary) is true
+ $hash{'a'} is true
+ $hash{'d'} is false
+ defined $hash{'d'} is true
+ defined $hash{'a'} is true
+ exists $hash{'a'} is true (Perl5 only)
+ grep ($_ eq 'a', keys %hash) is true
If you now say
- undef $ary{'a'}
+ undef $hash{'a'}
your table now reads:
and these conditions now hold; changes in caps:
- $ary{'a'} is FALSE
- $ary{'d'} is false
- defined $ary{'d'} is true
- defined $ary{'a'} is FALSE
- exists $ary{'a'} is true (perl5 only)
- grep ($_ eq 'a', keys %ary) is true
+ $hash{'a'} is FALSE
+ $hash{'d'} is false
+ defined $hash{'d'} is true
+ defined $hash{'a'} is FALSE
+ exists $hash{'a'} is true (Perl5 only)
+ grep ($_ eq 'a', keys %hash) is true
Notice the last two: you have an undef value, but a defined key!
Now, consider this:
- delete $ary{'a'}
+ delete $hash{'a'}
your table now reads:
and these conditions now hold; changes in caps:
- $ary{'a'} is false
- $ary{'d'} is false
- defined $ary{'d'} is true
- defined $ary{'a'} is false
- exists $ary{'a'} is FALSE (perl5 only)
- grep ($_ eq 'a', keys %ary) is FALSE
+ $hash{'a'} is false
+ $hash{'d'} is false
+ defined $hash{'d'} is true
+ defined $hash{'a'} is false
+ exists $hash{'a'} is FALSE (Perl5 only)
+ grep ($_ eq 'a', keys %hash) is FALSE
See, the whole entry is gone!
=head2 Why don't my tied hashes make the defined/exists distinction?
-They may or may not implement the EXISTS() and DEFINED() methods
-differently. For example, there isn't the concept of undef with hashes
-that are tied to DBM* files. This means the true/false tables above
-will give different results when used on such a hash. It also means
-that exists and defined do the same thing with a DBM* file, and what
-they end up doing is not what they do with ordinary hashes.
+This depends on the tied hash's implementation of EXISTS().
+For example, there isn't the concept of undef with hashes
+that are tied to DBM* files. It also means that exists() and
+defined() do the same thing with a DBM* file, and what they
+end up doing is not what they do with ordinary hashes.
=head2 How do I reset an each() operation part-way through?
=head2 How can I get the unique keys from two hashes?
-First you extract the keys from the hashes into arrays, and then solve
-the uniquifying the array problem described above. For example:
+First you extract the keys from the hashes into lists, then solve
+the "removing duplicates" problem described above. For example:
%seen = ();
for $element (keys(%foo), keys(%bar)) {
Use the Tie::IxHash from CPAN.
use Tie::IxHash;
- tie(%myhash, Tie::IxHash);
- for ($i=0; $i<20; $i++) {
+ tie my %myhash, 'Tie::IxHash';
+ for (my $i=0; $i<20; $i++) {
$myhash{$i} = 2*$i;
}
- @keys = keys %myhash;
+ my @keys = keys %myhash;
# @keys = (0,1,2,3,...)
=head2 Why does passing a subroutine an undefined element in a hash create it?
get scalars passed in by reference. If somefunc() modifies C<$_[0]>,
it has to be ready to write it back into the caller's version.
-This has been fixed as of perl5.004.
+This has been fixed as of Perl5.004.
Normally, merely accessing a key's value for a nonexistent key does
I<not> cause that key to be forever there. This is different than
=head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
-Use references (documented in L<perlref>). Examples of complex data
-structures are given in L<perldsc> and L<perllol>. Examples of
-structures and object-oriented classes are in L<perltoot>.
+Usually a hash ref, perhaps like this:
+
+ $record = {
+ NAME => "Jason",
+ EMPNO => 132,
+ TITLE => "deputy peon",
+ AGE => 23,
+ SALARY => 37_000,
+ PALS => [ "Norbert", "Rhys", "Phineas"],
+ };
+
+References are documented in L<perlref> and the upcoming L<perlreftut>.
+Examples of complex data structures are given in L<perldsc> and
+L<perllol>. Examples of structures and object-oriented classes are
+in L<perltoot>.
=head2 How can I use a reference as a hash key?
-You can't do this directly, but you could use the standard Tie::Refhash
-module distributed with perl.
+You can't do this directly, but you could use the standard Tie::RefHash
+module distributed with Perl.
=head1 Data: Misc
print "Your kernel is GNU-zip enabled!\n";
}
-On some systems, however, you have to play tedious games with "text"
-versus "binary" files. See L<perlfunc/"binmode">.
+On less elegant (read: Byzantine) systems, however, you have
+to play tedious games with "text" versus "binary" files. See
+L<perlfunc/"binmode"> or L<perlopentut>.
If you're concerned about 8-bit ASCII data, then see L<perllocale>.
Assuming that you don't care about IEEE notations like "NaN" or
"Infinity", you probably just want to use a regular expression.
- warn "has nondigits" if /\D/;
- warn "not a natural number" unless /^\d+$/; # rejects -3
- warn "not an integer" unless /^-?\d+$/; # rejects +3
- warn "not an integer" unless /^[+-]?\d+$/;
- warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2
- warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/;
- warn "not a C float"
- unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/;
-
-If you're on a POSIX system, Perl's supports the C<POSIX::strtod>
+ if (/\D/) { print "has nondigits\n" }
+ if (/^\d+$/) { print "is a whole number\n" }
+ if (/^-?\d+$/) { print "is an integer\n" }
+ if (/^[+-]?\d+$/) { print "is a +/- integer\n" }
+ if (/^-?\d+\.?\d*$/) { print "is a real number\n" }
+ if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" }
+ if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
+ { print "a C float\n" }
+
+There are also some commonly used modules for the task.
+L<Scalar::Util> (distributed with 5.8) provides access to perl's
+internal function C<looks_like_number> for determining
+whether a variable looks like a number. L<Data::Types>
+exports functions that validate data types using both the
+above and other regular expressions. Thirdly, there is
+C<Regexp::Common> which has regular expressions to match
+various types of numbers. Those three modules are available
+from the CPAN.
+
+If you're on a POSIX system, Perl supports the C<POSIX::strtod>
function. Its semantics are somewhat cumbersome, so here's a C<getnum>
wrapper function for more convenient access. This function takes
a string and returns the number it found, or C<undef> for input that
return undef;
} else {
return $num;
- }
- }
+ }
+ }
- sub is_numeric { defined &getnum }
+ sub is_numeric { defined getnum($_[0]) }
-Or you could check out
-http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz
-instead. The POSIX module (part of the standard Perl distribution)
-provides the C<strtol> and C<strtod> for converting strings to double
-and longs, respectively.
+Or you could check out the L<String::Scanf> module on the CPAN
+instead. The POSIX module (part of the standard Perl distribution) provides
+the C<strtod> and C<strtol> for converting strings to double and longs,
+respectively.
=head2 How do I keep persistent data across program calls?
For some specific applications, you can use one of the DBM modules.
-See L<AnyDBM_File>. More generically, you should consult the
-FreezeThaw, Storable, or Class::Eroot modules from CPAN.
+See L<AnyDBM_File>. More generically, you should consult the FreezeThaw
+or Storable modules from CPAN. Starting from Perl 5.8 Storable is part
+of the standard distribution. Here's one example using Storable's C<store>
+and C<retrieve> functions:
+
+ use Storable;
+ store(\%hash, "filename");
+
+ # later on...
+ $href = retrieve("filename"); # by ref
+ %hash = %{ retrieve("filename") }; # direct to hash
=head2 How do I print out or copy a recursive data structure?
-The Data::Dumper module on CPAN is nice for printing out
-data structures, and FreezeThaw for copying them. For example:
+The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great
+for printing out data structures. The Storable module on CPAN (or the
+5.8 release of Perl), provides a function called C<dclone> that recursively
+copies its argument.
- use FreezeThaw qw(freeze thaw);
- $new = thaw freeze $old;
+ use Storable qw(dclone);
+ $r2 = dclone($r1);
-Where $old can be (a reference to) any kind of data structure you'd like.
-It will be deeply copied.
+Where $r1 can be a reference to any kind of data structure you'd like.
+It will be deeply copied. Because C<dclone> takes and returns references,
+you'd have to add extra punctuation if you had a hash of arrays that
+you wanted to copy.
+
+ %newhash = %{ dclone(\%oldhash) };
=head2 How do I define methods for every class/object?
Get the Business::CreditCard module from CPAN.
+=head2 How do I pack arrays of doubles or floats for XS code?
+
+The kgbpack.c code in the PGPLOT module on CPAN does just this.
+If you're doing a lot of float or double processing, consider using
+the PDL module from CPAN instead--it makes number-crunching easy.
+
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997, 1998 Tom Christiansen and Nathan Torkington.
-All rights reserved.
+Copyright (c) 1997-2005 Tom Christiansen, Nathan Torkington, and
+other authors as noted. All rights reserved.
-When included as part of the Standard Version of Perl, or as part of
-its complete documentation whether printed or otherwise, this work
-may be distributed only under the terms of Perl's Artistic License.
-Any distribution of this file or derivatives thereof I<outside>
-of that package require that special arrangements be made with
-copyright holder.
+This documentation is free; you can redistribute it and/or modify it
+under the same terms as Perl itself.
Irrespective of its distribution, all code examples in this file
are hereby placed into the public domain. You are permitted and
projects / p5sagit/p5-mst-13.2.git / blobdiff