=head1 NAME
-perlfaq4 - Data Manipulation ($Revision: 1.15 $)
+perlfaq4 - Data Manipulation ($Revision: 1.40 $, $Date: 1999/01/08 04:26:39 $)
=head1 DESCRIPTION
=head1 Data: Numbers
+=head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
+
+The infinite set that a mathematician thinks of as the real numbers can
+only be approximate on a computer, since the computer only has a finite
+number of bits to store an infinite number of, um, numbers.
+
+Internally, your computer represents floating-point numbers in binary.
+Floating-point numbers read in from a file or appearing as literals
+in your program are converted from their decimal floating-point
+representation (eg, 19.95) to the internal binary representation.
+
+However, 19.95 can't be precisely represented as a binary
+floating-point number, just like 1/3 can't be exactly represented as a
+decimal floating-point number. The computer's binary representation
+of 19.95, therefore, isn't exactly 19.95.
+
+When a floating-point number gets printed, the binary floating-point
+representation is converted back to decimal. These decimal numbers
+are displayed in either the format you specify with printf(), or the
+current output format for numbers (see L<perlvar/"$#"> 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">.
+
=head2 Why isn't my octal data interpreted correctly?
Perl only understands octal and hex numbers as such when they occur
chmod(644, $file); # WRONG -- perl -w catches this
chmod(0644, $file); # right
-=head2 Does perl have a round function? What about ceil() and floor()?
-Trig functions?
+=head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions?
-For rounding to a certain number of digits, sprintf() or printf() is
-usually the easiest route.
+Remember that int() merely truncates toward 0. For rounding to a
+certain number of digits, sprintf() or printf() is usually the easiest
+route.
+
+ printf("%.3f", 3.1415926535); # prints 3.142
The POSIX module (part of the standard perl distribution) implements
ceil(), floor(), and a number of other mathematical and trigonometric
functions.
-The Math::Complex module (part of the standard perl distribution)
-defines a number of mathematical functions that can also work on real
-numbers. It's not as efficient as the POSIX library, but the POSIX
-library can't work with complex numbers.
+ use POSIX;
+ $ceil = ceil(3.5); # 4
+ $floor = floor(3.5); # 3
+
+In 5.000 to 5.003 Perls, trigonometry was done in the Math::Complex
+module. With 5.004, the Math::Trig module (part of the standard perl
+distribution) implements the trigonometric functions. Internally it
+uses the Math::Complex module and some functions can break out from
+the real axis into the complex plane, for example the inverse sine of
+2.
Rounding in financial applications can have serious implications, and
the rounding method used should be specified precisely. In these
being used by Perl, but to instead implement the rounding function you
need yourself.
+To see why, notice how you'll still have an issue on half-way-point
+alternation:
+
+ for ($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 bits into ints?
-To turn a string of 1s and 0s like '10110110' into a scalar containing
+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">):
$binary_string = join('', unpack('B*', "\x29"));
+=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<1>). 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?
Use the Math::Matrix or Math::MatrixReal modules (available from CPAN)
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));
}
=head2 How can I output Roman numerals?
=head2 Why aren't my random numbers random?
-The short explanation is that you're getting pseudorandom numbers, not
-random ones, because that's how these things work. A longer
-explanation is available on
+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.
+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 :-).
http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of Tom
-Phoenix.
+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.''
-You should also check out the Math::TrulyRandom module from CPAN.
+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://www.nr.com/ .
=head1 Data: Dates
$week_of_year = int($day_of_year / 7);
-Of course, this believes that weeks start at zero.
+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. Note that not
+all businesses consider ``week 1'' to be the same; for example,
+American businesses often consider the first week with a Monday
+in it to be Work Week #1, despite ISO 8601, which considers
+WW1 to be the first week with a Thursday in it.
-=head2 How can I compare two date strings?
+=head2 How can I compare two dates and find the difference?
-Use the Date::Manip or Date::DateCalc modules from CPAN.
+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.
=head2 How can I take a string and turn it into epoch seconds?
If it's a regular enough string that it always has the same format,
-you can split it up and pass the parts to timelocal in the standard
-Time::Local module. Otherwise, you should look into one of the
-Date modules from CPAN.
+you can split it up and pass the parts to C<timelocal> in the standard
+Time::Local module. Otherwise, you should look into the Date::Calc
+and Date::Manip modules from CPAN.
=head2 How can I find the Julian Day?
-Neither Date::Manip nor Date::DateCalc deal with Julian days.
-Instead, there is an example of Julian date calculation in
-http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz,
-which should help.
+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
+.
+
+=head2 How do I find yesterday's date?
+
+The C<time()> function returns the current time in seconds since the
+epoch. Take one day off that:
+
+ $yesterday = time() - ( 24 * 60 * 60 );
+
+Then you can pass this to C<localtime()> and get the individual year,
+month, day, hour, minute, seconds values.
-=head2 Does Perl have a year 2000 problem?
+=head2 Does Perl have a year 2000 problem? Is Perl Y2K compliant?
-Not unless you use Perl to create one. The date and time functions
-supplied with perl (gmtime and localtime) supply adequate information
-to determine the year well beyond 2000 (2038 is when trouble strikes).
-The year returned by these functions when used in an array context is
-the year minus 1900. For years between 1910 and 1999 this I<happens>
-to be a 2-digit decimal number. To avoid the year 2000 problem simply
-do not treat the year as a 2-digit number. It isn't.
+Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is
+Y2K compliant (whatever that means). The programmers you've hired to
+use it, however, probably are not.
-When gmtime() and localtime() are used in a scalar context they return
+Long answer: The question belies a true understanding of the issue.
+Perl is just as Y2K compliant as your pencil--no more, and no less.
+Can you use your pencil to write a non-Y2K-compliant memo? Of course
+you can. Is that the pencil's fault? Of course it isn't.
+
+The date and time functions supplied with perl (gmtime and localtime)
+supply adequate information to determine the year well beyond 2000
+(2038 is when trouble strikes for 32-bit machines). The year returned
+by these functions when used in an array context is the year minus 1900.
+For years between 1910 and 1999 this I<happens> to be a 2-digit decimal
+number. To avoid the year 2000 problem simply do not treat the year as
+a 2-digit number. It isn't.
+
+When gmtime() and localtime() are used in scalar context they return
a timestamp string that contains a fully-expanded year. For example,
C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00
2001". There's no year 2000 problem here.
+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
+a longer exposition.
+
=head1 Data: Strings
=head2 How do I validate input?
The answer to this question is usually a regular expression, perhaps
-with auxiliary logic. See the more specific questions (numbers, email
+with auxiliary logic. See the more specific questions (numbers, mail
addresses, etc.) for details.
=head2 How do I unescape a string?
-It depends just what you mean by "escape". URL escapes are dealt with
-in L<perlfaq9>. Shell escapes with the backslash (\)
+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:
s/\\(.)/$1/g;
-Note that this won't expand \n or \t or any other special escapes.
+This won't expand C<"\n"> or C<"\t"> or any other special escapes.
=head2 How do I remove consecutive pairs of characters?
-To turn "abbcccd" into "abccd":
+To turn C<"abbcccd"> into C<"abccd">:
s/(.)\1/$1/g;
This is documented in L<perlref>. In general, this is fraught with
quoting and readability problems, but it is possible. To interpolate
-a subroutine call (in a list context) into a string:
+a subroutine call (in list context) into a string:
print "My sub returned @{[mysub(1,2,3)]} that time.\n";
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 tackled in one regular expression, no
-matter how complicated. To find something between two single 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.
+This isn't something that can be done in one regular expression, no
+matter how complicated. To find something between two single
+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.
+
+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, the CPAN module Parse::Yapp, and Mark-Jason
+Dominus's excellent I<py> tool at http://www.plover.com/~mjd/perl/py/
+.
+
+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) {
+ # 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/);
+ print join("\n",@$[0..$#$]) if( $$[-1] );
=head2 How do I reverse a string?
-Use reverse() in a scalar context, as documented in
+Use reverse() in scalar context, as documented in
L<perlfunc/reverse>.
$reversed = reverse $string;
=head2 How do I expand tabs in a string?
-You can do it the old-fashioned way:
+You can do it yourself:
1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
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?
There are many ways. If you just want to grab a copy, use
-substr:
+substr():
$first_byte = substr($a, 0, 1);
substr($a, 0, 3) = "Tom";
-Although those with a regexp kind of thought process will likely prefer
+Although those with a pattern matching kind of thought process will
+likely prefer:
$a =~ s/^.../Tom/;
=head2 How do I change the Nth occurrence of something?
-You have to keep track. For example, let's say you want
-to change the fifth occurrence of "whoever" or "whomever"
-into "whosoever", case insensitively.
+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.
$count = 0;
s{((whom?)ever)}{
: $1 # renege and leave it there
}igex;
+In the more general case, you can use the C</g> modifier in a C<while>
+loop, keeping count of matches.
+
+ $WANT = 3;
+ $count = 0;
+ while (/(\w+)\s+fish\b/gi) {
+ if (++$count == $WANT) {
+ print "The third fish is a $1 one.\n";
+ # Warning: don't `last' out of this loop
+ }
+ }
+
+That prints out: C<"The third fish is a red one."> You can also use a
+repetition count and repeated pattern like this:
+
+ /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
+
=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
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";
=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):
+
+ $string =~ s/ (
+ (^\w) #at the beginning of the line
+ | # or
+ (\s\w) #preceded by whitespace
+ )
+ /\U$1/xg;
+ $string =~ /([\w']+)/\u\L$1/g;
+
To make the whole line upper case:
+
$line = uc($line);
To force each word to be lower case, with the first letter upper case:
+
$line =~ s/(\w+)/\u\L$1/g;
+You can (and probably should) enable locale awareness of those
+characters by placing a C<use locale> pragma in your program.
+See L<perllocale> for endless details on locales.
+
+This is sometimes referred to as putting something into "title
+case", but that's not quite accurate. Consdier the proper
+capitalization of the movie I<Dr. Strangelove or: How I Learned to
+Stop Worrying and Love the Bomb>, for example.
+
=head2 How can I split a [character] delimited string except when inside
[character]? (Comma-separated files)
}gx;
push(@new, undef) if substr($text,-1,1) eq ',';
+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.
+
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 module on CPAN.
+
=head2 How do I strip blank space from the beginning/end of a string?
-The simplest approach, albeit not the fastest, is probably like this:
+Although the simplest approach would seem to be:
$string =~ s/^\s*(.*?)\s*$/$1/;
-It would be faster to do this in two steps:
+This is unnecessarily slow, destructive, and fails with embedded newlines.
+It is much better faster to do this in two steps:
$string =~ s/^\s+//;
$string =~ s/\s+$//;
s/\s+$//;
}
+This idiom takes advantage of the C<foreach> loop's aliasing
+behavior to factor out common code. You can do this
+on several strings at once, or arrays, or even the
+values of a hash if you use a slide:
+
+ # trim whitespace in the scalar, the array,
+ # and all the values in the hash
+ foreach ($scalar, @array, @hash{keys %hash}) {
+ s/^\s+//;
+ s/\s+$//;
+ }
+
+=head2 How do I pad a string with blanks or pad a number with zeroes?
+
+(This answer contributed by Uri Guttman)
+
+In the following examples, C<$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.
+
+The simplest method use the C<sprintf> function. It can pad on the
+left or right with blanks and on the left with zeroes.
+
+ # Left padding with blank:
+ $padded = sprintf( "%${pad_len}s", $text ) ;
+
+ # Right padding with blank:
+ $padded = sprintf( "%${pad_len}s", $text ) ;
+
+ # Left padding with 0:
+ $padded = sprintf( "%0${pad_len}d", $num ) ;
+
+If you need to pad with a character other than blank or zero you can use
+one of the following methods.
+
+These methods generate a pad string with the C<x> operator and
+concatenate that with the original text.
+
+Left and right padding with any character:
+
+ $padded = $pad_char x ( $pad_len - length( $text ) ) . $text ;
+ $padded = $text . $pad_char x ( $pad_len - length( $text ) ) ;
+
+Or you can left or right pad $text directly:
+
+ $text .= $pad_char x ( $pad_len - length( $text ) ) ;
+ substr( $text, 0, 0 ) = $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,
+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 {
+ my(@positions) = @_;
+ my $template = '';
+ my $lastpos = 1;
+ for my $place (@positions) {
+ $template .= "A" . ($place - $lastpos) . " ";
+ $lastpos = $place;
+ }
+ $template .= "A*";
+ return $template;
+ }
=head2 How do I find the soundex value of a string?
Let's assume that you have a string like:
$text = 'this has a $foo in it and a $bar';
- $text =~ s/\$(\w+)/${$1}/g;
-Before version 5 of perl, this had to be done with a double-eval
-substitution:
+If those were both global variables, then this would
+suffice:
+
+ $text =~ s/\$(\w+)/${$1}/g; # no /e needed
+
+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 /ee, not /e
+
+It's probably better in the general case to treat those
+variables as entries in some special hash. For example:
+
+ %user_defs = (
+ foo => 23,
+ bar => 19,
+ );
+ $text =~ s/\$(\w+)/$user_defs{$1}/g;
-Which is bizarre enough that you'll probably actually need an EEG
-afterwards. :-)
+See also ``How do I expand function calls in a string?'' in this section
+of the FAQ.
=head2 What's wrong with always quoting "$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.
+don't want them to be. 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.
-=head2 Why don't my <<HERE documents work?
+Stringification also destroys arrays.
+
+ @lines = `command`;
+ print "@lines"; # WRONG - extra blanks
+ print @lines; # right
+
+=head2 Why don't my E<lt>E<lt>HERE documents work?
Check for these three things:
=back
+If you want to indent the text in the here document, you
+can do this:
+
+ # all in one
+ ($VAR = <<HERE_TARGET) =~ s/^\s+//gm;
+ your text
+ goes here
+ HERE_TARGET
+
+But the HERE_TARGET must still be flush against the margin.
+If you want that indented also, you'll have to quote
+in the indentation.
+
+ ($quote = <<' FINIS') =~ s/^\s+//gm;
+ ...we will have peace, when you and all your works have
+ perished--and the works of your dark master to whom you
+ 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--/;
+
+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
+subsequent line.
+
+ sub fix {
+ local $_ = shift;
+ my ($white, $leader); # common white space and common leading string
+ if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) {
+ ($white, $leader) = ($2, quotemeta($1));
+ } else {
+ ($white, $leader) = (/^(\s+)/, '');
+ }
+ s/^\s*?$leader(?:$white)?//gm;
+ return $_;
+ }
+
+This works with leading special strings, dynamically determined:
+
+ $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP';
+ @@@ int
+ @@@ runops() {
+ @@@ SAVEI32(runlevel);
+ @@@ runlevel++;
+ @@@ while ( op = (*op->op_ppaddr)() ) ;
+ @@@ TAINT_NOT;
+ @@@ return 0;
+ @@@ }
+ MAIN_INTERPRETER_LOOP
+
+Or with a fixed amount of leading white space, with remaining
+indentation correctly preserved:
+
+ $poem = fix<<EVER_ON_AND_ON;
+ Now far ahead the Road has gone,
+ And I must follow, if I can,
+ Pursuing it with eager feet,
+ Until it joins some larger way
+ Where many paths and errands meet.
+ And whither then? I cannot say.
+ --Bilbo in /usr/src/perl/pp_ctl.c
+ 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<@_>, 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 evaluates the
+left hand side, then evaluates and returns the left hand side. 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
=over 4
=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);
-This is nice in that it doesn't use much extra memory,
-simulating uniq(1)'s behavior of removing only adjacent
-duplicates.
+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.
=item b) If you don't know whether @in is sorted:
=back
-=head2 How can I tell whether an array contains a certain element?
+But perhaps you should have been using a hash all along, eh?
+
+=head2 How can I tell whether a list or array contains a certain element?
-There are several ways to approach this. If you are going to make
-this query many times and the values are arbitrary strings, the
-fastest way is probably to invert the original array and keep an
+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
+designed to answer this question quickly and efficiently. Arrays aren't.
+
+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.
@blues = qw/azure cerulean teal turquoise lapis-lazuli/;
@articles = ( 1..10, 150..2000, 2017 );
undef $read;
- grep (vec($read,$_,1) = 1, @articles);
+ for (@articles) { vec($read,$_,1) = 1 }
Now check whether C<vec($read,$n,1)> is true for some C<$n>.
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?).
+regexp 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 test whether two arrays or hashes are equal?
+
+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) = @_;
+ local $^W = 0; # silence spurious -w undef complaints
+ return 0 unless @$first == @$second;
+ for (my $i = 0; $i < @$first; $i++) {
+ return 0 if $first->[$i] ne $second->[$i];
+ }
+ return 1;
+ }
+
+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;
+ $b{EXTRA} = \%a;
+
+ printf "a and b contain %s hashes\n",
+ cmpStr(\%a, \%b) == 0 ? "the same" : "different";
+
+ printf "a and b contain %s hashes\n",
+ cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
+
+
+The first reports that both those the hashes contain the same data,
+while the second reports that they do not. Which you prefer is left as
+an exercise to the reader.
+
=head2 How do I find the first array element for which a condition is true?
You can use this if you care about the index:
- for ($i=0; $i < @array; $i++) {
+ for ($i= 0; $i < @array; $i++) {
if ($array[$i] eq "Waldo") {
$found_index = $i;
last;
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.
+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
+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 $node->{VALUE}, " ";
+ }
+ print "\n";
+
+You could grow the list this way:
+
+ my ($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?
-Here's a shuffling algorithm which works its way through the list,
+Use this:
+
+ # 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 $j = int rand ($i+1);
+ next if $i == $j;
+ @$array[$i,$j] = @$array[$j,$i];
+ }
+ }
+
+ fisher_yates_shuffle( \@array ); # permutes @array in place
+
+You've probably seen shuffling algorithms that works using splice,
randomly picking another element to swap the current element with:
srand;
push(@new, splice(@old, rand @old, 1));
}
-For large arrays, this avoids a lot of the reshuffling:
-
- srand;
- @new = ();
- @old = 1 .. 10000; # just a demo
- for( @old ){
- my $r = rand @new+1;
- push(@new,$new[$r]);
- $new[$r] = $_;
- }
+This is bad because splice is already O(N), and since you do it N times,
+you just invented a quadratic algorithm; that is, O(N**2). This does
+not scale, although Perl is so efficient that you probably won't notice
+this until you have rather largish arrays.
=head2 How do I process/modify each element of an array?
Use C<for>/C<foreach>:
for (@lines) {
- s/foo/bar/;
- tr[a-z][A-Z];
+ s/foo/bar/; # change that word
+ y/XZ/ZX/; # swap those letters
}
Here's another; let's compute spherical volumes:
- for (@radii) {
+ for (@volumes = @radii) { # @volumes has changed parts
$_ **= 3;
$_ *= (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:
+
+ for $orbit ( @orbits{keys %orbits} ) {
+ ($orbit **= 3) *= (4/3) * 3.14159;
+ }
+
=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.
+
=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 permut() function should work on any list:
+in the permute() function should work on any list:
#!/usr/bin/perl -n
- # permute - tchrist@perl.com
- permut([split], []);
- sub permut {
- my @head = @{ $_[0] };
- my @tail = @{ $_[1] };
- unless (@head) {
- # stop recursing when there are no elements in the head
- print "@tail\n";
+ # tsc-permute: permute each word of input
+ permute([split], []);
+ sub permute {
+ my @items = @{ $_[0] };
+ my @perms = @{ $_[1] };
+ unless (@items) {
+ print "@perms\n";
} else {
- # for all elements in @head, move one from @head to @tail
- # and call permut() on the new @head and @tail
- my(@newhead,@newtail,$i);
- foreach $i (0 .. $#head) {
- @newhead = @head;
- @newtail = @tail;
- unshift(@newtail, splice(@newhead, $i, 1));
- permut([@newhead], [@newtail]);
+ my(@newitems,@newperms,$i);
+ foreach $i (0 .. $#items) {
+ @newitems = @items;
+ @newperms = @perms;
+ unshift(@newperms, splice(@newitems, $i, 1));
+ permute([@newitems], [@newperms]);
}
}
}
@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 method gets faster the more sparse the bit vector is.
(Courtesy of Tim Bunce and Winfried Koenig.)
+Here's a demo on how to use 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), $bits);
+ print "bits are: @bytes\n\n";
+ }
+
=head2 Why does defined() return true on empty arrays and hashes?
-See L<perlfunc/defined> in the 5.004 release or later of Perl.
+The short story is that you should probably only use defined on scalars or
+functions, not on aggregates (arrays and hashes). See L<perlfunc/defined>
+in the 5.004 release or later of Perl for more detail.
=head1 Data: Hashes (Associative Arrays)
Use the each() function (see L<perlfunc/each>) if you don't care
whether it's sorted:
- while (($key,$value) = each %hash) {
+ while ( ($key, $value) = each %hash) {
print "$key = $value\n";
}
If you mean how many keys, then all you have to do is
take the scalar sense of the keys() function:
- $num_keys = scalar keys %hash;
+ $num_keys = scalar keys %hash;
In void context it just resets the iterator, which is faster
for tied hashes.
You can look into using the DB_File module and tie() using the
$DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">.
+The Tie::IxHash module from CPAN might also be instructive.
=head2 What's the difference between "delete" and "undef" with hashes?
=head2 How do I reset an each() operation part-way through?
-Using C<keys %hash> in a scalar context returns the number of keys in
+Using C<keys %hash> in scalar context returns the number of keys in
the hash I<and> resets the iterator associated with the hash. You may
need to do this if you use C<last> to exit a loop early so that when you
re-enter it, the hash iterator has been reset.
Use the Tie::IxHash from CPAN.
+ use Tie::IxHash;
+ tie(%myhash, Tie::IxHash);
+ for ($i=0; $i<20; $i++) {
+ $myhash{$i} = 2*$i;
+ }
+ @keys = keys %myhash;
+ # @keys = (0,1,2,3,...)
+
=head2 Why does passing a subroutine an undefined element in a hash create it?
If you say something like:
I<not> cause that key to be forever there. This is different than
awk's behavior.
-=head2 How can I make the Perl equivalent of a C structure/C++ class/hash
-or array of hashes or arrays?
+=head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
+
+Usually a hash ref, perhaps like this:
+
+ $record = {
+ NAME => "Jason",
+ EMPNO => 132,
+ TITLE => "deputy peon",
+ AGE => 23,
+ SALARY => 37_000,
+ PALS => [ "Norbert", "Rhys", "Phineas"],
+ };
-Use references (documented in L<perlref>). Examples of complex data
-structures are given in L<perldsc> and L<perllol>. Examples of
-structures and object-oriented classes are in L<perltoot>.
+References are documented in L<perlref> and the upcoming L<perlreftut>.
+Examples of complex data structures are given in L<perldsc> and
+L<perllol>. Examples of structures and object-oriented classes are
+in L<perltoot>.
=head2 How can I use a reference as a hash key?
print "Your kernel is GNU-zip enabled!\n";
}
-On some systems, however, you have to play tedious games with "text"
-versus "binary" files. See L<perlfunc/"binmode">.
+On some legacy systems, however, you have to play tedious games with
+"text" versus "binary" files. See L<perlfunc/"binmode">, or the upcoming
+L<perlopentut> manpage.
If you're concerned about 8-bit ASCII data, then see L<perllocale>.
-If you want to deal with multi-byte characters, however, there are
+If you want to deal with multibyte characters, however, there are
some gotchas. See the section on Regular Expressions.
=head2 How do I determine whether a scalar is a number/whole/integer/float?
Assuming that you don't care about IEEE notations like "NaN" or
"Infinity", you probably just want to use a regular expression.
- warn "has nondigits" if /\D/;
- warn "not a whole number" unless /^\d+$/;
- warn "not an integer" unless /^-?\d+$/; # reject +3
- warn "not an integer" unless /^[+-]?\d+$/;
- 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+))?$/;
+ if (/\D/) { print "has nondigits\n" }
+ if (/^\d+$/) { 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" }
+ if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/)
+ { print "a C float" }
+
+If you're on a POSIX system, Perl's 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
+isn't a C float. The C<is_numeric> function is a front end to C<getnum>
+if you just want to say, ``Is this a float?''
+
+ sub getnum {
+ use POSIX qw(strtod);
+ my $str = shift;
+ $str =~ s/^\s+//;
+ $str =~ s/\s+$//;
+ $! = 0;
+ my($num, $unparsed) = strtod($str);
+ if (($str eq '') || ($unparsed != 0) || $!) {
+ return undef;
+ } else {
+ return $num;
+ }
+ }
+
+ sub is_numeric { defined &getnum }
Or you could check out
http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz
=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,
+Storable, or Class::Eroot modules from CPAN. 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, found on CPAN,
+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 Tom Christiansen and Nathan Torkington.
-All rights reserved. See L<perlfaq> for distribution information.
+Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington.
+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.
+
+Irrespective of its distribution, all code examples in this file
+are hereby placed into the public domain. You are permitted and
+encouraged to use this code in your own programs for fun
+or for profit as you see fit. A simple comment in the code giving
+credit would be courteous but is not required.
+
projects / p5sagit/p5-mst-13.2.git / blobdiff