=head1 NAME
-perlfaq4 - Data Manipulation ($Revision: 1.15 $)
+perlfaq4 - Data Manipulation ($Revision: 1.26 $, $Date: 1998/08/05 12:04:00 $)
=head1 DESCRIPTION
=head1 Data: Numbers
+=head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
+
+The infinite set that a mathematician thinks of as the real numbers can
+only be approximate on a computer, since the computer only has a finite
+number of bits to store an infinite number of, um, numbers.
+
+Internally, your computer represents floating-point numbers in binary.
+Floating-point numbers read in from a file or appearing as literals
+in your program are converted from their decimal floating-point
+representation (eg, 19.95) to the internal binary representation.
+
+However, 19.95 can't be precisely represented as a binary
+floating-point number, just like 1/3 can't be exactly represented as a
+decimal floating-point number. The computer's binary representation
+of 19.95, therefore, isn't exactly 19.95.
+
+When a floating-point number gets printed, the binary floating-point
+representation is converted back to decimal. These decimal numbers
+are displayed in either the format you specify with printf(), or the
+current output format for numbers (see L<perlvar/"$#"> 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?
+
+Remember that int() merely truncates toward 0. For rounding to a
+certain number of digits, sprintf() or printf() is usually the easiest
+route.
-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
=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">):
=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
+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.
+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.
+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 .
=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 business consider ``week 1'' to be the same; for example,
+American business often consider the first week with a Monday
+in it to be Work Week #1, despite ISO 8601, which consider
+WW1 to be the frist 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 Does Perl have a year 2000 problem? Is Perl Y2K compliant?
-=head2 Does Perl have a year 2000 problem?
+Short answer: No, Perl does not have a Year 2000 problem. Yes,
+Perl is Y2K compliant. The programmers you've hired to use it,
+however, probably are not.
-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.
+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.
-When gmtime() and localtime() are used in a scalar context they return
+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, 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
+ }
=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$1/g;
+You can (and probably should) enable locale awareness of those
+characters by placing a C<use locale> pragma in your program.
+See L<perllocale> for endless details on locales.
+
=head2 How can I split a [character] delimited string except when inside
[character]? (Comma-separated files)
}gx;
push(@new, undef) if substr($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:
=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 unneccesarily 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 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';
+
+If those were both global variables, then this would
+suffice:
+
$text =~ s/\$(\w+)/${$1}/g;
-Before version 5 of perl, this had to be done with a double-eval
-substitution:
+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
+
+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"?
number, such as the magical C<++> autoincrement operator or the
syscall() function.
+Stringification also destroys arrays.
+
+ @lines = `command`;
+ print "@lines"; # WRONG - extra blanks
+ print @lines; # right
+
=head2 Why don't my <<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 $array[1] and @array[1]?
=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?
+=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>.
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
=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 { [ $_, uc((/\d+\s*(\S+)/ )[0] ] } @data;
If you need to sort on several fields, the following paradigm is useful.
Use the each() function (see L<perlfunc/each>) if you don't care
whether it's sorted:
- while (($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*$i;
+ }
+ @keys = keys %myhash;
+ # @keys = (0,1,2,3,...)
+
=head2 Why does passing a subroutine an undefined element in a hash create it?
If you say something like:
I<not> cause that key to be forever there. This is different than
awk's behavior.
-=head2 How can I make the Perl equivalent of a C structure/C++ class/hash
-or array of hashes or arrays?
+=head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
Use references (documented in L<perlref>). Examples of complex data
structures are given in L<perldsc> and L<perllol>. Examples of
If you're concerned about 8-bit ASCII data, then see L<perllocale>.
-If you want to deal with multi-byte characters, however, there are
+If you want to deal with multibyte characters, however, there are
some gotchas. See the section on Regular Expressions.
=head2 How do I determine whether a scalar is a number/whole/integer/float?
"Infinity", you probably just want to use a regular expression.
warn "has nondigits" if /\D/;
- warn "not a whole number" unless /^\d+$/;
- warn "not an integer" unless /^-?\d+$/; # reject +3
- warn "not an integer" unless /^[+-]?\d+$/;
+ warn "not a 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>
+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
instead. The POSIX module (part of the standard Perl distribution)
=head1 AUTHOR AND COPYRIGHT
-Copyright (c) 1997 Tom Christiansen and Nathan Torkington.
-All rights reserved. See L<perlfaq> for distribution information.
+Copyright (c) 1997, 1998 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