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diff --git a/pod/perlfaq4.pod b/pod/perlfaq4.pod index 326ec9180b..1c06b0b2c8 100644 --- a/pod/perlfaq4.pod +++ b/pod/perlfaq4.pod @@ -1,6 +1,6 @@ =head1 NAME -perlfaq4 - Data Manipulation ($Revision: 10394 $) +perlfaq4 - Data Manipulation =head1 DESCRIPTION @@ -48,35 +48,45 @@ numbers. What you think in the above as 'three' is really more like =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. Octal literals in perl must start with a -leading C<0> and hexadecimal literals must start with a leading C<0x>. -If they are read in from somewhere and assigned, no automatic -conversion takes place. You must explicitly use C<oct()> or C<hex()> if you -want the values converted to decimal. C<oct()> interprets hexadecimal (C<0x350>), -octal (C<0350> or even without the leading C<0>, like C<377>) and binary -(C<0b1010>) numbers, while C<hex()> only converts hexadecimal ones, with -or without a leading C<0x>, such as C<0x255>, C<3A>, C<ff>, or C<deadbeef>. -The inverse mapping from decimal to octal can be done with either the -<%o> or C<%O> C<sprintf()> formats. +(contributed by brian d foy) + +You're probably trying to convert a string to a number, which Perl only +converts as a decimal number. When Perl converts a string to a number, it +ignores leading spaces and zeroes, then assumes the rest of the digits +are in base 10: + + my $string = '0644'; + + print $string + 0; # prints 644 + + print $string + 44; # prints 688, certainly not octal! + +This problem usually involves one of the Perl built-ins that has the +same name a unix command that uses octal numbers as arguments on the +command line. In this example, C<chmod> on the command line knows that +its first argument is octal because that's what it does: + + %prompt> chmod 644 file + +If you want to use the same literal digits (644) in Perl, you have to tell +Perl to treat them as octal numbers either by prefixing the digits with +a C<0> or using C<oct>: + + chmod( 0644, $file); # right, has leading zero + chmod( oct(644), $file ); # also correct -This problem shows up most often when people try using C<chmod()>, -C<mkdir()>, C<umask()>, or C<sysopen()>, which by widespread tradition -typically take permissions in octal. +The problem comes in when you take your numbers from something that Perl +thinks is a string, such as a command line argument in C<@ARGV>: - chmod(644, $file); # WRONG - chmod(0644, $file); # right + chmod( $ARGV[0], $file); # wrong, even if "0644" -Note the mistake in the first line was specifying the decimal literal -C<644>, rather than the intended octal literal C<0644>. The problem can -be seen with: + chmod( oct($ARGV[0]), $file ); # correct, treat string as octal - printf("%#o",644); # prints 01204 +You can always check the value you're using by printing it in octal +notation to ensure it matches what you think it should be. Print it +in octal and decimal format: -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 C<0..7>. + printf "0%o %d", $number, $number; =head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions? @@ -363,7 +373,7 @@ pseudorandom generator than comes with your operating system, look at =head2 How do I get a random number between X and Y? To get a random number between two values, you can use the C<rand()> -builtin to get a random number between 0 and 1. From there, you shift +built-in to get a random number between 0 and 1. From there, you shift that into the range that you want. C<rand($x)> returns a number such that C<< 0 <= rand($x) < $x >>. Thus @@ -373,7 +383,7 @@ 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 ); + my $number = 10 + int rand( 15-10+1 ); # ( 10,11,12,13,14, or 15 ) Hence you derive the following simple function to abstract that. It selects a random integer between the two given @@ -478,6 +488,9 @@ Julian day) 31 =head2 How do I find yesterday's date? +X<date> X<yesterday> X<DateTime> X<Date::Calc> X<Time::Local> +X<daylight saving time> X<day> X<Today_and_Now> X<localtime> +X<timelocal> (contributed by brian d foy) @@ -504,6 +517,22 @@ dates, but that assumes that 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. Let the modules do the work. +If you absolutely must do it yourself (or can't use one of the +modules), here's a solution using C<Time::Local>, which comes with +Perl: + + # contributed by Gunnar Hjalmarsson + use Time::Local; + my $today = timelocal 0, 0, 12, ( localtime )[3..5]; + my ($d, $m, $y) = ( localtime $today-86400 )[3..5]; + printf "Yesterday: %d-%02d-%02d\n", $y+1900, $m+1, $d; + +In this case, you measure the day starting at noon, and subtract 24 +hours. Even if the length of the calendar day is 23 or 25 hours, +you'll still end up on the previous calendar day, although not at +noon. Since you don't care about the time, the one hour difference +doesn't matter and you end up with the previous date. + =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 @@ -533,15 +562,6 @@ not the language. At the risk of inflaming the NRA: "Perl doesn't break Y2K, people do." See http://www.perl.org/about/y2k.html for a longer exposition. -=head2 Does Perl have a Year 2038 problem? - -No, all of Perl's built in date and time functions and modules will -work to about 2 billion years before and after 1970. - -Many systems cannot count time past the year 2038. Older versions of -Perl were dependent on the system to do date calculation and thus -shared their 2038 bug. - =head1 Data: Strings =head2 How do I validate input? @@ -788,44 +808,22 @@ 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. Other times you might need a -more thorough solution (Suggested by brian d foy): - - $string =~ s/ ( - (^\w) #at the beginning of the line - | # or - (\s\w) #preceded by whitespace - ) - /\U$1/xg; - - $string =~ s/([\w']+)/\u\L$1/g; - -To make the whole line upper case: - - $line = uc($line); +=head2 Does Perl have a Year 2038 problem? -To force each word to be lower case, with the first letter upper case: +No, all of Perl's built in date and time functions and modules will +work to about 2 billion years before and after 1970. - $line =~ s/(\w+)/\u\L$1/g; +Many systems cannot count time past the year 2038. Older versions of +Perl were dependent on the system to do date calculation and thus +shared their 2038 bug. -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 do I capitalize all the words on one line? +X<Text::Autoformat> X<capitalize> X<case, title> X<case, sentence> -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. +(contributed by brian d foy) -Damian Conway's L<Text::Autoformat> module provides some smart -case transformations: +Damian Conway's L<Text::Autoformat> handles all of the thinking +for you. use Text::Autoformat; my $x = "Dr. Strangelove or: How I Learned to Stop ". @@ -836,6 +834,30 @@ case transformations: print autoformat($x, { case => $style }), "\n"; } +How do you want to capitalize those words? + + FRED AND BARNEY'S LODGE # all uppercase + Fred And Barney's Lodge # title case + Fred and Barney's Lodge # highlight case + +It's not as easy a problem as it looks. How many words do you think +are in there? Wait for it... wait for it.... If you answered 5 +you're right. Perl words are groups of C<\w+>, but that's not what +you want to capitalize. How is Perl supposed to know not to capitalize +that C<s> after the apostrophe? You could try a regular expression: + + $string =~ s/ ( + (^\w) #at the beginning of the line + | # or + (\s\w) #preceded by whitespace + ) + /\U$1/xg; + + $string =~ s/([\w']+)/\u\L$1/g; + +Now, what if you don't want to capitalize that "and"? Just use +L<Text::Autoformat> and get on with the next problem. :) + =head2 How can I split a [character] delimited string except when inside [character]? Several modules can handle this sort of parsing--C<Text::Balanced>, @@ -988,7 +1010,7 @@ appear as part of the data. If you want to work with comma-separated values, don't do this since that format is a bit more complicated. Use one of the modules that -handle that fornat, such as C<Text::CSV>, C<Text::CSV_XS>, or +handle that format, such as C<Text::CSV>, C<Text::CSV_XS>, or C<Text::CSV_PP>. If you want to break apart an entire line of fixed columns, you can use @@ -1038,7 +1060,7 @@ what's left in the string: The C</e> will also silently ignore violations of strict, replacing undefined variable names with the empty string. Since I'm using the -C</e> flag (twice even!), I have all of the same security problems I +C</e> flag (twice even!), I have all of the same security problems I have with C<eval> in its string form. If there's something odd in C<$foo>, perhaps something like C<@{[ system "rm -rf /" ]}>, then I could get myself in trouble. @@ -1050,7 +1072,7 @@ can replace the missing value with a marker, in this case C<???> to signal that I missed something: my $string = 'This has $foo and $bar'; - + my %Replacements = ( foo => 'Fred', ); @@ -1281,16 +1303,32 @@ same thing. =head2 How can I tell whether a certain element is contained in a list or array? -(portions of this answer contributed by Anno Siegel) +(portions of this answer contributed by Anno Siegel and brian d foy) 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 +That being said, there are several ways to approach this. In Perl 5.10 +and later, you can use the smart match operator to check that an item is +contained in an array or a hash: + + use 5.010; + + if( $item ~~ @array ) + { + say "The array contains $item" + } + + if( $item ~~ %hash ) + { + say "The hash contains $item" + } + +With earlier versions of Perl, you have to do a bit more work. 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 maintain a -hash whose keys are the first array's values. +hash whose keys are the first array's values: @blues = qw/azure cerulean teal turquoise lapis-lazuli/; %is_blue = (); @@ -1365,6 +1403,21 @@ in either A or in B but not in both. Think of it as an xor operation. =head2 How do I test whether two arrays or hashes are equal? +With Perl 5.10 and later, the smart match operator can give you the answer +with the least amount of work: + + use 5.010; + + if( @array1 ~~ @array2 ) + { + say "The arrays are the same"; + } + + if( %hash1 ~~ %hash2 ) # doesn't check values! + { + say "The hash keys are the same"; + } + 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. @@ -1495,14 +1548,24 @@ You could add to the list this way: But again, Perl's built-in are virtually always good enough. =head2 How do I handle circular lists? +X<circular> X<array> X<Tie::Cycle> X<Array::Iterator::Circular> +X<cycle> X<modulus> -Circular lists could be handled in the traditional fashion with linked -lists, or you could just do something like this with an array: +(contributed by brian d foy) + +If you want to cycle through an array endlessy, you can increment the +index modulo the number of elements in the array: - unshift(@array, pop(@array)); # the last shall be first - push(@array, shift(@array)); # and vice versa + my @array = qw( a b c ); + my $i = 0; + + while( 1 ) { + print $array[ $i++ % @array ], "\n"; + last if $i > 20; + } -You can also use C<Tie::Cycle>: +You can also use C<Tie::Cycle> to use a scalar that always has the +next element of the circular array: use Tie::Cycle; @@ -1512,6 +1575,19 @@ You can also use C<Tie::Cycle>: print $cycle; # 000000 print $cycle; # FFFF00 +The C<Array::Iterator::Circular> creates an iterator object for +circular arrays: + + use Array::Iterator::Circular; + + my $color_iterator = Array::Iterator::Circular->new( + qw(red green blue orange) + ); + + foreach ( 1 .. 20 ) { + print $color_iterator->next, "\n"; + } + =head2 How do I shuffle an array randomly? If you either have Perl 5.8.0 or later installed, or if you have @@ -1525,6 +1601,8 @@ If not, you can use a Fisher-Yates shuffle. sub fisher_yates_shuffle { my $deck = shift; # $deck is a reference to an array + return unless @$deck; # must not be empty! + my $i = @$deck; while (--$i) { my $j = int rand ($i+1); @@ -1664,7 +1742,7 @@ C<NextPermute> uses string order and C<NextPermuteNum> numeric order, so you can enumerate all the permutations of C<0..9> like this: use Algorithm::Loops qw(NextPermuteNum); - + my @list= 0..9; do { print "@list\n" } while NextPermuteNum @list; @@ -1721,14 +1799,23 @@ See also the question later in L<perlfaq4> on sorting hashes. Use C<pack()> and C<unpack()>, or else C<vec()> and the bitwise operations. -For example, this sets C<$vec> to have bit N set if C<$ints[N]> was -set: +For example, you don't have to store individual bits in an array +(which would mean that you're wasting a lot of space). To convert an +array of bits to a string, use C<vec()> to set the right bits. This +sets C<$vec> to have bit N set only if C<$ints[N]> was set: + @ints = (...); # array of bits, e.g. ( 1, 0, 0, 1, 1, 0 ... ) $vec = ''; - foreach(@ints) { vec($vec,$_,1) = 1 } + foreach( 0 .. $#ints ) { + vec($vec,$_,1) = 1 if $ints[$_]; + } -Here's how, given a vector in C<$vec>, you can get those bits into your -C<@ints> array: +The string C<$vec> only takes up as many bits as it needs. For +instance, if you had 16 entries in C<@ints>, C<$vec> only needs two +bytes to store them (not counting the scalar variable overhead). + +Here's how, given a vector in C<$vec>, you can get those bits into +your C<@ints> array: sub bitvec_to_list { my $vec = shift; @@ -1850,7 +1937,7 @@ can then get the value through the particular key you're processing: } Once you have the list of keys, you can process that list before you -process the hashh elements. For instance, you can sort the keys so you +process the hash elements. For instance, you can sort the keys so you can process them in lexical order: foreach my $key ( sort keys %hash ) { @@ -1868,7 +1955,7 @@ those using C<grep>: } If the hash is very large, you might not want to create a long list of -keys. To save some memory, you can grab on key-value pair at a time using +keys. To save some memory, you can grab one key-value pair at a time using C<each()>, which returns a pair you haven't seen yet: while( my( $key, $value ) = each( %hash ) ) { @@ -1886,6 +1973,62 @@ you use C<keys>, C<values>, or C<each> on the same hash, you can reset the iterator and mess up your processing. See the C<each> entry in L<perlfunc> for more details. +=head2 How do I merge two hashes? +X<hash> X<merge> X<slice, hash> + +(contributed by brian d foy) + +Before you decide to merge two hashes, you have to decide what to do +if both hashes contain keys that are the same and if you want to leave +the original hashes as they were. + +If you want to preserve the original hashes, copy one hash (C<%hash1>) +to a new hash (C<%new_hash>), then add the keys from the other hash +(C<%hash2> to the new hash. Checking that the key already exists in +C<%new_hash> gives you a chance to decide what to do with the +duplicates: + + my %new_hash = %hash1; # make a copy; leave %hash1 alone + + foreach my $key2 ( keys %hash2 ) + { + if( exists $new_hash{$key2} ) + { + warn "Key [$key2] is in both hashes!"; + # handle the duplicate (perhaps only warning) + ... + next; + } + else + { + $new_hash{$key2} = $hash2{$key2}; + } + } + +If you don't want to create a new hash, you can still use this looping +technique; just change the C<%new_hash> to C<%hash1>. + + foreach my $key2 ( keys %hash2 ) + { + if( exists $hash1{$key2} ) + { + warn "Key [$key2] is in both hashes!"; + # handle the duplicate (perhaps only warning) + ... + next; + } + else + { + $hash1{$key2} = $hash2{$key2}; + } + } + +If you don't care that one hash overwrites keys and values from the other, you +could just use a hash slice to add one hash to another. In this case, values +from C<%hash2> replace values from C<%hash1> when they have keys in common: + + @hash1{ keys %hash2 } = values %hash2; + =head2 What happens if I add or remove keys from a hash while iterating over it? (contributed by brian d foy) @@ -1922,14 +2065,35 @@ worry you, you can always reverse the hash into a hash of arrays instead: =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 -use the keys() function in a scalar context: +(contributed by brian d foy) + +This is very similar to "How do I process an entire hash?", also in +L<perlfaq4>, but a bit simpler in the common cases. + +You can use the C<keys()> built-in function in scalar context to find out +have many entries you have in a hash: - $num_keys = keys %hash; + my $key_count = keys %hash; # must be scalar context! + +If you want to find out how many entries have a defined value, that's +a bit different. You have to check each value. A C<grep> is handy: + + my $defined_value_count = grep { defined } values %hash; -The keys() function also resets the iterator, which means that you may +You can use that same structure to count the entries any way that +you like. If you want the count of the keys with vowels in them, +you just test for that instead: + + my $vowel_count = grep { /[aeiou]/ } keys %hash; + +The C<grep> in scalar context returns the count. If you want the list +of matching items, just use it in list context instead: + + my @defined_values = grep { defined } values %hash; + +The C<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(). +such as C<each()>. =head2 How do I sort a hash (optionally by value instead of key)? @@ -1945,7 +2109,7 @@ create a report which lists the keys in ASCIIbetical order. foreach my $key ( @keys ) { - printf "%-20s %6d\n", $key, $hash{$value}; + printf "%-20s %6d\n", $key, $hash{$key}; } We could get more fancy in the C<sort()> block though. Instead of @@ -2139,20 +2303,44 @@ Use the C<Tie::IxHash> from CPAN. =head2 Why does passing a subroutine an undefined element in a hash create it? -If you say something like: +(contributed by brian d foy) + +Are you using a really old version of Perl? + +Normally, accessing a hash key's value for a nonexistent key will +I<not> create the key. + + my %hash = (); + my $value = $hash{ 'foo' }; + print "This won't print\n" if exists $hash{ 'foo' }; + +Passing C<$hash{ 'foo' }> to a subroutine used to be a special case, though. +Since you could assign directly to C<$_[0]>, Perl had to be ready to +make that assignment so it created the hash key ahead of time: + + my_sub( $hash{ 'foo' } ); + print "This will print before 5.004\n" if exists $hash{ 'foo' }; - somefunc($hash{"nonesuch key here"}); + sub my_sub { + # $_[0] = 'bar'; # create hash key in case you do this + 1; + } + +Since Perl 5.004, however, this situation is a special case and Perl +creates the hash key only when you make the assignment: -Then that element "autovivifies"; that is, it springs into existence -whether you store something there or not. That's because functions -get scalars passed in by reference. If somefunc() modifies C<$_[0]>, -it has to be ready to write it back into the caller's version. + my_sub( $hash{ 'foo' } ); + print "This will print, even after 5.004\n" if exists $hash{ 'foo' }; + + sub my_sub { + $_[0] = 'bar'; + } -This has been fixed as of Perl5.004. +However, if you want the old behavior (and think carefully about that +because it's a weird side effect), you can pass a hash slice instead. +Perl 5.004 didn't make this a special case: -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 -awk's behavior. + my_sub( @hash{ qw/foo/ } ); =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? @@ -2174,18 +2362,31 @@ in L<perltoot>. =head2 How can I use a reference as a hash key? -(contributed by brian d foy) +(contributed by brian d foy and Ben Morrow) Hash keys are strings, so you can't really use a reference as the key. When you try to do that, perl turns the reference into its stringified form (for instance, C<HASH(0xDEADBEEF)>). From there you can't get back the reference from the stringified form, at least without doing -some extra work on your own. Also remember that hash keys must be -unique, but two different variables can store the same reference (and -those variables can change later). - -The C<Tie::RefHash> module, which is distributed with perl, might be -what you want. It handles that extra work. +some extra work on your own. + +Remember that the entry in the hash will still be there even if +the referenced variable goes out of scope, and that it is entirely +possible for Perl to subsequently allocate a different variable at +the same address. This will mean a new variable might accidentally +be associated with the value for an old. + +If you have Perl 5.10 or later, and you just want to store a value +against the reference for lookup later, you can use the core +Hash::Util::Fieldhash module. This will also handle renaming the +keys if you use multiple threads (which causes all variables to be +reallocated at new addresses, changing their stringification), and +garbage-collecting the entries when the referenced variable goes out +of scope. + +If you actually need to be able to get a real reference back from +each hash entry, you can use the Tie::RefHash module, which does the +required work for you. =head1 Data: Misc @@ -2288,7 +2489,14 @@ you wanted to copy. =head2 How do I define methods for every class/object? -Use the C<UNIVERSAL> class (see L<UNIVERSAL>). +(contributed by Ben Morrow) + +You can use the C<UNIVERSAL> class (see L<UNIVERSAL>). However, please +be very careful to consider the consequences of doing this: adding +methods to every object is very likely to have unintended +consequences. If possible, it would be better to have all your object +inherit from some common base class, or to use an object system like +Moose that supports roles. =head2 How do I verify a credit card checksum? @@ -2296,21 +2504,23 @@ Get the C<Business::CreditCard> module from CPAN. =head2 How do I pack arrays of doubles or floats for XS code? -The kgbpack.c code in the C<PGPLOT> module on CPAN does just this. +The arrays.h/arrays.c code in the C<PGPLOT> module on CPAN does just this. If you're doing a lot of float or double processing, consider using the C<PDL> module from CPAN instead--it makes number-crunching easy. +See L<http://search.cpan.org/dist/PGPLOT> for the code. + =head1 REVISION -Revision: $Revision: 10394 $ +Revision: $Revision$ -Date: $Date: 2007-12-09 18:47:15 +0100 (Sun, 09 Dec 2007) $ +Date: $Date$ See L<perlfaq> for source control details and availability. =head1 AUTHOR AND COPYRIGHT -Copyright (c) 1997-2007 Tom Christiansen, Nathan Torkington, and +Copyright (c) 1997-2009 Tom Christiansen, Nathan Torkington, and other authors as noted. All rights reserved. This documentation is free; you can redistribute it and/or modify it |