From: Perl 5 Porters Date: Sun, 7 Jul 1996 20:21:32 +0000 (+0000) Subject: perl 5.003_01: pod/perldsc.pod X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=commitdiff_plain;h=4973169ddd8a4845f55c50a6bb1be8d5c1558ea1;p=p5sagit%2Fp5-mst-13.2.git perl 5.003_01: pod/perldsc.pod Typos corrected; whitespace trimmed Update references to debugger --- diff --git a/pod/perldsc.pod b/pod/perldsc.pod index 7e18e74..6d31976 100644 --- a/pod/perldsc.pod +++ b/pod/perldsc.pod @@ -21,7 +21,7 @@ with three dimensions! for $x (1 .. 10) { for $y (1 .. 10) { for $z (1 .. 10) { - $LoL[$x][$y][$z] = + $LoL[$x][$y][$z] = $x ** $y + $z; } } @@ -34,7 +34,7 @@ How do you print it out? Why can't you just say C? How do you sort it? How can you pass it to a function or get one of these back from a function? Is is an object? Can you save it to disk to read back later? How do you access whole rows or columns of that matrix? Do -all the values have to be numeric? +all the values have to be numeric? As you see, it's quite easy to become confused. While some small portion of the blame for this can be attributed to the reference-based @@ -69,7 +69,7 @@ documents on each of the following: =back But for now, let's look at some of the general issues common to all -of these types of data structures. +of these types of data structures. =head1 REFERENCES @@ -84,14 +84,14 @@ You can't use a reference to a array or hash in quite the same way that you would a real array or hash. For C or C++ programmers unused to distinguishing between arrays and pointers to the same, this can be confusing. If so, just think of it as the difference between a structure and a pointer to a -structure. +structure. You can (and should) read more about references in the perlref(1) man page. Briefly, references are rather like pointers that know what they point to. (Objects are also a kind of reference, but we won't be needing -them right away--if ever.) That means that when you have something that -looks to you like an access to two-or-more-dimensional array and/or hash, -that what's really going on is that in all these cases, the base type is +them right away--if ever.) This means that when you have something which +looks to you like an access to a two-or-more-dimensional array and/or hash, +what's really going on is that the base type is merely a one-dimensional entity that contains references to the next level. It's just that you can I it as though it were a two-dimensional one. This is actually the way almost all C @@ -130,7 +130,7 @@ of a nested array: for $i (1..10) { @list = somefunc($i); $LoL[$i] = @list; # WRONG! - } + } That's just the simple case of assigning a list to a scalar and getting its element count. If that's what you really and truly want, then you @@ -139,7 +139,7 @@ might do well to consider being a tad more explicit about it, like this: for $i (1..10) { @list = somefunc($i); $counts[$i] = scalar @list; - } + } Here's the case of taking a reference to the same memory location again and again: @@ -147,7 +147,7 @@ again and again: for $i (1..10) { @list = somefunc($i); $LoL[$i] = \@list; # WRONG! - } + } So, just what's the big problem with that? It looks right, doesn't it? After all, I just told you that you need an array of references, so by @@ -164,29 +164,29 @@ the following C program: rp = getpwnam("root"); dp = getpwnam("daemon"); - printf("daemon name is %s\nroot name is %s\n", + printf("daemon name is %s\nroot name is %s\n", dp->pw_name, rp->pw_name); } Which will print daemon name is daemon - root name is daemon + root name is daemon The problem is that both C and C are pointers to the same location in memory! In C, you'd have to remember to malloc() yourself some new memory. In Perl, you'll want to use the array constructor C<[]> or the hash constructor C<{}> instead. Here's the right way to do the preceding -broken code fragments +broken code fragments: for $i (1..10) { @list = somefunc($i); $LoL[$i] = [ @list ]; - } + } The square brackets make a reference to a new array with a I of what's in @list at the time of the assignment. This is what -you want. +you want. Note that this will produce something similar, but it's much harder to read: @@ -194,7 +194,7 @@ much harder to read: for $i (1..10) { @list = 0 .. $i; @{$LoL[$i]} = @list; - } + } Is it the same? Well, maybe so--and maybe not. The subtle difference is that when you assign something in square brackets, you know for sure @@ -220,7 +220,7 @@ they're disturbingly more apt to mean that it's "annoying", So just remember to always use the array or hash constructors with C<[]> or C<{}>, and you'll be fine, although it's not always optimally -efficient. +efficient. Surprisingly, the following dangerous-looking construct will actually work out fine: @@ -228,7 +228,7 @@ actually work out fine: for $i (1..10) { my @list = somefunc($i); $LoL[$i] = \@list; - } + } That's because my() is more of a run-time statement than it is a compile-time declaration I. This means that the my() variable is @@ -251,7 +251,7 @@ In summary: @{ $LoL[$i] } = @list; # way too tricky for most programmers -=head1 CAVEAT ON PRECEDENCE +=head1 CAVEAT ON PRECEDENCE Speaking of things like C<@{$LoL[$i]}>, the following are actually the same thing: @@ -303,16 +303,11 @@ variable, and it would thereby remind you to instead write: =head1 DEBUGGING -The standard Perl debugger in 5.001 doesn't do a very nice job of -printing out complex data structures. However, the perl5db that -Ilya Zakharevich EFE -wrote, which is accessible at - - ftp://ftp.perl.com/pub/perl/ext/perl5db-kit-0.9.tar.gz - -has several new features, including command line editing as well -as the C command to dump out complex data structures. For example, -given the assignment to $LoL above, here's the debugger output: +Before 5.002, the standard Perl debugger didn't do a very nice job of +printing out complex data structures. With version 5.002 or above, the +debugger includes several new features, including command line editing as +well as the C command to dump out complex data structures. For +example, given the assignment to $LoL above, here's the debugger output: DB<1> X $LoL $LoL = ARRAY(0x13b5a0) @@ -338,7 +333,7 @@ There's also a lower-case B command which is nearly the same. =head1 CODE EXAMPLES Presented with little comment (these will get their own man pages someday) -here are short code examples illustrating access of various +here are short code examples illustrating access of various types of data structures. =head1 LISTS OF LISTS @@ -356,18 +351,18 @@ types of data structures. # reading from file while ( <> ) { push @LoL, [ split ]; - + } # calling a function for $i ( 1 .. 10 ) { $LoL[$i] = [ somefunc($i) ]; - + } # using temp vars for $i ( 1 .. 10 ) { @tmp = somefunc($i); $LoL[$i] = [ @tmp ]; - + } # add to an existing row push @{ $LoL[0] }, "wilma", "betty"; @@ -383,19 +378,19 @@ types of data structures. # print the whole thing with refs for $aref ( @LoL ) { print "\t [ @$aref ],\n"; - + } # print the whole thing with indices for $i ( 0 .. $#LoL ) { print "\t [ @{$LoL[$i]} ],\n"; - + } # print the whole thing one at a time for $i ( 0 .. $#LoL ) { for $j ( 0 .. $#{$LoL[$i]} ) { print "elt $i $j is $LoL[$i][$j]\n"; } - + } =head1 HASHES OF LISTS @@ -414,7 +409,7 @@ types of data structures. while ( <> ) { next unless s/^(.*?):\s*//; $HoL{$1} = [ split ]; - + } # reading from file; more temps # flintstones: fred barney wilma dino @@ -422,18 +417,18 @@ types of data structures. ($who, $rest) = split /:\s*/, $line, 2; @fields = split ' ', $rest; $HoL{$who} = [ @fields ]; - + } # calling a function that returns a list for $group ( "simpsons", "jetsons", "flintstones" ) { $HoL{$group} = [ get_family($group) ]; - + } # likewise, but using temps for $group ( "simpsons", "jetsons", "flintstones" ) { @members = get_family($group); $HoL{$group} = [ @members ]; - + } # append new members to an existing family push @{ $HoL{"flintstones"} }, "wilma", "betty"; @@ -449,7 +444,7 @@ types of data structures. # print the whole thing foreach $family ( keys %HoL ) { print "$family: @{ $HoL{$family} }\n" - + } # print the whole thing with indices foreach $family ( keys %HoL ) { @@ -458,15 +453,17 @@ types of data structures. print " $i = $HoL{$family}[$i]"; } print "\n"; - + } # print the whole thing sorted by number of members foreach $family ( sort { @{$HoL{$b}} <=> @{$HoL{$b}} } keys %HoL ) { print "$family: @{ $HoL{$family} }\n" + } # print the whole thing sorted by number of members and name foreach $family ( sort { @{$HoL{$b}} <=> @{$HoL{$a}} } keys %HoL ) { print "$family: ", join(", ", sort @{ $HoL{$family}), "\n"; + } =head1 LISTS OF HASHES @@ -474,8 +471,8 @@ types of data structures. @LoH = ( { - Lead => "fred", - Friend => "barney", + Lead => "fred", + Friend => "barney", }, { Lead => "george", @@ -500,6 +497,7 @@ types of data structures. $rec->{$key} = $value; } push @LoH, $rec; + } # reading from file @@ -507,30 +505,30 @@ types of data structures. # no temp while ( <> ) { push @LoH, { split /[\s+=]/ }; - + } # calling a function that returns a key,value list, like # "lead","fred","daughter","pebbles" while ( %fields = getnextpairset() ) push @LoH, { %fields }; - + } # likewise, but using no temp vars while (<>) { push @LoH, { parsepairs($_) }; - + } # add key/value to an element - $LoH[0]{"pet"} = "dino"; - $LoH[2]{"pet"} = "santa's little helper"; + $LoH[0]{pet} = "dino"; + $LoH[2]{pet} = "santa's little helper"; =head2 Access and Printing of a LIST OF HASHES # one element - $LoH[0]{"lead"} = "fred"; + $LoH[0]{lead} = "fred"; # another element - $LoH[1]{"lead"} =~ s/(\w)/\u$1/; + $LoH[1]{lead} =~ s/(\w)/\u$1/; # print the whole thing with refs for $href ( @LoH ) { @@ -539,7 +537,7 @@ types of data structures. print "$role=$href->{$role} "; } print "}\n"; - + } # print the whole thing with indices for $i ( 0 .. $#LoH ) { @@ -548,13 +546,14 @@ types of data structures. print "$role=$LoH[$i]{$role} "; } print "}\n"; - + } # print the whole thing one at a time for $i ( 0 .. $#LoH ) { for $role ( keys %{ $LoH[$i] } ) { print "elt $i $role is $LoH[$i]{$role}\n"; } + } =head1 HASHES OF HASHES @@ -566,15 +565,16 @@ types of data structures. "pal" => "barney", }, "jetsons" => { - "lead" => "george", - "wife" => "jane", - "his boy"=> "elroy", - } + "lead" => "george", + "wife" => "jane", + "his boy" => "elroy", + }, "simpsons" => { "lead" => "homer", "wife" => "marge", "kid" => "bart", - ); + }, + ); =head2 Generation of a HASH OF HASHES @@ -599,38 +599,39 @@ types of data structures. ($key, $value) = split /=/, $field; $rec->{$key} = $value; } - + } # calling a function that returns a key,value list, like # "lead","fred","daughter","pebbles" while ( %fields = getnextpairset() ) push @a, { %fields }; - + } # calling a function that returns a key,value hash for $group ( "simpsons", "jetsons", "flintstones" ) { $HoH{$group} = { get_family($group) }; - + } # likewise, but using temps for $group ( "simpsons", "jetsons", "flintstones" ) { %members = get_family($group); $HoH{$group} = { %members }; - + } # append new members to an existing family %new_folks = ( "wife" => "wilma", "pet" => "dino"; ); + for $what (keys %new_folks) { $HoH{flintstones}{$what} = $new_folks{$what}; - + } =head2 Access and Printing of a HASH OF HASHES # one element - $HoH{"flintstones"}{"wife"} = "wilma"; + $HoH{flintstones}{wife} = "wilma"; # another element $HoH{simpsons}{lead} =~ s/(\w)/\u$1/; @@ -638,29 +639,30 @@ types of data structures. # print the whole thing foreach $family ( keys %HoH ) { print "$family: "; - for $role ( keys %{ $HoH{$family} } { + for $role ( keys %{ $HoH{$family} } ) { print "$role=$HoH{$family}{$role} "; } print "}\n"; - + } # print the whole thing somewhat sorted foreach $family ( sort keys %HoH ) { print "$family: "; - for $role ( sort keys %{ $HoH{$family} } { + for $role ( sort keys %{ $HoH{$family} } ) { print "$role=$HoH{$family}{$role} "; } print "}\n"; + } # print the whole thing sorted by number of members foreach $family ( sort { keys %{$HoH{$b}} <=> keys %{$HoH{$b}} } keys %HoH ) { print "$family: "; - for $role ( sort keys %{ $HoH{$family} } { + for $role ( sort keys %{ $HoH{$family} } ) { print "$role=$HoH{$family}{$role} "; } print "}\n"; - + } # establish a sort order (rank) for each role $i = 0; @@ -670,10 +672,11 @@ types of data structures. foreach $family ( sort { keys %{$HoH{$b}} <=> keys %{$HoH{$b}} } keys %HoH ) { print "$family: "; # and print these according to rank order - for $role ( sort { $rank{$a} <=> $rank{$b} keys %{ $HoH{$family} } { + for $role ( sort { $rank{$a} <=> $rank{$b} keys %{ $HoH{$family} } ) { print "$role=$HoH{$family}{$role} "; } print "}\n"; + } =head1 MORE ELABORATE RECORDS @@ -684,48 +687,48 @@ Here's a sample showing how to create and use a record whose fields are of many different sorts: $rec = { - STRING => $string, - LIST => [ @old_values ], - LOOKUP => { %some_table }, - FUNC => \&some_function, - FANON => sub { $_[0] ** $_[1] }, - FH => \*STDOUT, + TEXT => $string, + SEQUENCE => [ @old_values ], + LOOKUP => { %some_table }, + THATCODE => \&some_function, + THISCODE => sub { $_[0] ** $_[1] }, + HANDLE => \*STDOUT, }; - print $rec->{STRING}; + print $rec->{TEXT}; print $rec->{LIST}[0]; - $last = pop @ { $rec->{LIST} }; + $last = pop @ { $rec->{SEQUENCE} }; print $rec->{LOOKUP}{"key"}; ($first_k, $first_v) = each %{ $rec->{LOOKUP} }; - $answer = &{ $rec->{FUNC} }($arg); - $answer = &{ $rec->{FANON} }($arg1, $arg2); + $answer = &{ $rec->{THATCODE} }($arg); + $answer = &{ $rec->{THISCODE} }($arg1, $arg2); # careful of extra block braces on fh ref - print { $rec->{FH} } "a string\n"; + print { $rec->{HANDLE} } "a string\n"; use FileHandle; - $rec->{FH}->autoflush(1); - $rec->{FH}->print(" a string\n"); + $rec->{HANDLE}->autoflush(1); + $rec->{HANDLE}->print(" a string\n"); =head2 Declaration of a HASH OF COMPLEX RECORDS %TV = ( "flintstones" => { series => "flintstones", - nights => [ qw(monday thursday friday) ]; + nights => [ qw(monday thursday friday) ], members => [ { name => "fred", role => "lead", age => 36, }, { name => "wilma", role => "wife", age => 31, }, - { name => "pebbles", role => "kid", age => 4, }, + { name => "pebbles", role => "kid", age => 4, }, ], }, "jetsons" => { series => "jetsons", - nights => [ qw(wednesday saturday) ]; + nights => [ qw(wednesday saturday) ], members => [ { name => "george", role => "lead", age => 41, }, { name => "jane", role => "wife", age => 39, }, @@ -735,7 +738,7 @@ many different sorts: "simpsons" => { series => "simpsons", - nights => [ qw(monday) ]; + nights => [ qw(monday) ], members => [ { name => "homer", role => "lead", age => 34, }, { name => "marge", role => "wife", age => 37, }, @@ -832,6 +835,5 @@ L, L, L, L Tom Christiansen EFE -Last update: -Tue Dec 12 09:20:26 MST 1995 - +Last update: +Mon Jul 8 05:22:49 MDT 1996