=head1 NAME
+X<data structure> X<complex data structure> X<struct>
perldsc - Perl Data Structures Cookbook
to develop and maintain--to put it mildly.
The 5.0 release of Perl let us have complex data structures. You
-may now write something like this and all of a sudden, you'd have a array
+may now write something like this and all of a sudden, you'd have an array
with three dimensions!
- my @AoA;
- for my $x (1 .. 10) {
- for my $y (1 .. 10) {
- for my $z (1 .. 10) {
- $AoA[$x][$y][$z] = $x ** $y + $z;
+ for $x (1 .. 10) {
+ for $y (1 .. 10) {
+ for $z (1 .. 10) {
+ $AoA[$x][$y][$z] =
+ $x ** $y + $z;
}
}
}
How do you print it out? Why can't you say just C<print @AoA>? 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
+from a function? Is it 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?
these types of data structures.
=head1 REFERENCES
+X<reference> X<dereference> X<dereferencing> X<pointer>
The most important thing to understand about all data structures in Perl
-- including multidimensional arrays--is that even though they might
one-dimensional. They can hold only scalar values (meaning a string,
number, or a reference). They cannot directly contain other arrays or
hashes, but instead contain I<references> to other arrays or hashes.
+X<multidimensional array> X<array, multidimensional>
-You can't use a reference to a array or hash in quite the same way that you
+You can't use a reference to an 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
out your array in with a simple print() function, you'll get something
that doesn't look very nice, like this:
- my @AoA = (
- [2, 3, ],
- [4, 5, 7],
- [0, ],
- );
+ @AoA = ( [2, 3], [4, 5, 7], [0] );
print $AoA[1][2];
7
print @AoA;
repeatedly. Here's the case where you just get the count instead
of a nested array:
- my @AoA;
- for my $i (1..10) {
- my @array = somefunc($i);
- $AoA[$i] = @array; # WRONG!
+ for $i (1..10) {
+ @array = somefunc($i);
+ $AoA[$i] = @array; # WRONG!
}
That's just the simple case of assigning an array to a scalar and getting
its element count. If that's what you really and truly want, then you
might do well to consider being a tad more explicit about it, like this:
- my @counts;
- for my $i (1..10) {
- my @array = somefunc($i);
- $counts[$i] = scalar @array;
+ for $i (1..10) {
+ @array = somefunc($i);
+ $counts[$i] = scalar @array;
}
-Here's the right way to do the reference C<@array>:
+Here's the case of taking a reference to the same memory location
+again and again:
- my @AoA
- for my $i (1..10) {
- my @array = somefunc($i);
- $AoA[$i] = [ @array ];
+ for $i (1..10) {
+ @array = somefunc($i);
+ $AoA[$i] = \@array; # WRONG!
+ }
+
+So, 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
+golly, you've made me one!
+
+Unfortunately, while this is true, it's still broken. All the references
+in @AoA refer to the I<very same place>, and they will therefore all hold
+whatever was last in @array! It's similar to the problem demonstrated in
+the following C program:
+
+ #include <pwd.h>
+ main() {
+ struct passwd *getpwnam(), *rp, *dp;
+ rp = getpwnam("root");
+ dp = getpwnam("daemon");
+
+ 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
+
+The problem is that both C<rp> and C<dp> 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:
+X<[]> X<{}>
+
+ for $i (1..10) {
+ @array = somefunc($i);
+ $AoA[$i] = [ @array ];
}
The square brackets make a reference to a new array with a I<copy>
-of what's in C<@array>.
+of what's in @array at the time of the assignment. This is what
+you want.
Note that this will produce something similar, but it's
much harder to read:
- my @AoA;
- for my $i (1..10) {
- my @array = somefunc($i);
- @{ $AoA[$i] } = @array;
+ for $i (1..10) {
+ @array = 0 .. $i;
+ @{$AoA[$i]} = @array;
}
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
it's always a brand new reference with a new I<copy> of the data.
-Something else could be going on in this new case with the C<@{ $AoA[$i]} }>
+Something else could be going on in this new case with the C<@{$AoA[$i]}>
dereference on the left-hand-side of the assignment. It all depends on
whether C<$AoA[$i]> had been undefined to start with, or whether it
already contained a reference. If you had already populated @AoA with
Then the assignment with the indirection on the left-hand-side would
use the existing reference that was already there:
- @{ $AoA[3] } = @array;
+ @{$AoA[3]} = @array;
Of course, this I<would> have the "interesting" effect of clobbering
@another_array. (Have you ever noticed how when a programmer says
Surprisingly, the following dangerous-looking construct will
actually work out fine:
- my @AoA;
- for my $i (1..10) {
- my @array = somefunc($i);
- $AoA[$i] = \@array;
+ for $i (1..10) {
+ my @array = somefunc($i);
+ $AoA[$i] = \@array;
}
That's because my() is more of a run-time statement than it is a
In summary:
- $AoA[$i] = [ @array ]; # usually best
- $AoA[$i] = \@array; # perilous; just how my() is that array?
- @{ $AoA[$i] } = @array; # way too tricky for most programmers
+ $AoA[$i] = [ @array ]; # usually best
+ $AoA[$i] = \@array; # perilous; just how my() was that array?
+ @{ $AoA[$i] } = @array; # way too tricky for most programmers
=head1 CAVEAT ON PRECEDENCE
+X<dereference, precedence> X<dereferencing, precedence>
-Speaking of things like C<@{ $AoA[$i] }>, the following are actually the
+Speaking of things like C<@{$AoA[$i]}>, the following are actually the
same thing:
+X<< -> >>
$aref->[2][2] # clear
$$aref[2][2] # confusing
this:
my $aref = [
- [ 'fred', 'barney', 'pebbles', 'bambam', 'dino', ],
- [ 'homer', 'bart', 'marge', 'maggie', ],
- [ 'george', 'jane', 'elroy', 'judy', ],
+ [ "fred", "barney", "pebbles", "bambam", "dino", ],
+ [ "homer", "bart", "marge", "maggie", ],
+ [ "george", "jane", "elroy", "judy", ],
];
print $aref[2][2];
print $aref->[2][2]
=head1 DEBUGGING
+X<data structure, debugging> X<complex data structure, debugging>
+X<AoA, debugging> X<HoA, debugging> X<AoH, debugging> X<HoH, debugging>
+X<array of arrays, debugging> X<hash of arrays, debugging>
+X<array of hashes, debugging> X<hash of hashes, debugging>
Before version 5.002, the standard Perl debugger didn't do a very nice job of
printing out complex data structures. With 5.002 or above, the
types of data structures.
=head1 ARRAYS OF ARRAYS
+X<array of arrays> X<AoA>
-=head2 Declaration of a ARRAY OF ARRAYS
+=head2 Declaration of an ARRAY OF ARRAYS
- my @AoA = (
- [ 'fred', 'barney' ],
- [ 'george', 'jane', 'elroy' ],
- [ 'homer', 'marge', 'bart' ],
+ @AoA = (
+ [ "fred", "barney" ],
+ [ "george", "jane", "elroy" ],
+ [ "homer", "marge", "bart" ],
);
-=head2 Generation of a ARRAY OF ARRAYS
+=head2 Generation of an ARRAY OF ARRAYS
# reading from file
- my @AoA;
while ( <> ) {
push @AoA, [ split ];
}
# calling a function
- my @AoA;
- foreach my $i ( 1 .. 10 ) {
+ for $i ( 1 .. 10 ) {
$AoA[$i] = [ somefunc($i) ];
}
# using temp vars
- my @AoA;
- foreach my $i ( 1 .. 10 ) {
- my @tmp = somefunc($i);
- $AoA[$i] = [ @tmp ];
+ for $i ( 1 .. 10 ) {
+ @tmp = somefunc($i);
+ $AoA[$i] = [ @tmp ];
}
# add to an existing row
- push @{ $AoA[0] }, 'wilma', 'betty';
-
-=head2 Access and Printing of a ARRAY OF ARRAYS
+ push @{ $AoA[0] }, "wilma", "betty";
- my @AoA;
+=head2 Access and Printing of an ARRAY OF ARRAYS
# one element
- $AoA[0][0] = 'Fred';
+ $AoA[0][0] = "Fred";
# another element
$AoA[1][1] =~ s/(\w)/\u$1/;
# print the whole thing with refs
- foreach my $aref ( @AoA ) {
+ for $aref ( @AoA ) {
print "\t [ @$aref ],\n";
}
# print the whole thing with indices
- foreach my $i ( 0 .. $#AoA ) {
- print "\t [ @{ $AoA[$i] } ],\n";
+ for $i ( 0 .. $#AoA ) {
+ print "\t [ @{$AoA[$i]} ],\n";
}
# print the whole thing one at a time
- foreach my $i ( 0 .. $#AoA ) {
- foreach my $j ( 0 .. $#{ $AoA[$i] } ) {
- print "element $i $j is $AoA[$i][$j]\n";
+ for $i ( 0 .. $#AoA ) {
+ for $j ( 0 .. $#{ $AoA[$i] } ) {
+ print "elt $i $j is $AoA[$i][$j]\n";
}
}
=head1 HASHES OF ARRAYS
+X<hash of arrays> X<HoA>
=head2 Declaration of a HASH OF ARRAYS
- my %HoA = (
- flintstones => [ 'fred', 'barney' ],
- jetsons => [ 'george', 'jane', 'elroy' ],
- simpsons => [ 'homer', 'marge', 'bart' ],
+ %HoA = (
+ flintstones => [ "fred", "barney" ],
+ jetsons => [ "george", "jane", "elroy" ],
+ simpsons => [ "homer", "marge", "bart" ],
);
=head2 Generation of a HASH OF ARRAYS
# reading from file
# flintstones: fred barney wilma dino
- my %HoA;
while ( <> ) {
- next unless s/^([^:]*):\s*//;
+ next unless s/^(.*?):\s*//;
$HoA{$1} = [ split ];
}
# reading from file; more temps
# flintstones: fred barney wilma dino
- my %HoA;
- while ( my $line = <> ) {
- my ($who, $rest) = split /:\s*/, $line, 2;
- my @fields = split ' ', $rest;
- $HoA{$who} = [ @fields ];
+ while ( $line = <> ) {
+ ($who, $rest) = split /:\s*/, $line, 2;
+ @fields = split ' ', $rest;
+ $HoA{$who} = [ @fields ];
}
# calling a function that returns a list
- my %HoA;
- foreach my $group ( 'simpsons', 'jetsons', 'flintstones' ) {
+ for $group ( "simpsons", "jetsons", "flintstones" ) {
$HoA{$group} = [ get_family($group) ];
}
# likewise, but using temps
- my %HoA;
- foreach my $group ( 'simpsons', 'jetsons', 'flintstones' ) {
- my @members = get_family($group);
- $HoA{$group} = [ @members ];
+ for $group ( "simpsons", "jetsons", "flintstones" ) {
+ @members = get_family($group);
+ $HoA{$group} = [ @members ];
}
# append new members to an existing family
- push @{ $HoA{flintstones} }, 'wilma', 'betty';
+ push @{ $HoA{"flintstones"} }, "wilma", "betty";
=head2 Access and Printing of a HASH OF ARRAYS
- my %HoA;
-
# one element
- $HoA{flintstones}[0] = 'Fred';
+ $HoA{flintstones}[0] = "Fred";
# another element
$HoA{simpsons}[1] =~ s/(\w)/\u$1/;
# print the whole thing
- foreach my $family ( keys %HoA ) {
- print "$family: @{ $HoA{$family} }\n";
+ foreach $family ( keys %HoA ) {
+ print "$family: @{ $HoA{$family} }\n"
}
# print the whole thing with indices
- foreach my $family ( keys %HoA ) {
- print 'family: ';
- foreach my $i ( 0 .. $#{ $HoA{$family} } ) {
+ foreach $family ( keys %HoA ) {
+ print "family: ";
+ foreach $i ( 0 .. $#{ $HoA{$family} } ) {
print " $i = $HoA{$family}[$i]";
}
print "\n";
}
# print the whole thing sorted by number of members
- sub num_members {
- @{ $HoA{$b} } <=> @{ $HoA{$a} }
- }
- foreach my $family ( sort num_members keys %HoA ) {
+ foreach $family ( sort { @{$HoA{$b}} <=> @{$HoA{$a}} } keys %HoA ) {
print "$family: @{ $HoA{$family} }\n"
}
# print the whole thing sorted by number of members and name
- sub members_and_name {
- @{ $HoA{$b} } <=> @{ $HoA{$a} }
- ||
- $a cmp $b
- }
- foreach my $family ( sort members_and_name keys %HoA ) {
+ foreach $family ( sort {
+ @{$HoA{$b}} <=> @{$HoA{$a}}
+ ||
+ $a cmp $b
+ } keys %HoA )
+ {
print "$family: ", join(", ", sort @{ $HoA{$family} }), "\n";
}
=head1 ARRAYS OF HASHES
+X<array of hashes> X<AoH>
-=head2 Declaration of a ARRAY OF HASHES
+=head2 Declaration of an ARRAY OF HASHES
- my @AoH = (
+ @AoH = (
{
- Lead => 'fred',
- Friend => 'barney',
+ Lead => "fred",
+ Friend => "barney",
},
{
- Lead => 'george',
- Wife => 'jane',
- Son => 'elroy',
+ Lead => "george",
+ Wife => "jane",
+ Son => "elroy",
},
{
- Lead => 'homer',
- Wife => 'marge',
- Son => 'bart',
+ Lead => "homer",
+ Wife => "marge",
+ Son => "bart",
}
);
-=head2 Generation of a ARRAY OF HASHES
+=head2 Generation of an ARRAY OF HASHES
# reading from file
# format: LEAD=fred FRIEND=barney
- my @AoH;
while ( <> ) {
- my $rec = {};
- foreach my $field ( split ) {
- my($key, $value) = split /=/, $field;
- $rec->{$key} = $value;
+ $rec = {};
+ for $field ( split ) {
+ ($key, $value) = split /=/, $field;
+ $rec->{$key} = $value;
}
push @AoH, $rec;
}
# reading from file
# format: LEAD=fred FRIEND=barney
# no temp
- my @AoH;
while ( <> ) {
push @AoH, { split /[\s+=]/ };
}
# calling a function that returns a key/value pair list, like
- # lead => 'fred', daughter => 'pebbles'
- my @AoH;
- while ( my %fields = getnextpairset() ) {
+ # "lead","fred","daughter","pebbles"
+ while ( %fields = getnextpairset() ) {
push @AoH, { %fields };
}
# likewise, but using no temp vars
- my @AoH;
while (<>) {
push @AoH, { parsepairs($_) };
}
# add key/value to an element
- $AoH[0]{pet} = 'dino';
+ $AoH[0]{pet} = "dino";
$AoH[2]{pet} = "santa's little helper";
-=head2 Access and Printing of a ARRAY OF HASHES
-
- my @AoH;
+=head2 Access and Printing of an ARRAY OF HASHES
# one element
- $AoH[0]{lead} = 'fred';
+ $AoH[0]{lead} = "fred";
# another element
$AoH[1]{lead} =~ s/(\w)/\u$1/;
# print the whole thing with refs
- foreach my $href ( @AoH ) {
- print '{ ';
- foreach my $role ( keys %$href ) {
- print "$role = $href->{$role} ";
+ for $href ( @AoH ) {
+ print "{ ";
+ for $role ( keys %$href ) {
+ print "$role=$href->{$role} ";
}
print "}\n";
}
# print the whole thing with indices
- foreach my $i ( 0 .. $#AoH ) {
+ for $i ( 0 .. $#AoH ) {
print "$i is { ";
- foreach my $role ( keys %{ $AoH[$i] } ) {
- print "$role = $AoH[$i]{$role} ";
+ for $role ( keys %{ $AoH[$i] } ) {
+ print "$role=$AoH[$i]{$role} ";
}
print "}\n";
}
# print the whole thing one at a time
- foreach my $i ( 0 .. $#AoH ) {
- foreach my $role ( keys %{ $AoH[$i] } ) {
- print "element $i $role is $AoH[$i]{$role}\n";
+ for $i ( 0 .. $#AoH ) {
+ for $role ( keys %{ $AoH[$i] } ) {
+ print "elt $i $role is $AoH[$i]{$role}\n";
}
}
=head1 HASHES OF HASHES
+X<hass of hashes> X<HoH>
=head2 Declaration of a HASH OF HASHES
- my %HoH = (
+ %HoH = (
flintstones => {
- lead => 'fred',
- pal => 'barney',
+ lead => "fred",
+ pal => "barney",
},
jetsons => {
- lead => 'george',
- wife => 'jane',
- 'his boy' => 'elroy',
+ lead => "george",
+ wife => "jane",
+ "his boy" => "elroy",
},
simpsons => {
- lead => 'homer',
- wife => 'marge',
- kid => 'bart',
+ lead => "homer",
+ wife => "marge",
+ kid => "bart",
},
);
# reading from file
# flintstones: lead=fred pal=barney wife=wilma pet=dino
- my %HoH;
while ( <> ) {
- next unless s/^([^:]*):\s*//;
- my $who = $1;
- for my $field ( split ) {
- my($key, $value) = split /=/, $field;
+ next unless s/^(.*?):\s*//;
+ $who = $1;
+ for $field ( split ) {
+ ($key, $value) = split /=/, $field;
$HoH{$who}{$key} = $value;
}
# reading from file; more temps
- my %HoH;
while ( <> ) {
- next unless s/^([^:]*):\s*//;
- my $who = $1;
- my $rec = {};
+ next unless s/^(.*?):\s*//;
+ $who = $1;
+ $rec = {};
$HoH{$who} = $rec;
- foreach my $field ( split ) {
- my($key, $value) = split /=/, $field;
- $rec->{$key} = $value;
+ for $field ( split ) {
+ ($key, $value) = split /=/, $field;
+ $rec->{$key} = $value;
}
}
# calling a function that returns a key,value hash
- my %HoH;
- foreach my $group ( 'simpsons', 'jetsons', 'flintstones' ) {
+ for $group ( "simpsons", "jetsons", "flintstones" ) {
$HoH{$group} = { get_family($group) };
}
# likewise, but using temps
- my %HoH;
- foreach my $group ( 'simpsons', 'jetsons', 'flintstones' ) {
- my %members = get_family($group);
+ for $group ( "simpsons", "jetsons", "flintstones" ) {
+ %members = get_family($group);
$HoH{$group} = { %members };
}
# append new members to an existing family
- my %HoH;
- my %new_folks = (
- wife => 'wilma',
- pet => 'dino',
+ %new_folks = (
+ wife => "wilma",
+ pet => "dino",
);
- foreach my $what (keys %new_folks) {
+ for $what (keys %new_folks) {
$HoH{flintstones}{$what} = $new_folks{$what};
}
=head2 Access and Printing of a HASH OF HASHES
- %HoH;
-
# one element
- $HoH{flintstones}{wife} = 'wilma';
+ $HoH{flintstones}{wife} = "wilma";
# another element
$HoH{simpsons}{lead} =~ s/(\w)/\u$1/;
# print the whole thing
- foreach my $family ( keys %HoH ) {
+ foreach $family ( keys %HoH ) {
print "$family: { ";
- foreach my $role ( keys %{ $HoH{$family} } ) {
- print "$role = $HoH{$family}{$role} ";
+ for $role ( keys %{ $HoH{$family} } ) {
+ print "$role=$HoH{$family}{$role} ";
}
print "}\n";
}
# print the whole thing somewhat sorted
- foreach my $family ( sort keys %HoH ) {
+ foreach $family ( sort keys %HoH ) {
print "$family: { ";
- foreach my $role ( sort keys %{ $HoH{$family} } ) {
- print "$role = $HoH{$family}{$role} ";
+ for $role ( sort keys %{ $HoH{$family} } ) {
+ print "$role=$HoH{$family}{$role} ";
}
print "}\n";
}
+
# print the whole thing sorted by number of members
- sub num_members {
- keys %{ $HoH{$b} } <=> keys %{ $HoH{$a} }
- }
- foreach my $family ( sort num_members keys %HoH ) {
+ foreach $family ( sort { keys %{$HoH{$b}} <=> keys %{$HoH{$a}} } keys %HoH ) {
print "$family: { ";
- foreach my $role ( sort keys %{ $HoH{$family} } ) {
- print "$role = $HoH{$family}{$role} ";
+ for $role ( sort keys %{ $HoH{$family} } ) {
+ print "$role=$HoH{$family}{$role} ";
}
print "}\n";
}
# establish a sort order (rank) for each role
- my %rank;
- my $i = 0;
- foreach ( qw(lead wife son daughter pal pet) ) {
- $rank{$_} = ++$i;
- }
+ $i = 0;
+ for ( qw(lead wife son daughter pal pet) ) { $rank{$_} = ++$i }
# now print the whole thing sorted by number of members
- sub num_members {
- keys %{ $HoH{$b} } <=> keys %{ $HoH{$a} }
- }
- sub rank {
- $rank{$a} <=> $rank{$b}
- }
-
- foreach my $family ( sort num_members keys %HoH ) {
+ foreach $family ( sort { keys %{ $HoH{$b} } <=> keys %{ $HoH{$a} } } keys %HoH ) {
print "$family: { ";
# and print these according to rank order
- foreach my $role ( sort rank keys %{ $HoH{$family} } ) {
- print "$role = $HoH{$family}{$role} ";
+ for $role ( sort { $rank{$a} <=> $rank{$b} } keys %{ $HoH{$family} } ) {
+ print "$role=$HoH{$family}{$role} ";
}
print "}\n";
}
=head1 MORE ELABORATE RECORDS
+X<record> X<structure> X<struct>
=head2 Declaration of MORE ELABORATE RECORDS
Here's a sample showing how to create and use a record whose fields are of
many different sorts:
- my $rec = {
+ $rec = {
TEXT => $string,
SEQUENCE => [ @old_values ],
LOOKUP => { %some_table },
print $rec->{TEXT};
- print $rec->{SEQUENCE}->[0];
- my $last = pop @{ $rec->{SEQUENCE} };
+ print $rec->{SEQUENCE}[0];
+ $last = pop @ { $rec->{SEQUENCE} };
- print $rec->{LOOKUP}->{key};
- my($first_k, $first_v) = each %{ $rec->{LOOKUP} };
+ print $rec->{LOOKUP}{"key"};
+ ($first_k, $first_v) = each %{ $rec->{LOOKUP} };
- my $answer = $rec->{THATCODE}->($arg);
- my $result = $rec->{THISCODE}->($arg1, $arg2);
+ $answer = $rec->{THATCODE}->($arg);
+ $answer = $rec->{THISCODE}->($arg1, $arg2);
# careful of extra block braces on fh ref
print { $rec->{HANDLE} } "a string\n";
=head2 Declaration of a HASH OF COMPLEX RECORDS
- my %TV = (
+ %TV = (
flintstones => {
- series => 'flintstones',
+ series => "flintstones",
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 => "fred", role => "lead", age => 36, },
+ { name => "wilma", role => "wife", age => 31, },
+ { name => "pebbles", role => "kid", age => 4, },
],
},
jetsons => {
- series => 'jetsons',
+ series => "jetsons",
nights => [ qw(wednesday saturday) ],
members => [
- { name => 'george", role => 'lead', age => 41, },
- { name => 'jane", role => 'wife', age => 39, },
- { name => 'elroy", role => 'kid', age => 9, },
+ { name => "george", role => "lead", age => 41, },
+ { name => "jane", role => "wife", age => 39, },
+ { name => "elroy", role => "kid", age => 9, },
],
},
simpsons => {
- series => 'simpsons',
+ series => "simpsons",
nights => [ qw(monday) ],
members => [
- { name => 'homer', role => 'lead', age => 34, },
- { name => 'marge', role => 'wife', age => 37, },
- { name => 'bart', role => 'kid', age => 11, },
+ { name => "homer", role => "lead", age => 34, },
+ { name => "marge", role => "wife", age => 37, },
+ { name => "bart", role => "kid", age => 11, },
],
},
);
=head2 Generation of a HASH OF COMPLEX RECORDS
-Here's a piece by piece build up of a hash of complex records. We'll
-read in a file that has our data in it.
+ # reading from file
+ # this is most easily done by having the file itself be
+ # in the raw data format as shown above. perl is happy
+ # to parse complex data structures if declared as data, so
+ # sometimes it's easiest to do that
- my %TV = ();
- my $rec = {};
- $rec->{series} = 'flintstones';
+ # here's a piece by piece build up
+ $rec = {};
+ $rec->{series} = "flintstones";
$rec->{nights} = [ find_days() ];
- my @members = ();
+ @members = ();
# assume this file in field=value syntax
- while ( <> ) {
- my %fields = split /[\s=]+/, $_;
+ while (<>) {
+ %fields = split /[\s=]+/;
push @members, { %fields };
}
$rec->{members} = [ @members ];
# now remember the whole thing
$TV{ $rec->{series} } = $rec;
-Now, you might want to make interesting extra fields that
-include pointers back into the same data structure so if
-change one piece, it changes everywhere, like for example
-if you wanted a 'kids' field that was a reference
-to an array of the kids' records without having duplicate
-records and thus update problems.
-
- foreach my $family ( keys %TV ) {
- my $rec = $TV{$family}; # $rec points to $TV{$family}
- my @kids = ();
- foreach my $person ( @{ $rec->{members} } ) {
- if ( $person->{role} =~ /kid|son|daughter/ ) {
+ ###########################################################
+ # now, you might want to make interesting extra fields that
+ # include pointers back into the same data structure so if
+ # change one piece, it changes everywhere, like for example
+ # if you wanted a {kids} field that was a reference
+ # to an array of the kids' records without having duplicate
+ # records and thus update problems.
+ ###########################################################
+ foreach $family (keys %TV) {
+ $rec = $TV{$family}; # temp pointer
+ @kids = ();
+ for $person ( @{ $rec->{members} } ) {
+ if ($person->{role} =~ /kid|son|daughter/) {
push @kids, $person;
}
}
$rec->{kids} = [ @kids ];
}
-You copied the array, but the array itself contains pointers
-to uncopied objects. This means that if you make bart get
-older via
+ # you copied the array, but the array itself contains pointers
+ # to uncopied objects. this means that if you make bart get
+ # older via
$TV{simpsons}{kids}[0]{age}++;
-Then this would also change in C<$TV{simpsons}{members}[2]{age}>
-because C<$TV{simpsons}{kids}[0]> and C<$TV{simpsons}{members}[2]>
-both point to the same underlying anonymous hash table.
+ # then this would also change in
+ print $TV{simpsons}{members}[2]{age};
- # print the whole thing
- foreach my $family ( keys %TV ) {
- print "the $family is on during @{ $TV{$family}{nights} }\n",
- "its members are:\n";
+ # because $TV{simpsons}{kids}[0] and $TV{simpsons}{members}[2]
+ # both point to the same underlying anonymous hash table
- foraech my $who ( @{ $TV{$family}{members} } ) {
+ # print the whole thing
+ foreach $family ( keys %TV ) {
+ print "the $family";
+ print " is on during @{ $TV{$family}{nights} }\n";
+ print "its members are:\n";
+ for $who ( @{ $TV{$family}{members} } ) {
print " $who->{name} ($who->{role}), age $who->{age}\n";
}
-
- print "it turns out that $TV{$family}{lead} has ",
- scalar ( @{ $TV{$family}{kids} } ),
- ' kids named ',
- join(
- ', ',
- map { $_->{name} } @{ $TV{$family}{kids} }
- ),
- "\n";
+ print "it turns out that $TV{$family}{lead} has ";
+ print scalar ( @{ $TV{$family}{kids} } ), " kids named ";
+ print join (", ", map { $_->{name} } @{ $TV{$family}{kids} } );
+ print "\n";
}
=head1 Database Ties
Tom Christiansen <F<tchrist@perl.com>>
-Last update (by Tom):
+Last update:
Wed Oct 23 04:57:50 MET DST 1996
-
-Last update (by Casey West, <F<casey@geeknest.com>>
-Mon Sep 17 13:33:41 EDT 2001
projects / p5sagit/p5-mst-13.2.git / blobdiff