출처
http://www.cs.mcgill.ca/~abatko/computers/programming/perl/howto/hash/
- Perl Hash Howto
- Initialize (clear, or empty) a hash
- Add a key/value pair to a hash
- Add several key/value pairs to a hash
- Copy a hash
- Delete a single key/value pair
- Perform an action on each key/value pair in a hash
- Get the size of a hash
- Use hash references
- Function to build a hash of hashes; return a reference
- Access and print a reference to a hash of hashes
- Function to build a hash of hashes of hashes; return a reference
- Access and print a reference to a hash of hashes of hashes
- Print the keys and values of a hash, given a hash reference
- Determine whether a hash value exists, is defined, or is true
- AUTHOR
- SEE ALSO
Perl Hash Howto
This how-to comes with no guaratees other than the fact that these code segments were copy/pasted from code that I wrote and ran successfully.
Initialize (clear, or empty) a hash
Assigning an empty list is the fastest method.
Solution
my %hash = ();
Note
People have asked how to initialize a hash reference (aka hash ref). It's just like any scalar variable; you can use my alone, or you can assign a value.
my $hash_ref; my $hash_ref = 0; # zero
Add a key/value pair to a hash
In the solutions below, quotes around the keys can be omitted when the keys are identifiers.
Hash:
Solution
$hash{ 'key' } = 'value'; # hash $hash{ $key } = $value; # hash, using variables
Hash reference:
Solution
$href->{ 'key' } = 'value'; # hash ref $href->{ $key } = $value; # hash ref, using variables
Add several key/value pairs to a hash
Solution
The following statements are equivalent, though the second one is more readable:
%hash = ( 'key1', 'value1', 'key2', 'value2', 'key3', 'value3' ); %hash = ( key1 => 'value1', key2 => 'value2', key3 => 'value3', );
Copy a hash
Solution
my %hash_copy = %hash; # copy a hash my $href_copy = $href; # copy a hash ref
Delete a single key/value pair
The solution differs for a hash and a hash reference, but both cases can use the delete function.
Solution
Hash:
delete $hash{$key};
Hash reference:
delete $hash_ref->{$key};
Perform an action on each key/value pair in a hash
The actions below print the key/value pairs.
Solution
Use each within a while loop. Note that each iterates over entries in an apparently random order, but that order is guaranteed to be the same for the functions keys and values.
while ( my ($key, $value) = each(%hash) ) { print "$key => $value\n"; }
A hash reference would be only slightly different:
while ( my ($key, $value) = each(%$hash_ref) ) { print "$key => $value\n"; }
Solution
Use keys with a for loop.
for my $key ( keys %hash ) { my $value = $hash{$key}; print "$key => $value\n"; }
Example
my $file = $ARGV[0] || "-"; my %from = (); open FILE, "< $file" or die "Can't open $file : $!"; while( <FILE> ) { if (/^From: (.*)/) { $from{$1}++ } # count recurrences of sender } close FILE; for my $sender ( sort keys %from ) { print "$sender: $from{$sender}\n"; }
Get the size of a hash
Solution
print "size of hash: " . keys( %hash ) . ".\n";
Solution
my $i = 0; $i += scalar keys %$hash_ref; # method 1: explicit scalar context $i += keys %$hash_ref; # method 2: implicit scalar context
Use hash references
Solution
sub foo { my $hash_ref; $hash_ref->{ 'key1' } = 'value1'; $hash_ref->{ 'key2' } = 'value2'; $hash_ref->{ 'key3' } = 'value3'; return $hash_ref; } my $hash_ref = foo(); print "the keys... ", sort keys %$hash_ref, "...\n";
Function to build a hash of hashes; return a reference
Solution
sub foo { my ( $login, $p, $uid, $gid, $gecos, $dir, $s ); my %HoH = (); my $file = '/etc/passwd'; open( PASSWD, "< $file" ) or die "Can't open $file : $!"; while( <PASSWD> ) { ( $login, $p, $uid, $gid, $gecos, $dir, $s ) = split( ':' ); $HoH{ $login }{ 'uid' } = $uid; $HoH{ $login }{ 'gid' } = $gid; $HoH{ $login }{ 'dir' } = $dir; } close PASSWD; return \%HoH; }
Access and print a reference to a hash of hashes
Solution
my $rHoH = foo(); my( $uid, $gid, $dir ); for my $login ( keys %$rHoH ) { $uid = $rHoH->{ $login }->{ 'uid' }; # method 1 most readable $gid = ${ $rHoH->{ $login } }{ 'gid' }; # method 2 $dir = ${ ${ $rHoH }{ $login } }{ 'dir' }; # method 3 least readable print "uid: $uid, gid: $gid, dir, $dir.\n"; }
Solution
my $rHoH = foo(); for my $k1 ( sort keys %$rHoH ) { print "k1: $k1\n"; for my $k2 ( keys %{$rHoH->{ $k1 }} ) { print "k2: $k2 $rHoH->{ $k1 }{ $k2 }\n"; } }
Function to build a hash of hashes of hashes; return a reference
Solution
sub foo { my %HoHoH = (); while( ... ) { if( /LOCATION:/ ) { ... } elsif( /MODULE:/ ) { $HoHoH{ $loc }{ $module_type }{ MODULE_NAME } = $module_name; } elsif( $ARGS_ALLOWED ) { $HoHoH{ $loc }{ $module_type }{ $arg_name } = $arg_value; } } return \%HoHoH; }
Access and print a reference to a hash of hashes of hashes
Solution
my $rHoHoH = foo(); for my $k1 ( sort keys %$rHoHoH ) { print "$k1\n"; for my $k2 ( sort keys %{$rHoHoH->{ $k1 }} ) { print "\t$k2\n"; for my $k3 ( sort keys %{$rHoHoH->{ $k1 }->{ $k2 }} ) { print "\t\t$k3 => $rHoHoH->{ $k1 }->{ $k2 }->{ $k3 }\n"; } } }
Print the keys and values of a hash, given a hash reference
Solution
while( my ($k, $v) = each %$hash_ref ) { print "key: $k, value: $v.\n"; }
Determine whether a hash value exists, is defined, or is true
Solution
print "Value EXISTS, but may be undefined.\n" if exists $hash{ $key }; print "Value is DEFINED, but may be false.\n" if defined $hash{ $key }; print "Value is TRUE at hash key $key.\n" if $hash{ $key };
Example
Let's say we execute an sql query where some of the resulting values may be NULL. Before attempting to use any of the values we should first check whether they are defined, as in the following code. Note that the subroutine sql_fetch_hashref()
takes care of connecting to the database, preparing the statement, executing it, and returning the resulting row as a hash reference using DBI's fetchrow_hashref()
method.
my $answers = 'a,b,c,d,e'; my $sql = "select max_time, $answers from questions " . 'where question_number=?'; my $hash_ref = sql_fetch_hashref( $sql, $q ); my @answers = split ',', $answers; my $max_time = $hash_ref->{max_time} || '60'; my $hash_ref_ans; for my $letter ( @answers ) { $hash_ref_ans->{ $letter } = $hash_ref->{ $letter } if defined $hash_ref->{ $letter }; }
The for loop made a new hash of only defined key/value pairs.
AUTHOR
Alex BATKO <abatko AT cs.mcgill.ca>
Thanks to all those who have written with suggestions and comments.
SEE ALSO
http://www.cs.mcgill.ca/~abatko/computers/programming/perl/howto/
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