Perl中对hash的操作

lixiaolong

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 = ();
　　

Initialize (clear, or empty) a hash reference
　　People have asked how to initialize a hash reference (aka hash ref and href). This is the way to go:
　　Solution

    my $hash_ref = {};  # a reference to an empty hash, ref will return HASH
　　The great thing about this is that if before performing an actual assignment, you want to determine (using the ref operator) the type of thingy that a reference is pointing to, you can!... and you can expect it to be a HASH built-in type, because that is what the line above initializes it to be.
　　Note
　　If you treat the variable just as any scalar variable; and use the my declaration alone, or assign a value, ref will return false.

    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";
　　

Create a hash of hashes; via references
　　The following two solutions are equivalent, except for the way the look. In my opinion the second approach is clearer.
　　Solution

    $requiredPatches_href->{ $patch }->{ os }    = $os;
$requiredPatches_href->{ $patch }->{ arch }  = $arch;
$requiredPatches_href->{ $patch }->{ info }  = $info;
　　Solution

    $requiredPatches_href->{ $patch } = {
os    => $os,
arch  => $arch,
info  => $info,
};
　　

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.