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5.4 Hash Functions

Here are some functions for hashes.

5.4.1 The keys Function

The keys( %hashname ) function yields a list of all the current keys in the hash %hashname . In other words, it's like the odd-numbered (first, third, fifth, and so on) elements of the list returned by unwinding %hashname in an array context, and in fact, returns them in that order. If there are no elements to the hash, then keys returns an empty list.

Here's an example using the hash from the previous examples:

$fred{"aaa"} = "bbb";
$fred{234.5} = 456.7;
@list = keys(%fred); # @list gets ("aaa",234.5) or
                     # (234.5,"aaa")

As with all other built-in functions, the parentheses are optional: keys %fred is like keys( %fred ) . For example:

foreach $key (keys %fred) { # once for each key of %fred
        print "at $key we have $fred{$key}\n"; # show key and value
}

This example also shows that individual hash elements can be interpolated into double-quoted strings. You cannot interpolate the entire hash, however.[ 3 ]

[3] Well, you can , using a slice, but we don't talk about slices here.

In a scalar context, the keys function gives the number of elements (key-value pairs) in the hash. For example, you can find out whether a hash is empty:



if (keys(

%somehash

)) { # if keys() not zero:
         ...;           # hash is non empty
}
                        # ... or ...


while (keys(

%somehash

) < 10) {
        ...;         # keep looping while we have less than 10 elements
} 

In fact, merely using %somehash in a scalar context will reveal whether the hash is empty or not:

if (

%somehash

) { # if true, then something's in it 
    # do something with it
}

5.4.2 The values Function

The values( %hashname ) function returns a list of all the current values of the %hashname , in the same order as the keys returned by the keys( %hashname ) function. As always, the parentheses are optional. For example:

%lastname = 

();                 # force %lastname empty
$lastname{"fred"} = "flintstone";
$lastname{"barney"} = "rubble";
@lastnames = values(%lastname); # grab the values

At this point, @lastnames contains either ("flintstone" , "rubble") or ("rubble" , "flintstone") .

5.4.3 The each Function

To iterate over (that is, examine every element of) an entire hash, use keys , looking up each returned key to get the corresponding value. Although this method is frequently used, a more efficient way is to use each( %hashname ) , which returns a key-value pair as a two-element list. On each evaluation of this function for the same hash, the next successive key-value pair is returned until all the elements have been accessed. When there are no more pairs, each returns an empty list.

So, for example, to step through the %lastname hash from the previous example, do something like this:



while (($first,$last) = each(%lastname)) {
        print "The last name of $first is $last\n";
}

Assigning a new value to the entire hash resets the each function to the beginning. Adding or deleting elements of the hash is quite likely to confuse each (and possibly you as well).

5.4.4 The delete Function

So far, with what you know, you can add elements to a hash, but you cannot remove them (other than by assigning a new value to the entire hash). Perl provides the delete function to remove hash elements. The operand of delete is a hash reference, just as if you were merely looking at a particular value. Perl removes the key-value pair from the hash. For example:

%fred = ("aaa","bbb",234.5,34.56); # give %fred two elements
delete $fred{"aaa"};
# %fred is now just one key-value pair










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