Clever Geek Handbook

Updated: Initializing and clearing multiple hashes in one line

How to initialize and clear multiple hashes on one line.

Example:

my %hash1 = ();
my %hash2 = ();
my %hash3 = ();
my %hash4 = ();

to

my ( %hash1, %hash2, %hash3, %hash4 ) = ?
+3


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4 answers


It looks like (from your comments) that you really want to empty hashes that already have things in them. You can do it like this:

(%hash1,%hash2,%hash3) = ();

Complete example:

use strict;
use warnings;

my %hash1 = ('foo' => 1);
my %hash2 = ('bar' => 1);
my %hash3 = ('baz' => 1);

(%hash1,%hash2,%hash3) = ();

print (%hash1,%hash2,%hash3);

A variable declaration always gives an empty variable, so there is no need to set it to empty. This is true even in a loop:

for (0..100)
{
    my $x;
    $x++;
    print $x;
}


This will print 1

over and over; even though you might expect it $x

to retain its value, it is not.

Explanation: Perl allows you to assign a list, eg ($foo,$bar) = (1,2)

. If the list on the right is shorter, all other items get undef

. So assigning an empty list to a list of variables makes them all undefined.

Another useful way to set a bunch of things is with the operator x

:

my ($x,$y,$z) = (100)x3;

This sets all three variables to 100. It doesn't work that well for hashes, because everyone needs a list assigned to it.

+5


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It's as easy as doing

my ( %hash1, %hash2, %hash3, %hash4 );

and they won't contain any keys or values ​​at that point.



The same technique applies to scalars and arrays.

For undef

multiple hashes, you could do

undef %$_ for ( \%hash1, \%hash2 );
+5


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You can initialize it like:

my% hash1 =% hash2 =% hash3 =% hash4 = ();

0


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You don't need to assign anything to new to make sure it's empty. All variables are empty if nothing is assigned to them.

my %hash;  # hash contains nothing
%hash = () # hash still contains nothing

The only time it would be useful to assign an empty list to a hash is if you want to remove previously assigned values. And even then it would only be a useful task if it couldn't be solved by applying the correct size limit for the hash.

my (%hash1, %hash2);
while (something) {
    # some code
    (%hash1,%hash2) = ();   # delete old values
}

Emptying hashes. Better written as:

while (something) {
    my (%hash1, %hash2);    # all values are now local to the while loop
    # some code
}
0


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