Deleting from a Hash

Problem

You want to remove an entry from a hash so that it doesn't show up with keys, values, or each. If you were using a hash to associate salaries with employees, and an employee resigned, you'd want to remove their entry from the hash.

Solution

Use the delete function:

# remove $KEY and its value from %HASH delete($HASH{$KEY});

Discussion

Sometimes people mistakenly try to use undef to remove an entry from a hash. undef $hash{$key} and $hash{$key} = undef both make %hash have an entry with key $key and value undef.

The delete function is the only way to remove a specific entry from a hash. Once you've deleted a key, it no longer shows up in a keys list or an each iteration, and exists will return false for that key.

This demonstrates the difference between undef and delete:

# %food_color as per Introduction sub print_foods {
 my @foods = keys %food_color; my $food;
print "Keys: @foods\n";
print "Values: "; foreach $food (@foods) {
 my $color = $food_color{$food};
 if (defined $color) {
 print "$color ";
}
else {
 print "(undef) ";
}
} print "\n";
}
print "Initially:\n"; print_foods(); print "\nWith Banana undef\n"; undef $food_color{"Banana"};
 print_foods(); print "\nWith Banana deleted\n"; delete $food_color{"Banana"};
 print_foods(); Initially: Keys: Banana Apple Carrot Lemon Values: yellow red orange yellow  With Banana undef Keys: Banana Apple Carrot Lemon Values: (undef) red orange yellow  With Banana deleted Keys: Apple Carrot Lemon Values: red orange yellow

As you see, if we set $food_color{"Banana"} to undef, "Banana" still shows up as a key in the hash. The entry is still there; we only succeeded in making the value undef. On the other hand, delete actually removed it from the hash - "Banana" is no longer in the list returned by keys.

delete can also take a hash slice, deleting all listed keys at once:

delete @food_color{"Banana", "Apple", "Cabbage"};