Perl Cookbook (2024)

You need to know whether a hash has a particular key, regardless ofany possible associated value.

Use the exists function.

# does %HASH have a value for $KEY ?if (exists($HASH{$KEY})) { # it exists} else { # it doesn't}

This code uses exists to check whether a key is inthe %food_color hash:

# %food_color per the introductionforeach $name ("Banana", "Martini") { if (exists $food_color{$name}) { print "$name is a food.\n"; } else { print "$name is a drink.\n"; }} Banana is a food. Martini is a drink.

The exists function tests whether a key is in thehash. It doesn’t test whether the value corresponding to thatkey is defined, nor whether the value is true or false. We may besplitting hairs, but problems caused by confusing existence,definedness, and truth can multiply like rabbits. Take this code:

%age = ();$age{"Toddler"} = 3;$age{"Unborn"} = 0;$age{"Phantasm"} = undef;foreach $thing ("Toddler", "Unborn", "Phantasm", "Relic") { print "$thing: "; print "Exists " if exists $age{$thing}; print "Defined " if defined $age{$thing}; print "True " if $age{$thing}; print "\n";} Toddler: Exists Defined True  Unborn: Exists Defined  Phantasm: Exists  Relic: 

$age{"Toddler"} passes the existence, definedness,and truth tests. It exists because we gave"Toddler" a value in the hash, it’s definedbecause that value isn’t undef, andit’s true because the value isn’t one of Perl’sfalse values.

$age{"Unborn"} passes ...