How to create and use a hash in Perl

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A hash, or more technically an associative array, assigns a key to a value. For example, let's say you have the following key:value pairs.

Key Value
Name Jeremy Canfield
ID 123456
Occupation Engineer

 

In this example, a variable named %hash is created, and the %hash variable contains the key:value pairs.

my %hash = ('name' => Jeremy, 'id' => 123456, 'occupation' => Engineer);

 


Print

You can print the contents of the entire hash.

print %hash;

 

print %hash will print all of the keys and values in the hash, like this.

Name Jeremy Canfield ID 123456 Occupation Engineer

 

Or, you can print the value of a key. For example, the following will print the value "Jeremy Canfield in the "name" key.

print $hash{name};

 


Start with an empty hash

Following are both valid examples of how to create an empty hash (does not contain any key value pairs).

my %hash;
my %hash = ();

 

Then, somewhere in the script key:value pairs will be created. In this example, a key called "name" is created with a value of "Jeremy" and another key called "occupation" with a value of "engineer".

$hash{name}       = "Jeremy";
$hash{occupation} = "Engineer";

 

You can then print the values associated with a hash key.

print $hash{name};
print $hash{id};
print $hash{occupation};

 


Variables

You can create a variable for one or more of the key.

$username       = $hash{name};
$userid         = $hash{id};
$useroccupation = $hash{occupation};

 

You can then print the value's associated with each key.

print $username;
print $userid;
print $useroccupation;

 


Arrays

Each hash key will contain a single value, which is to say that a hash key cannot contain multiple values. For example, if you were to do something like this,  $hash{name} will only contain a value of Winston.

$hash{name} = "Jeremy";
$hash{name} = "Winston";

 

A solution to this change is to create an array that contains multiple values. In this example, the @{$hash{name}} array will contain both Jeremy and Winston.

push ( @{$hash{name}}, Jeremy );
push ( @{$hash{name}}, Winston );

 

You can then use a foreach loop to print all of the values in the array.

foreach my $key (keys %hash) {
    print @{$hash{$key}};
}


 


Loop through keys

You will usually want to loop through the hash. The following foreach loop will print the keys.

foreach (keys %hash) {
    print $_;
}

 

The following foreach loop will print the value associated with each key.

foreach (keys %hash) {
    print $hash{$_};
}

 


Loop through values

The following foreach loop with print the values.

foreach (values %hash) {
    print $_;
}

 

This will print the values.

Jeremy Canfield
123456
Engineer

 



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