Arrays and slices


You should now be able to create very simple scripts that can take input from the keyboard, assign it to scalar variables, and echo it back to the screen with print. Time for some technicalities.

Perl is often (and wrongly!) termed a weakly typed language. For those of you coming from bondage and discipline languages like C or Java, this means Perl does not require you to explicitly point out what kind of variable your variables are: floats, doubles, integers, characters, booleans, strings, etc.

For those of you who are learning programming here, this means you don’t have to worry about what you put into a variable like $name, in particular, you don’t need to worry if its a little integral number, a big fat floating point number, a single character, a string, or anything else, and you are free to assign different values, and different kinds of value dynamically as the program runs.

Perl does have data types though. However, what perl worries about is not the distinctions a computer might like to make (in terms of how big a chunk of data is in memory), but the distinction between singular and plural, much like many human languages. In Perl, singular data is called scalar data, and comes in variables starting with a $. Plural data is called array (or list) data, and is stored in variables starting with a @.


Scalar data is $ingular data. Scalar variables always start with a $ for $calar, and can contain one of pretty much anything. $name could contain an integer (2), a floating point number (12.045e+89), a character ("K") or a string ("or Pretty much anything else"). They can even contain pointers to other sorts of data (“references”, but we’re getting ahead of ourselves). To create a scalar variable, you just write:

$string = "Some string or other";
$number = 12453;

You don’t need to, and indeed should not, put quotes around numbers.


In contrast to scalar data, array data is plur@l data. Array variables start with a @ for @rray, and store a list of scalars. Creating an array could look like any of the following:

@numbers  = ( 1, 3.05, 4, 2e-10, 23 );
@strings  = ( "Hello", 'everybody', "I'm Dr. Nick Riviera\n" );
@allsorts = ( 12.5, 'plop', "some tabs\t\t\t\t", $chocolate, 56, "C" );

When you create an array, you need to put the (suitably quoted, scalar) values in a list between parentheses, separated by commas, i.e. arrays are created like this:

@array = ( $scalar1, $scalar2, $scalar3 );

To access the individual members of an array, imaginatively called @array, we need to use square brackets. The syntax is:

@array = ( 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22 );
$the_9th_member_of_the_array = $array[8];
print $the_9th_member_of_the_array;

And $the_9th_member_of_the_array will print out ’16’. There are two gotchas here.

Firstly, what has happened to the @ of @array?

And secondly, why 8, not 9?

The $ and @ ‘sigils’ aren’t really part of the name of the variable. They are more a way of telling perl whether the thing you’re talking about is a single thing or a list of things. When you create an array, you use a @ sign, because you want  perl to put a list into it. However, when you want to access an individual member of an array, you’re trying to get a singular piece of data, a scalar, so this gets an $.

This syntax is a little weird, but you’re currently stuck with it: when you’re talking about a list of data, you need a @, as in @array = ( blah... ); but to access a single bit of array data, you need to a $, as in $array[index_number];

Which brings me to the second point. In Perl, arrays index from the number 0, so the 9th element in an array is at index number 8: $array[8]. If you just put your brain into perl/C gear and think of the 1st element of an array as being the ‘zeroth’, 0th, member, you may find this easier. So:

@beans = ( "adzuki", "haricot", "mung" );
    # anything following a '#' is a comment
    # perl will ignore everything from the '#' to the end of the line
print "$beans[0]\n";  # this is adzuki
print "$beans[1]\n";  # this is haricot
print "$beans[2]\n";  # this is mung
print "$beans[-1]\n"; # this is also mung

Negative indices are counted back from the end of the array, so the last item is the minus-1th.


Another useful way to access pieces of arrays is to take slices of them:

@cake = ( "flour", "eggs", "milk", "sugar", "butter", "sultanas", "water" );
@slice_of_cake = @cake[ 3, 6 ];
print "@slice_of_cake\n";
sugar water

@slice_of_cake will contain the list ( “sugar”, “water” ). Note that the slice syntax @slice_of_cake needs a @, because you are creating a list. Likewise, @cake also gets a @, because you’re creating plur@l data, not a $ingle piece of data. Slices are useful for getting at multiple bits of data simultaneously, without tediously having to write single accesses for each bit. You can also assign things to a slice:

@array = ( "hello", "everybody", "I'm", "Dr.", "Nick", "Riviera" );
print "@array\n";
@array[ 3, 4, 5 ] = ( "a", "complete", "charlatan" );
print "@array\n";

You can see from the previous few scripts that, like scalar variables, arrays also interpolate their values when put in double quotes. Furthermore, the items will be helpfully padded with spaces too.

Next up…hashes.

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