Array variables

A slightly more interesting kind of variable is the array variable which is a list of scalars (ie numbers and strings). Array variables have the same format as scalar variables except that they are prefixed by an @ symbol. The statement

@food  = ("apples", "pears", "eels");
@music = ("whistle", "flute");

assigns a three element list to the array variable @food and a two element list to the array variable @music.

The array is accessed by using indices starting from 0, and square brackets are used to specify the index. The expression

$food[2]

returns eels. Notice that the @ has changed to a $ because eels is a scalar.

Array assignments

As in all of Perl, the same expression in a different context can produce a different result. The first assignment below explodes the @music variable so that it is equivalent to the second assignment.

@moremusic = ("organ", @music, "harp");
@moremusic = ("organ", "whistle", "flute", "harp");

This should suggest a way of adding elements to an array. A neater way of adding elements is to use the statement

push(@food, "eggs");

which pushes eggs onto the end of the array @food. To push two or more items onto the array use one of the following forms:

push(@food, "eggs", "lard");
push(@food, ("eggs", "lard"));
push(@food, @morefood);

The push function returns the length of the new list.

To remove the last item from a list and return it use the pop function. From our original list the pop function returns eels and @food now has two elements:

$grub = pop(@food);	# Now $grub = "eels"

It is also possible to assign an array to a scalar variable. As usual context is important. The line

$f = @food;

assigns the length of @food, but

$f = "@food";

turns the list into a string with a space between each element. This space can be replaced by any other string by changing the value of the special $" variable. This variable is just one of Perl's many special variables, most of which have odd names.

Arrays can also be used to make multiple assignments to scalar variables:

($a, $b) = ($c, $d);		# Same as $a=$c; $b=$d;
($a, $b) = @food;		# $a and $b are the first two
				# items of @food.
($a, @somefood) = @food;	# $a is the first item of @food
				# @somefood is a list of the
				# others.
(@somefood, $a) = @food;	# @somefood is @food and
				# $a is undefined.

The last assignment occurs because arrays are greedy, and @somefood will swallow up as much of @food as it can. Therefore that form is best avoided.

Finally, you may want to find the index of the last element of a list. To do this for the @food array use the expression

$#food

Displaying arrays

Since context is important, it shouldn't be too surprising that the following all produce different results:

print @food;	# By itself
print "@food";	# Embedded in double quotes
print @food."";	# In a scalar context

Exercise

Try out each of the above three print statements to see what they do.