Learning Perl, 5th Edition

List Literals

A list (the way you represent a list value within your program) is a list of comma-separated values enclosed in parentheses. These values form the elements of the list. For example:

(1, 2, 3)      # list of three values 1, 2, and 3
(1, 2, 3,)     # the same three values (the trailing comma is ignored)
("fred", 4.5)  # two values, "fred" and 4.5
( )            # empty list - zero elements
(1..100)       # list of 100 integers

That last one uses the .. range operator, which is seen here for the first time. That operator creates a list of values by counting from the left scalar up to the right scalar by ones. For example:

(1..5)            # same as (1, 2, 3, 4, 5)
(1.7..5.7)        # same thing - both values are truncated
(5..1)            # empty list - .. only counts "uphill"
(0, 2..6, 10, 12) # same as (0, 2, 3, 4, 5, 6, 10, 12)
($m..$n)          # range determined by current values of $m and $n
(0..$#rocks)      # the indices of the rocks array from the previous section

As you can see from those last two items, the elements of a list literal are not necessarily constants—they can be expressions that will be newly evaluated each time the literal is used. For example:

($m, 17)       # two values: the current value of $m, and 17
($m+$o, $p+$q) # two values

Of course, a list may have any scalar values, like this typical list of strings:

("fred", "barney", "betty", "wilma", "dino")

The qw Shortcut

It turns out that lists of simple words (like the previous example) are frequently needed in Perl programs. The qw shortcut makes it easy to generate them without ...