Полезная информация

Learning Perl

Learning PerlSearch this book
Previous: 3.1 What Is a List or Array?Chapter 3
Arrays and List Data
Next: 3.3 Variables

3.2 Literal Representation

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

(1,2,3)             # array of three values 1, 2, and 3
("fred",4.5)        # two values, "fred" and 4.5

The elements of a list are not necessarily constants; they can be expressions that will be reevaluated each time the literal is used. For example:

($a,17);            # two values: the current value of $a, and 17
($b+$c,$d+$e)       # two values

The empty list (one of no elements) is represented by an empty pair of parentheses:

() # the empty list (zero elements)

An item of the list literal can include the list constructor operator, indicated by two scalar values separated by two consecutive periods. This operator creates a list of values starting at the left scalar value up through the right scalar value, incrementing by one each time. For example:

(1 .. 5)            # same as (1, 2, 3, 4, 5)
(1.2 .. 5.2)        # same as (1.2, 2.2, 3.2, 4.2, 5.2)
(2 .. 6,10,12)      # same as (2,3,4,5,6,10,12)
($a .. $b)          # range determined by current values of $a and $b

Having the right scalar less than the left scalar results in an empty list; you can't count down by switching the order of the values. If the final value is not a whole number of steps above the initial value, the list stops just before the next value would have been outside the range:

(1.3 .. 6.1) # same as (1.3,2.3,3.3,4.3,5.3)

List literals with lots of short text strings start to look pretty noisy with all the quotes and commas:

@a = ("fred","barney","betty","wilma"); # ugh!

So there's a shortcut: the "quote word" function, which creates a list from the nonwhitespace parts between the parentheses:[1]

@a = qw(fred barney betty wilma); # better!
@a = qw(
);                                # same thing

[1] Actually, like the pattern-matching functions we'll learn about later, you could use any nonwhitespace, nonalphanumeric character as the delimiter instead of parentheses.

One use of a list literal is as arguments to the print function introduced earlier. Elements of the list are printed out without any intervening whitespace:

print("The answer is ",@a,"\n");

This statement prints The answer is followed by a space, the value of @a, and a newline. Stay tuned for other uses for list literals.

Previous: 3.1 What Is a List or Array?Learning PerlNext: 3.3 Variables
3.1 What Is a List or Array?Book Index3.3 Variables