4.10 Scalar and List Context

Many Perl operations behave differently depending on the context in which they are used. Perl has scalar context and list context; both are listed in Example 4-8.

Example 4-8. Scalar context and list context

#!/usr/bin/perl -w
# Demonstration of "scalar context" and "list context"

@bases = ('A', 'C', 'G', 'T');

print "@bases\n";

$a = @bases;

print $a, "\n";

($a) = @bases;

print $a, "\n";

exit;

Here's the output of Example 4-8:

A C G T
4
A

First, Example 4-8 declares an array of the four bases. Then the assignment statement tries to assign an array (which is a kind of list) to a scalar variable $a:

$a = @bases;

In this kind of scalar context , an array evaluates to the size of the array, that is, the number of elements in the array. The scalar context is supplied by the scalar variable on the left side of the statement.

Next, Example 4-8 tries to assign an array (to repeat, a kind of list) to another list, in this case, having just one variable, $a:

($a) = @bases;

In this kind of list context , an array evaluates to a list of its elements. The list context is supplied by the list in parentheses on the left side of the statement. If there aren't enough variables on the left side to assign to, only part of the array gets assigned to variables. This behavior of Perl pops up in many situations; by design, many features of Perl behave differently depending on whether they are in scalar or list context. See Appendix B for more about scalar and list content.

Now you've seen the use of strings and arrays to hold sequence and file data, and learned the basic syntax of Perl, including variables, assignment, printing, and reading files. You've transcribed DNA to RNA and calculated the reverse complement of a strand of DNA. By the end of Chapter 5, you'll have covered the essentials of Perl programming.