9.6 && and || as Control Structures

These look like punctuation characters or parts of expressions. Can they really be considered control structures? Well, in Perl-think, almost anything is possible, so let's see what we're talking about here.

Often, you run across "if this, then that." We've previously seen these two forms:

if (this) { that; } # one way
that if this;       # another way

Here's a third (and believe it or not, there are still others):

this && that;

Why does this work? Isn't that the logical-and operator? Check out what happens when this takes on each value of true or false:

• If this is true, then the value of the entire expression is still not known, because it depends on the value of that. So that has to be evaluated.

• If this is false, there's no point in looking at that, because the value of the whole expression has to be false. Since there's no point to evaluating that, we might as well skip it.

And in fact, this is what Perl does. Perl evaluates that only when this is true, making it equivalent to the previous two forms.

Likewise, the logical-or works like the unless statement (or unless modifier). So you can replace:

unless (this) { that; }

with:

this || that;

If you're familiar with using these operators in shell programming to control conditional execution commands, you'll see that they work similarly in Perl.

Which one should you use? It depends on your mood, sometimes, or how big each of the expression parts are, or whether you need to parenthesize the expressions because of precedence conflicts. Look at other people's programs, and see what they do. You'll probably see a little of each. Larry suggests that you put the most important part of the expression first, so that it stands out.