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 statement 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. Because you don't have to evaluate that , we might as well skip it.

And in fact, Perl does just that. Perl evaluates that only when this is true, making the form equivalent to the previous two examples.

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

unless (

this

) { 

that

; }

with

this



 || 

that

;

Finally, the C-like ternary operator:

exp1

 ? 

exp2

 : 

exp3

;

evaluates to exp2 if exp1 is true, and to exp3 in all other cases. You might have used:

if (

exp1

) { 

exp2

; } else { 

exp3

; }

but you could have eliminated all of that punctuation. For example, you could write:

($a < 10) ? ($b = $a) : ($a = $b);

Which one should you use? Your choice depends on your mood, sometimes, or on how big each of the expression parts are, or on 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.