Programming Perl, 4th Edition

Unary Arithmetic Operators

As if $variable += 1 weren’t short enough, Perl borrows from C an even shorter way to increment a variable. The autoincrement (and autodecrement) operators simply add (or subtract) one from the value of the variable. They can be placed on either side of the variable, depending on when you want them to be evaluated; see Table 1-3.

Table 1-3. Increment operators

Example	Name	Result
++$a, $a++	Autoincrement	Add 1 to $a
––$a, $a––	Autodecrement	Subtract 1 from $a

If you place one of these “auto” operators before the variable, it is known as a preincremented (predecremented) variable. Its value will be changed before it is referenced. If it is placed after the variable, it is known as a postincremented (postdecremented) variable, and its value is changed after it is used. For example:

$a = 5;        # $a is assigned 5
$b = ++$a;     # $b is assigned the incremented value of $a, 6
$c = $a––;     # $c is assigned 6, then $a is decremented to 5

Line 19 of our Average Example increments the number of scores by one so that we’ll know how many scores we’re averaging. It uses a postincrement operator ($scores++), but in this case it doesn’t matter since the expression is in void context, which is just a funny way of saying that the expression is being evaluated only for the side effect of incrementing the variable. The value returned is being thrown away.^[26]

^[26] The optimizer will notice this and optimize the postincrement into a preincrement, because that’s a bit faster to execute. (You didn’t ...