9.5 Expression Modifiers

As Yet Another Way to indicate "if this, then that," Perl allows you to tag an if modifier onto an expression that is a standalone statement, like this:

some_expression

 if 

control_expression

;

In this case, control_expression is evaluated first for its truth value (using the same rules as always), and if true, some_expression is evaluated next. This method is roughly equivalent to:

if (

control_expression

) {
        

some_expression

;
}

except that you don't need the extra punctuation, the statement reads backwards, and the expression must be a simple expression (not a block of statements). Many times, however, this inverted description turns out to be the most natural way to state the problem. For example, here's how you can exit from a loop when a certain condition arises:

LINE: while (<STDIN>) {
        

last LINE if /^From: /;
}

See how much easier that is to write? And you can even read it in a normal English way: "last line if it begins with From."

Other parallel forms include the following:

exp2

 

unless 

exp1

;# like: unless (

exp1

) { 

exp2

; }


exp2

 

while 

exp1

; # like: while (

exp1

) { 

exp2

; }


exp2

 

until 

exp1

; # like: until (

exp1

) { 

exp2

; }

All of these forms evaluate exp1 first, and based on that evaluation, do or don't do something with exp2 .

For example, here's how to find the first power of two greater than a given number:

chomp($n = <STDIN>);
$i = 1;                   # initial guess
$i *= 2 until $i > $n;    # iterate until we find it

Once again, we gain some clarity and reduce the clutter.

These forms don't nest: you can't say exp3 while exp2 if exp1 . This restriction is because the form exp2 if exp1 is no longer an expression, but a full-blown statement, and you can't tack one of these modifiers on after a statement.