Perl for Web Site Management

if Statements

The next section of the script introduces another common programming construct: the if statement. An if statement is similar to the foreach loop we just looked at, in that it allows you to do something special with a block of code. An if statement allows you to make execution of the code conditional upon the outcome of a test.

Another term for an if statement is a conditional statement. Conditional statements in Perl look like this:

if (test) {
    do something...
}

Other, fancier versions of Perl’s conditional statements look like this:

if (test) {
    do something...
} else {
    do something else...
}

or this:

if (test) {
    do something...
} elsif (another test) {
    do something else...
} elsif (yet another test) {
    do some other something else...
} else {
    do still some other something else...
}

and so on. The trickiest part for beginners (besides getting all those curly braces in the right place) is understanding how Perl evaluates those (test) statements for “truth.” To understand that, see True or False?.

True or False?

The basic rule Perl uses for figuring out whether the logical test inside the parentheses at the beginning of an if block is true is as follows.

If the stuff inside the parentheses evaluates to (in programmer-speak, returns) the number 0, or the string "0", or the empty string (which you’ll sometimes see written as ''), or an undefined value, it’s false. In all other cases, it’s true.

If you put a scalar variable inside the parentheses, it just returns whatever ...