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The if Control Structure

2.7 The if Control Structure

Once you can compare two values, you'll probably want your program to make decisions based upon that comparison. Like all similar languages, Perl has an if control structure:

if ($name gt 'fred') {

 print "'$name' comes after 'fred' in sorted order.\n";

If you need an alternative choice, the else keyword provides that as well:

if ($name gt 'fred') {

 print "'$name' comes after 'fred' in sorted order.\n";

} else {

 print "'$name' does not come after 'fred'.\n";

 print "Maybe it's the same string, in fact.\n";

Unlike in C, those block curly braces are required around the conditional code. It's a good idea to indent the contents of the blocks of code as we show here; that makes it easier to see what's going on. If you're using a programmers' text editor (as discussed in Chapter 1 ), it'll do most of the work for you.

2.7.1 Boolean Values

You may actually use any scalar value as the conditional of the if control structure. That's handy if you want to store a true or false value into a variable, like this:

$is_bigger = $name gt 'fred';

if ($is_bigger) { ... }

But how does Perl decide whether a given value is true or false? Perl doesn't have a separate Boolean data type, like some languages have. Instead, it uses a few simple rules:

1. The special value undef is false. (We'll see this a little later in this section.)

2. Zero is false; all other numbers are true.

3. The empty string ( '' ) is false; all other strings are normally true.

4. The one exception: since numbers and strings are equivalent, the string form of zero, '0' , has the same value as its numeric form: false.

So, if your scalar value is undef , 0 , '' , or '0' , it's false. All other scalars are true—including all of the types of scalars that we haven't told you about yet.

If you need to get the opposite of any Boolean value, use the unary not operator, ! . If what follows it is a true value, it returns false; if what follows is false, it returns true:

if (! $is_bigger) {

 # Do something when $is_bigger is not true

2.8 Getting User Input

At this point, you're probably wondering how to get a value from the keyboard into a Perl program. Here's the simplest way: use the line-input operator, <STDIN> . ^[24] Each time you use <STDIN> in a place where a scalar value is expected, Perl reads the next complete text line from standard input (up to the first newline), and uses that string as the value of <STDIN> . Standard input can mean many things, but unless you do something uncommon, it means the keyboard of the user who invoked your program (probably you). If there's nothing waiting to be read (typically the case, unless you type ahead a complete line), the Perl program will stop and wait for you to enter some characters followed by a newline (return). ^[25]

^[24] This is actually a line-input operator working on the filehandle STDIN , but we can't tell you about that until we get to filehandles (in Chapter 11 ).

^[25] To be honest, it's normally your system that waits for the input; Perl waits for your system. Although the details depend upon your system and its configuration, you can generally correct your mistyping with a backspace key before you press return—your system handles that, not Perl itself. If you need more control over the input, get the Term::ReadLine module from CPAN.

The string value of <STDIN> typically has a newline character on the end of it. ^[26] So you could do something like this:

^[26] The exception is if the standard input stream somehow runs out in the middle of a line. But that's not a proper text file, of course!