Perl 6 and Parrot Essentials, 2nd Edition

Control Structures

The simplest flow of control is linear—one statement follows the next in a straight line to the end of the program. Since this is far too limiting for most development tasks, languages provide ways to alter the control flow.

Selection

Selection executes one set of actions out of many possible sets. The selection control structures are if, unless, and given/when.

The if statement

The if statement checks a condition and executes its associated block only if that condition is true. The condition can be any expression that evaluates to a truth value. Parentheses around the condition are optional:

if $blue {
    print "True Blue.";
}

The if statement can also have an unlimited number of elsif statements that check additional conditions when the preceding conditions are false. The final else statement executes if all preceding if and elsif conditions are false:

if $blue {
    print "True Blue.";
} elsif $green {
    print "Green, green, green they say . . . ";
} else {
    print "Colorless green ideas sleep furiously.";
}

The unless statement

The unless statement is the logical opposite of if. Its block executes only when the tested condition is false:

unless $fire {
    print "All's well.";
}

There is no elsunless statement, though else works with unless.

The switch statement

The switch statement selects an action by comparing a given expression (the switch) to a series of when statements (the cases). When a case matches the switch, its block is executed:

given $bugblatter { when Beast::Trall ...