Learning Perl, Second Edition

4.4. The for Statement

Another Perl iteration construct is the for statement, which looks suspiciously like C or Java's for statement and works roughly the same way. Here it is:

for ( initial_exp; test_exp; re-init_exp ) {
    statement_1;
    statement_2;
    statement_3;
}

Unraveled into forms we've seen before, this turns out as:

initial_exp;
while (test_exp) {
    statement_1;
    statement_2;
    statement_3;
    re-init_exp;
}

In either case, the initial_exp expression is evaluated first. This expression typically assigns an initial value to an iterator variable, but there are no restrictions on what it can contain; in fact, it may even be empty (doing nothing). Then the test_exp expression is evaluated for truth or falsehood. If the value is true, the body is executed, followed by the re-init_exp (typically, but not solely, used to increment the iterator). Perl then reevaluates the test_exp, repeating as necessary.

This example prints the numbers 1 through 10, each followed by a space:

for ($i = 1; $i <= 10; $i++) {
    print "$i ";
}

Initially, the variable $i is set to 1. Then, this variable is compared with 10, which it is indeed less than or equal to. The body of the loop (the single print statement) is executed, and then the re-init expression (the autoincrement expression $i++) is executed, changing the value in $i to 2. Because this is still less than or equal to 10, we repeat the process until the last iteration where the value of 10 in $i gets changed to 11. This is then no longer less ...