How to use redo to restart the current iteration in Perl?

In Perl, the redo statement is a lesser-known but very useful control-flow keyword for loops. It allows you to restart the current iteration of a loop from the beginning, skipping any loop control checks or expressions (like increment steps or condition tests). This can be handy when you want to re-execute the loop body for the same iteration without advancing to the next one.

Understanding redo in Perl Loops

The redo keyword causes the current iteration of a for, foreach, while, or until loop to immediately restart from the beginning of the loop block. Unlike next, last, or goto, redo does not evaluate the loop's conditional expression or execute the loop's increment step.

• redo; restarts the current iteration without condition checks.
• next; skips to the next iteration (evaluates condition and increment).
• last; exits the loop completely.

Since redo jumps back to the start of the loop block itself, it can be used to "retry" an iteration if some condition inside the loop is not met, for example, prompting for input again or reprocessing data.

Key points regarding redo

• Does not re-evaluate the loop's conditional or increment expressions.
• Does not change the loop variable or iteration count.
• Used mainly inside loops where you might need to "repeat" work on the current item.

Example of Using redo to Restart an Iteration

Below is a self-contained Perl script that demonstrates redo inside a foreach loop. It simulates processing a list of items, and if an item doesn't meet a certain condition (here, it must be an even number), the iteration is restarted after modifying the item:

use strict;
use warnings;

my @numbers = (1, 2, 3, 4, 5);

foreach my $num (@numbers) {
    print "Processing number: $num\n";
    
    # If the number is not even, increment and redo the iteration
    if ($num % 2 != 0) {
        print "  Number $num is not even. Incrementing and retrying...\n";
        $num++;
        redo;  # Restart this iteration with the updated $num
    }
    
    print "  Final even number: $num\n";
}

print "Done processing all numbers.\n";

Explanation:

• We loop over @numbers, printing each value.
• If the current number is odd, we increment it and use redo to restart the loop body with the updated number.
• The loop variable $num is modified directly (thanks to aliasing in foreach loops).
• When the modified number is even, the loop prints and advances to the next item.

This example highlights how redo lets you retry processing the same iteration with updated data, without moving forward or reevaluating the loop condition.

Common Gotchas

• Using redo inside for/foreach loops modifies the loop variable directly because it aliases the iterated value. This can be useful or surprising.
• redo does not evaluate the loop's conditional test again, which may cause infinite loops if the termination condition depends on external variables changed during the "redo".
• Do not confuse redo with next — next moves to the next iteration, redo retries the current iteration without increment.

Summary

Use redo to restart the current iteration of a loop if you want to re-run the loop body without going to the next iteration or re-evaluating the loop condition. It’s a powerful tool for loop control but should be used with care to avoid infinite loops.