Control Loops in PERL

There may be a situation when you need to execute a block of code several times. Perl provide various control structures that allow for more complicated execution paths.

Perl provides the following types of loop to handle the looping requirements.

while loop

This loop is used to repeat a statement or group of statements while a given condition is true. It tests the condition before entering the block.

#!/usr/bin/perl my $counter = 10; while($counter > 0){ print("$counter\n"); $counter--; }

#!/usr/bin/perl my $num; my @numbers = (); print "Enter numbers, each per line :\n"; print "ctrl-z (windows) or ctrl-d(Linux) to exit\n>"; while(my $input = <>) { print(">"); chomp $input; $num = int($input); push(@numbers, $num); } print "You entered: @numbers\n";

until loop

This loop is also used to repeat a statement or group of statements until a given condition becomes true. It tests the condition before entering the block.

#!/usr/bin/perl my $counter = 5; until($counter == 0){ print("$counter \n"); $counter--; }

#!/usr/bin/perl my $cmd; print("Enter a command, enter exit to quit.\n"); do { print(">"); chomp($cmd = <STDIN>); $cmd = lc($cmd); print("You entered:$cmd\n"); }until($cmd eq "exit");

for loop

This loop also executes a sequence of statements multiple times and abbreviates the code that manages the loop variable.

#!/usr/bin/perl my @a = (1..9); for my $i (@a){ print("$i","\n"); }

#!/usr/bin/perl my @c = (1..6); for(my $i = 0; $i <= $#c; $i++){ print("$c[$i] \n"); }

foreach loop

The foreach loop iterates over a normal list value and sets the variable to be each element of the list in turn.

#!/usr/bin/perl my @a = (1..9); foreach(@a){ print("$_","\n"); }

do...while loop

Like a while statement, except that it tests the condition at the end of the loop body

#!/usr/bin/perl my $command; print("Enter a command, bye to quit.\n"); do { print(">"); chomp($command = <STDIN>); $command = lc($command); # display the command print("$command\n"); }while($command ne "bye");

Loop Control Statements

Different loop control statements used in Perl are discussed below.

next statement

It is used to skip the remaining lines of it's enclosing loop and moves the control to the next iteration immediately.

#!/usr/bin/perl use warnings; use strict; my @data = (1,4,3,2,5,6,8,7,9); my $key = 0; do{ print "Enter a number to search(1-9):\n"; $key = int(<STDIN>); }until($key >= 1 && $key <= 9); my $pos = -1; for my $i (@data){ $pos++; next if($i != $key); print("Found number $key at position $pos\n"); }

last statement

It terminates the loop statement and transfers execution to the statement immediately following the loop.

#!/usr/bin/perl use warnings; use strict; my ($key, $value); my %h = ("apple" => 1, "orange" => 2, "mango" => 3, "coconut" => 4); print("Please enter a key to search:\n"); $key = <STDIN>; chomp($key); $value = 0; # searching foreach(keys %h){ if($_ eq $key){ $value = $h{$_}; last; # stop searching if found } } # print the result if($value > 0){ print("element $key found with value: $value\n"); }else{ print("element $key not found\n"); }

continue statement

Let us try the following code to understand continue statement.

$i=1; while ($i <= 10) { if ($i == 5) { print "\$i == $i\n"; next; } print "$i "; }continue {$i++;}

redo statement

The redo command restarts the loop without evaluating the condition. The continue block, if any, is not executed.

$number = 1; while ( $number <= 5 ) { if ( $number <= 10 ) { print "$number "; ++$number; redo; } } print "\nStopped when \$number became $number.\n";

goto statement

Perl supports a goto command with three forms: goto label, goto expr, and goto &name.

#!/usr/local/bin/perl NEXTLINE: $line = <STDIN>; if ($line ne "") { print ($line); goto NEXTLINE; }

The Infinite Loop

If the condition of a loop never becomes false, the loop runs for infinite time.

#!/usr/local/bin/perl for( ; ; ) { printf "This loop will run forever.\n"; }

You can terminate the above infinite loop by pressing the Ctrl + C keys.

When the conditional expression is absent, it is assumed to be true. You may have an initialization and increment expression, but as a programmer more commonly use the for (;;) construct to signify an infinite loop.

Happy Exploring!