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    Control Loops in PERL

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    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.

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


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    #!/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 = <>) {
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    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.


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


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    #!/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");
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    for loop

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


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


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

    foreach loop

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


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

    do...while loop

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


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

    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.


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    #!/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";
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    last statement

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


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    #!/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");
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    continue statement

    Let us try the following code to understand continue statement.


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

    redo statement

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


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    $number = 1;
    while ( $number <= 5 ) {
    if ( $number <= 10 ) {
    print "$number ";
    ++$number;
    redo;
    }
    }
    XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

    goto statement

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


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

    The Infinite Loop

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


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

    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!

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