Perl Loops

Perl - Conditional Statemnts - IF...ELSE

Perl conditional statements helps in decision making which require the programmer specifies one or more conditions to be evaluated or tested by the program, along with a statement or statements to be executed if the condition is determined to be true, and optionally, other statements to be executed if the condition is determined to be false.

Following is the general from of a typical decision making structure found in most of the programming languages:

The number 0, the strings '0' and "" , the empty list () , and undef are all falsein a boolean context and all other values are true. Negation of a true value by ! or not returns a special false value.

Perl programming language provides following types of conditional statements. Click the following links to check their detail.

Statement	Description
if statement	An if statement consists of a boolean expression followed by one or more statements.
if...else statement	An if statement can be followed by an optional else statement.
if...elsif...else statement	An if statement can be followed by an optional elsif statement and then by an optional else statement.
unless statement	An unless statement consists of a boolean expression followed by one or more statements.
unless...else statement	An unless statement can be followed by an optionalelse statement.
unless...elsif..else statement	An unless statement can be followed by an optionalelsif statement and then by an optional else statement.
switch statement	With latest versions of Perl, you can make use of switchstatment which allows a simple way of comparing a variable value against various conditions.

The ? : Operator

Let's check conditional operator ? : which can be used to replace if...else statements. It has the following general form:

Exp1 ? Exp2 : Exp3;

Where Exp1, Exp2, and Exp3 are expressions. Notice the use and placement of the colon.

The value of a ? expression is determined like this: Exp1 is evaluated. If it is true, then Exp2 is evaluated and becomes the value of the entire ? expression. If Exp1 is false, then Exp3 is evaluated and its value becomes the value of the expression. Below is a simple example making use of this operator:

#!/usr/local/bin/perl

$name = "Ali";

$age = 10;

$status = ($age > 60 )? "A senior citizen" : "Not a senior citizen";

print "$name is  - $status\n";

This will produce following result:

Ali is - Not a senior citizen

Perl - Loops

There may be a situation when you need to execute a block of code several number of times. In general statements are executed sequentially: The first statement in a function is executed first, followed by the second, and so on.

Programming languages provide various control structures that allow for more complicated execution paths.

A loop statement allows us to execute a statement or group of statements multiple times and following is the general from of a loop statement in most of the programming languages:

Perl programming language provides following types of loop to handle looping requirements. Click the following links to check their detail.

Loop Type	Description
while loop	Repeats a statement or group of statements while a given condition is true. It tests the condition before executing the loop body.
until loop	Repeats a statement or group of statements until a given condition becomes true. It tests the condition before executing the loop body.
for loop	Execute a sequence of statements multiple times and abbreviates the code that manages the loop variable.
foreach loop	The foreach loop iterates over a normal list value and sets the variable VAR to be each element of the list in turn.
do...while loop	Like a while statement, except that it tests the condition at the end of the loop body
nested loops	You can use one or more loop inside any another while, for or do..while loop.

Loop Control Statements:

Loop control statements change execution from its normal sequence. When execution leaves a scope, all automatic objects that were created in that scope are destroyed.

C supports the following control statements. Click the following links to check their detail.

Control Statement	Description
next statement	Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating.
last statement	Terminates the loop statement and transfers execution to the statement immediately following the loop.
continue statement	A continue BLOCK, it is always executed just before the conditional is about to be evaluated again.
redo statement	The redo command restarts the loop block without evaluating the conditional again. The continue block, if any, is not executed.
goto statement	Perl supports a goto command with three forms: goto label, goto expr, and goto &name.

The Infinite Loop:

A loop becomes infinite loop if a condition never becomes false. The for loop is traditionally used for this purpose. Since none of the three expressions that form the for loop are required, you can make an endless loop by leaving the conditional expression empty.

#!/usr/local/bin/perl

for( ; ; )

{

   printf "This loop will run forever.\n";

}

You can terminate above infinite loop by pressing 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.