Friday, 24 August 2012

PHP Tutorial Lesson #10 Looping


PHP Loops

Often when you write code, you want the same block of code to run over and over again in a row. Instead of adding several almost equal lines in a script we can use loops to perform a task like this.
In PHP, we have the following looping statements:
  • while - loops through a block of code while a specified condition is true
  • do...while - loops through a block of code once, and then repeats the loop as long as a specified condition is true
  • for - loops through a block of code a specified number of times
  • foreach - loops through a block of code for each element in an array

The while Loop

The while loop executes a block of code while a condition is true.

Syntax

while (condition)
  {
  code to be executed;
  }

Example

The example below defines a loop that starts with i=1. The loop will continue to run as long as i is less than, or equal to 5. i will increase by 1 each time the loop runs:
<html>
<body>

<?php
$i=1;
while($i<=5)
  {
  echo "The number is " . $i . "<br />";
  $i++;
  }
?>

</body>
</html>
Output:
The number is 1
The number is 2
The number is 3
The number is 4
The number is 5


The do...while Statement

The do...while statement will always execute the block of code once, it will then check the condition, and repeat the loop while the condition is true.

Syntax

do
  {
  code to be executed;
  
}
while (condition);

Example

The example below defines a loop that starts with i=1. It will then increment i with 1, and write some output. Then the condition is checked, and the loop will continue to run as long as i is less than, or equal to 5:

<html>
<body>

<?php
$i=1;
do
  {
  $i++;
  echo "The number is " . $i . "<br />";
  }
while ($i<=5);
?>

</body>
</html>

Output:

The number is 2
The number is 3
The number is 4
The number is 5
The number is 6

The for Loop

The for loop is used when you know in advance how many times the script should run.

Syntax

for (init; condition; increment)
  {
  code to be executed;
  }

Parameters:
  • init: Mostly used to set a counter (but can be any code to be executed once at the beginning of the loop)
  • condition: Evaluated for each loop iteration. If it evaluates to TRUE, the loop continues. If it evaluates to FALSE, the loop ends.
  • increment: Mostly used to increment a counter (but can be any code to be executed at the end of the loop)
Note: Each of the parameters above can be empty, or have multiple expressions (separated by commas).

Example

The example below defines a loop that starts with i=1. The loop will continue to run as long as i is less than, or equal to 5. i will increase by 1 each time the loop runs:
<html>
<body>

<?php
for ($i=1; $i<=5; $i++)
  {
  echo "The number is " . $i . "<br />";
  }
?>

</body>
</html>
Output:
The number is 1
The number is 2
The number is 3
The number is 4
The number is 5


The foreach Loop

The foreach loop is used to loop through arrays.

Syntax

foreach ($array as $value)
  {
  code to be executed;
  }

For every loop iteration, the value of the current array element is assigned to $value (and the array pointer is moved by one) - so on the next loop iteration, you'll be looking at the next array value.

Example

The following example demonstrates a loop that will print the values of the given array:

<html>
<body>

<?php
$x=array("one","two","three");
foreach ($x as $value)
  {
  echo $value . "<br />";
  }
?>

</body>
</html>

Output:

one
two
three

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