For-Loop Structures
The for statement loop is used when you know how many times you
want to execute a statement or a list of statements. For this reason,
the for loop is known as a definite loop.
The basic for loop syntax is shown below:
for (intialization; condition; increment) {
code to be executed;
}
The for statement includes three parameters. The first parameter is used to
initialize variables, the second holds the condition, and the third parameter
contains any increments required to implement the loop. The code block associated
with the for statement is always enclosed within the { opening and }
closing brace symbols.
The following example demonstrates a for loop that displays the message
"Welcome to PHP" 4 times:
<?php
for ($counter=1; $counter < 5; $counter++) {
echo "Welcome to PHP <br>";
}
?>
In the above example, the for loop includes three parameters that perform
the following tasks:
- The variable
$counter is initialized to 1.
$counter < 5 establishes the condition that must be true for
the loop to execute.$counter++ increments the variable $counter each time
the loop runs
Below is the output generated by the sample loop:
Welcome to PHP
Welcome to PHP
Welcome to PHP
Welcome to PHP
During the first run, the value of $counter is initialized to 1. Since 1 is
less than 5, $counter is incremented to 2. Next,
the echo statement is used to display the string
"Welcome to PHP". A <br> is concatenated to the display to create a carriage
return each time the loop is processed. During the second iteration, the value of
$counter is equal to 2. Since 2 is less than 5,
the echo statement again displays the string
"Welcome to PHP". This process continues while the value of
$counter is 3 and 4. When
$counter is equal to 5, the condition is no longer
true and the while loop ends.
for loops are also used as a convenient way to
iterate through the values of an array. Recall from
3.2 Array Variables section that arrays consists of elements and
indexes. Each element has an associated index. The first index of a numerically indexed
array is 0. Until now, when printing array elements, it was necessary to print each
element separately. For large arrays, this can be very time consuming and cumbersome.
for loops solve this problem. In the following example,
an array containing 5 elements is created. Next, a for
loop is used to display each of the array values.
<?php
//Create a new array containing 5 color values
$colors = array('red', 'green', 'blue', 'yellow','white');
//Use a for loop to iterate through the array and display each element
for ($i = 0; $i < sizeof($colors); $i++) {
echo "Array value $i+1 is $colors[$i].";
}
?>
The $colors array contains five elements -
$colors[0] = "red", $colors[1] = "green",
$colors[2] = "blue", $colors[3] = "yellow", and
$colors[4] = "white". A for loop
is created; the counter variable $i is initially set
to 0 to correspond with the first element of the array. Next, the condition is set by
checked to see if the value of the counter, $i, is less
than sizeof($colors) or the total number of elements in the array. Here
we know the size of the array is 5, however, in most cases the size of the array at
runtime is unknown. Finally, the counter variable is incremented by 1 during each
iteration. Each time the loop runs, the string - "Array value $i+1 is
$colors[$i] is displayed. The values of $i+1
and $colors[$i] are expanded and produce literal values when printed.
The loop runs until the the counter exceeds the number of elements or the size of
the array. The output is show below:
Array value 1 is red.
Array value 2 is green.
Array value 3 is blue.
Array value 4 is yellow.
Array value 5 is white.
Loops are useful constructs that will be used extensively throughout future tutorials.