Programming:PHP/for loop

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[edit] The FOR loop

The for loop is one of the basic looping structures in most modern programming languages. Like the while loop, for loops execute a given code block until a certain condition is met.

[edit] Syntax

The basic syntax of the for loop in PHP is similar to the c syntax:

for([initialization]; [condition]; [step])

Initialization happens the first time the loop is run. It is used to initialize variables or perform other actions that are to be performed before the first execution of the body of the loop.
The Condition is evaluated before each execution of the body of the loop; if the condition is true, the body of the loop will be executed, if it is false, the loop is exited and program execution resumes at the first line after the body of the loop.
Step specifies an action that is to be performed after each execution of the loop body.

for($i = 0; $i < 5; $i++)
 {
  echo($i . "<nowiki><br /></nowiki>");
 }

0<br />1<br />2<br />3<br />4<br />

 0
 1
 2
 3
 4

the loop can also be formatted without using concatenation, according to personal preference:

for($i = 0; $i < 5; $i++)
 {
  echo "$i <nowiki><br /></nowiki>";
 }

[edit] Explanation

The variable $i is initialized as 0. When the loop ran once for the first time, it printed the original value of $i, 0. Each time the loop ran, it incremented $i, then checked to see if $i was still less than 5. If it was, it continued looping. When $i finally reached 5, it was no longer less than 5, so PHP stopped looping and went to the next line after the loop code block (none, in this case).

[edit] Using FOR loops to traverse arrays

In the section on while loops the sort() example uses a while loop to print out the contents of the array. Generally programmers use for loops for this kind of job.

[edit] Example

NOTE: Use of indices like below is highly discouraged and poor programming style. Use the key-value for-loop construct.

$menu = array("Toast and Jam", "Bacon and Eggs", "Homefries", "Skillet", "Milk and Cereal");
 // note to self: get breakfast after writing this article
 
 for($i = 0; $i < count($menu); $i++)
 {
  echo($i + 1 . ". " . $menu[$i] . "<nowiki><br /></nowiki>");
 }

Again, this can be formatted without concatenation if you prefer:

for($i = 0; $i < count($menu); $i++)
 {
  $j=$i+1;
  echo "$j. $menu[$i] <nowiki><br /></nowiki>";
 }

[edit] Explanation

for($i = 0; $i < count($menu); $i++)

This line sets up the loop. It initializes the counter, $i, to 0 at the start, adds one every time the loop goes through, and checks that $i is less than the size of the array (hence the nice count function, which returns the size of an array).

{
  echo($i + 1 . ". " . $menu[$i] . "<nowiki><br /></nowiki>");
 }

The echo statement is pretty self-explanatory, except perhaps the bit at the start, where we echo $i + 1. We do that because, as you may recall, arrays start at 0 and end at n - 1 (where n is their length), so to get a numbered list starting at one, we have to add one to the counter each time we print it.

Of course, as I mentioned before, both pieces of code produce the following output:

1. Toast and Jam
2. Bacon and Eggs
3. Homefries
4. Skillet
5. Milk and Cereal

Believe it or not, there's actually a way to traverse arrays that requires even less typing. (And isn't that the goal?) Check out the FOREACH loop for another way of doing what we did here.

The above example is not ideal for real world applications, as each time the loop is run, it runs the evaluation of count($menu). This means that it will count the menu the same number of times as the inside loop. Ideally, this should be a variable which is defined before the for loop so that this is not a problem.

[edit] For More Information

PHP Manual: FOR loops