PHP Programming/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.
PHP Code:
for($i = 0; $i < 5; $i++) { echo($i . "<br />"); }
PHP Output:
0<br />1<br />2<br />3<br />4<br />
HTML Render:
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 <br />"; }
[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).
Do note that the Initialization, Condition and Step of the For-loop can be empty. In this case, you will get an infinite loop, unless you use the "break" keyword inside the loop somwhere.
Also note that the Initialization and Step parts of the For-loop can hold more than one instruction. More info can be found via the link to the PHP manual at the end of this page.
[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. 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 $count = count($menu); for($i = 0; $i < $count; $i++) { echo($i + 1 . ". " . $menu[$i] . "<br />"); }
Again, this can be formatted without concatenation if you prefer:
for($i = 0; $i < $count; $i++) { $j=$i+1; echo "$j. {$menu[$i]} <br />"; }
[edit] Explanation
$count = count($menu);
We define the count before the for loop for more efficient processing. This is because each time the for loop is run (whilst $i < $count) it evaluates both sides of the equation and executes any functions. If we put $i < count($menu), This would evaluate count($menu) each time the process is executed which is costly when dealing with large arrays.
for($i = 0; $i < $count; $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 $i is less than the size of the array.
{ echo($i + 1 . ". " . $menu[$i] . "<br />"); }
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.
