(PHP 4, PHP 5, PHP 7)
for loops are the most complex loops in PHP. They behave like their C counterparts. The syntax of a for loop is:
for (expr1; expr2; expr3)
statement
The first expression (expr1) is evaluated (executed) once unconditionally at the beginning of the loop.
In the beginning of each iteration,
expr2 is evaluated. If it evaluates to
TRUE, the loop continues and the nested
statement(s) are executed. If it evaluates to
FALSE, the execution of the loop ends.
At the end of each iteration, expr3 is evaluated (executed).
Each of the expressions can be empty or contain multiple
expressions separated by commas. In expr2, all
expressions separated by a comma are evaluated but the result is taken
from the last part.
expr2 being empty means the loop should
be run indefinitely (PHP implicitly considers it as
TRUE, like C). This may not be as useless as
you might think, since often you'd want to end the loop using a
conditional break
statement instead of using the for truth
expression.
Consider the following examples. All of them display the numbers 1 through 10:
<?php
/* example 1 */
for ($i = 1; $i <= 10; $i++) {
echo $i;
}
/* example 2 */
for ($i = 1; ; $i++) {
if ($i > 10) {
break;
}
echo $i;
}
/* example 3 */
$i = 1;
for (; ; ) {
if ($i > 10) {
break;
}
echo $i;
$i++;
}
/* example 4 */
for ($i = 1, $j = 0; $i <= 10; $j += $i, print $i, $i++);
?>
Of course, the first example appears to be the nicest one (or perhaps the fourth), but you may find that being able to use empty expressions in for loops comes in handy in many occasions.
PHP also supports the alternate "colon syntax" for for loops.
for (expr1; expr2; expr3):
statement
...
endfor;
It's a common thing to many users to iterate through arrays like in the example below.
<?php
/*
* This is an array with some data we want to modify
* when running through the for loop.
*/
$people = array(
array('name' => 'Kalle', 'salt' => 856412),
array('name' => 'Pierre', 'salt' => 215863)
);
for($i = 0; $i < count($people); ++$i) {
$people[$i]['salt'] = mt_rand(000000, 999999);
}
?>
The above code can be slow, because the array size is fetched on every iteration. Since the size never changes, the loop be easily optimized by using an intermediate variable to store the size instead of repeatedly calling count():
<?php
$people = array(
array('name' => 'Kalle', 'salt' => 856412),
array('name' => 'Pierre', 'salt' => 215863)
);
for($i = 0, $size = count($people); $i < $size; ++$i) {
$people[$i]['salt'] = mt_rand(000000, 999999);
}
?>
Looping through letters is possible. I'm amazed at how few people know that.
for($col = 'R'; $col != 'AD'; $col++) {
echo $col.' ';
}
returns: R S T U V W X Y Z AA AB AC
Take note that you can't use $col < 'AD'. It only works with !=
Very convenient when working with excel columns.
The point about the speed in loops is, that the middle and the last expression are executed EVERY time it loops.
So you should try to take everything that doesn't change out of the loop.
Often you use a function to check the maximum of times it should loop. Like here:
<?php
for ($i = 0; $i <= somewhat_calcMax(); $i++) {
somewhat_doSomethingWith($i);
}
?>
Faster would be:
<?php
$maxI = somewhat_calcMax();
for ($i = 0; $i <= $maxI; $i++) {
somewhat_doSomethingWith($i);
}
?>
And here a little trick:
<?php
$maxI = somewhat_calcMax();
for ($i = 0; $i <= $maxI; somewhat_doSomethingWith($i++)) ;
?>
The $i gets changed after the copy for the function (post-increment).
You can use strtotime with for loops to loop through dates
<?php
for ($date = strtotime("2014-01-01"); $date < strtotime("2014-02-01"); $date = strtotime("+1 day", $date)) {
echo date("Y-m-d", $date)."<br />";
}
?>
Also acceptable:
<?php
for($letter = ord('a'); $letter <= ord('z'); $letter++)
print chr($letter);
?>
<?php
//this is a different way to use the 'for'
//Essa é uma maneira diferente de usar o 'for'
for($i = $x = $z = 1; $i <= 10;$i++,$x+=2,$z=&$p){
$p = $i + $x;
print "\$i = $i , \$x = $x , \$z = $z <br />";
}
?>
In this code:
<?php
$array = array(
'pop0',
'pop1',
'pop2',
'pop3',
'pop4',
'pop5',
'pop6',
'pop7',
'pop8'
);
echo "Tot Before: ".count($array)."<br><br>";
for ($i=0; $i<count($array); $i++) {
if ($i === 3) {
unset($array[$i]);
}
echo "Count: ".count($array). " - Position: ".$i."<br>";
}
echo "<br> Tot After: ".count($array)."<br>";
?>
The result is:
---
Tot Before: 9
Count: 9 - Position: 0
Count: 9 - Position: 1
Count: 9 - Position: 2
Count: 8 - Position: 3
Count: 8 - Position: 4
Count: 8 - Position: 5
Count: 8 - Position: 6
Count: 8 - Position: 7
Tot After: 8
---
The position 8 is skipped, because the "expr2" {{ $i<count($array) }} is evaluated again, for each cycle.
The solution is:
<?php
$array = array(
'pop0',
'pop1',
'pop2',
'pop3',
'pop4',
'pop5',
'pop6',
'pop7',
'pop8'
);
echo "Tot Before: ".count($array)."<br><br>";
$count = count($array);
for ($i=0; $i<$count; $i++) {
if ($i === 3) {
unset($array[$i]);
}
echo "Count: ".count($array). " - Position: ".$i."<br>";
}
echo "<br> Tot After: ".count($array)."<br>";
?>
On the combination problem again...
It seems to me like it would make more sense to go through systematically. That would take nested for loops, where each number was put through all of it's potentials sequentially.
The following would give you all of the potential combinations of a four-digit decimal combination, printed in a comma delimited format:
<?php
for($a=0;$a<10;$a++){
for($b=0;$b<10;$b++){
for($c=0;$c<10;$c++){
for($d=0;$d<10;$d++){
echo $a.$b.$c.$d.", ";
}
}
}
}
?>
Of course, if you know that the numbers you had used were in a smaller subset, you could just plunk your possible numbers into arrays $a, $b, $c, and $d and then do nested foreach loops as above.
- Elizabeth
Remember that for-loops don't always need to go 'forwards'. For example, let's say I have the following code:
<?php
for ($i = 0; $i < calculateLoopLength(); $i++) {
doSomethingWith($i);
}
>?
As other comments have pointed out, if "calculateLoopLength" will keep giving back the same value, it can be moved outside the loop:
<?php
$loopLength = calculateLoopLength();
for ($i=0; $i < $loopLength; $i++) {
doSomethingWith($i);
}
?>
However, if the order the looping doesn't matter (ie. each iteration is independent) then we don't need to use an extra variable either, we can just count down (ie. loop 'backwards') instead:
<?php
for ($i=calculateLoopLength(); $i > 0; $i--) {
doSomething($i);
}
?>
In fact, we can simplify this even more, since "$i > 0" is equivalent to "$i" (due to type casting):
<?php
for ($i=calculateLoopLength(); $i; $i--) {
doSomething($i);
}
?>
Finally, we can switch to a 'pre-decrement' instead of a 'post-decrement' to be slightly more efficient (see, for example, http://dfox.me/2011/04/php-most-common-mistakes-part-2-using-post-increment-instead-of-pre-increment/ ):
<?php
for ($i = calculateLoopLength(); $i; --$i) {
doSomething($i);
}
?>
In this case we could also replace the entire loop with a map, which might make your algorithm clearer (although this won't work if calculateLoopLength() == 0):
<?php
array_map('doSomething',
range(0, calculateLoopLength() - 1));
?>
<html>
<head>
<title>
drikhoek
</title>
</head>
<body bgcolor="#99FF66">
<form action="driehoek.php" method="post" >
<input type="text" name="breedte" />
<input type="submit" value="klikken" />
</form>
<?
$hoogte=$_POST["breedte"];
print(" ");
for($teller=1; $teller<=$hoogte; $teller++)
{
for($breedte=1; $breedte<=$teller; $breedte++)
{ print("*");
}
print(" </br>");
}
?>
it's an example!!!!!
</body>
</html>
Note, that, because the first line is executed everytime, it is not only slow to put a function there, it can also lead to problems like:
<?php
$array = array(0 => "a", 1 => "b", 2 => "c", 3 => "d");
for($i = 0; $i < count($array); $i++){
echo $array[$i];
unset($array[$i]);
}
?>
This will only output the half of the elements, because the array is becoming shorter everytime the for-expression counts it.
For those who are having issues with needing to evaluate multiple items in expression two, please note that it cannot be chained like expressions one and three can. Although many have stated this fact, most have not stated that there is still a way to do this:
<?php
for($i = 0, $x = $nums['x_val'], $n = 15; ($i < 23 && $number != 24); $i++, $x + 5;) {
// Do Something with All Those Fun Numbers
}
?>
You can also work with arrays. For example, say you want to generate an array of 12 unique 2-letter strings:
<?php
for ($names = array(); count($names) < 12; $names = array_unique($names)) {
/**
* we assume here we have some $faker object
* which generates n-letter strings
*/
$names[] = $faker->word(2);
}
print_r($names);
?>
will print something like:
Array
(
[0] => cc
[1] => cb
[2] => dd
[3] => db
[4] => bb
[6] => cd
[8] => aa
[9] => ad
[10] => ca
[11] => ac
[12] => dc
[15] => ab
)
If you're already using the fastest algorithms you can find (on the order of O(1), O(n), or O(n log n)), and you're still worried about loop speed, unroll your loops using e.g., Duff's Device:
<?php
$n = $ITERATIONS % 8;
while ($n--) $val++;
$n = (int)($ITERATIONS / 8);
while ($n--) {
$val++;
$val++;
$val++;
$val++;
$val++;
$val++;
$val++;
$val++;
}
?>
(This is a modified form of Duff's original device, because PHP doesn't understand the original's egregious syntax.)
That's algorithmically equivalent to the common form:
<?php
for ($i = 0; $i < $ITERATIONS; $i++) {
$val++;
}
?>
$val++ can be whatever operation you need to perform ITERATIONS number of times.
On my box, with no users, average run time across 100 samples with ITERATIONS = 10000000 (10 million) is:
Duff version: 7.9857 s
Obvious version: 27.608 s
As a note for people just starting out and wanting to know if you can do some thing like this...
<?php For( $a = 0; $a < 10; $a++ ) { ?>
//Random html elements you would like to duplicate.
<?php } ?>
Then yes you can. It works like a charm.
Here is another simple example for " for loops"
<?php
$text="Welcome to PHP";
$searchchar="e";
$count="0"; //zero
for($i="0"; $i<strlen($text); $i=$i+1){
if(substr($text,$i,1)==$searchchar){
$count=$count+1;
}
}
echo $count
?>
this will be count how many "e" characters in that text (Welcome to PHP)
Here is another simple example for " for loops"
<?php
$text="Welcome to PHP";
$searchchar="e";
$count="0"; //zero
for($i="0"; $i<strlen($text); $i=$i+1){
if(substr($text,$i,1)==$searchchar){
$count=$count+1;
}
}
echo $count
?>
this will be count how many "e" characters in that text (Welcome to PHP)
