Please note that following code is working:
$reverse = TRUE; //iteration direction switch
for ($i=$reverse?($N-1):0; $reverse?($i>-1):($i<$N); $reverse?$i--:$i++) {
... your code here
}
(Using PHP 5.4.45)
(PHP 4, PHP 5, PHP 7)
for 循环是 PHP 中最复杂的循环结构。它的行为和 C 语言的相似。 for 循环的语法是：
for (expr1; expr2; expr3)
statement
第一个表达式（expr1）在循环开始前无条件求值（并执行）一次。
expr2 在每次循环开始前求值。如果值为
TRUE，则继续循环，执行嵌套的循环语句。如果值为 FALSE，则终止循环。
expr3 在每次循环之后被求值（并执行）。
每个表达式都可以为空或包括逗号分隔的多个表达式。表达式 expr2
中，所有用逗号分隔的表达式都会计算，但只取最后一个结果。expr2
为空意味着将无限循环下去（和 C 一样，PHP 暗中认为其值为
TRUE）。这可能不像想象中那样没有用，因为经常会希望用有条件的 break
语句来结束循环而不是用 for 的表达式真值判断。
考虑以下的例子，它们都显示数字 1 到 10：
<?php
/* example 1 */
for ($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++);
?>
当然，第一个例子看上去最简洁（或者有人认为是第四个），但用户可能会发现在 for 循环中用空的表达式在很多场合下会很方便。
PHP 也支持用冒号的 for 循环的替代语法。
for (expr1; expr2; expr3):
statement;
...
endfor;
有时经常需要像下面这样例子一样对数组进行遍历：
<?php
/*
* 此数组将在遍历的过程中改变其中某些单元的值
*/
$people = Array(
Array('name' => 'Kalle', 'salt' => 856412),
Array('name' => 'Pierre', 'salt' => 215863)
);
for($i = 0; $i < count($people); ++$i)
{
$people[$i]['salt'] = rand(000000, 999999);
}
?>
以上代码可能执行很慢，因为每次循环时都要计算一遍数组的长度。由于数组的长度始终不变，可以用一个中间变量来储存数组长度以优化而不是不停调用 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'] = rand(000000, 999999);
}
?>
Please note that following code is working:
$reverse = TRUE; //iteration direction switch
for ($i=$reverse?($N-1):0; $reverse?($i>-1):($i<$N); $reverse?$i--:$i++) {
... your code here
}
(Using PHP 5.4.45)
Please note that following code is working:
for ($i=$reverse?($N-1):0; $reverse?($i>-1):($i<$N); $reverse?$i--:$i++) {
... your code here
}
(Using PHP 5.4.45)
I've tried to search for a results on internet for a basic array which contain letters A to Z inside
<!DOCTYPE html>
<html>
<body>
<?php
$letter = array();
for ($letters = 'A'; $letters != 'AA'; $letters++)
{
array_push($letter, $letters);
}
echo '<pre>' . var_export($letter, true) . '</pre>';
?>
</body>
</html>
Adding Letters from A to Z inside an array.
You should test it out.
<!DOCTYPE html>
<html>
<body>
<?php
$letter = array();
for ($letters = 'A'; $letters != 'AA'; $letters++)
{
array_push($letter, $letters);
}
echo '<pre>' . var_export($letter, true) . '</pre>';
?>
</body>
</html>
Warning about using the function "strlen" i a for-loop:
This note should might be under the "strlen" manual page, but there is a better chance for more paying attention here (nevertheless I have made a short note over there allso).
A loop function that test for the string length at each iteration takes forever (possibly due to "strlen" searches for the C-style string terminator - a binary 0 - every time..
So loops like this, using "strlen" in the for...
for ($i = 0; $i < strlen($crc); $i++) .....
Will benefit tremendously in speed by a short step that saves the string length once and use that in the loop.
$clen = strlen($crc);
for ($i = 0; $i < $clen ; $i++) .....
Note: as a real hard-core programmer You are aware, that this is only valid if you don't change the string content inside the loop (and hereby allso the length). If the change is only occationly , You could just refresh the length variable or else just live with a quite slow loop.
This "discovery" was made from using an example of 16 bit crc calculation over at the "crc32" function manual page, that do exactly that..
when iterating a multidimentional array like this:
for ($i = 0; $i < $size_x; $i++) {
for ($j = 0; $j < $size_y; $j++) {
do_something($a[$i][$j]);
}
}
it is faster to use $a[$i][$j] than using $a[$j][$i]
for ($i = 0; $i < $size_x; $i++) {
for ($j = 0; $j < $size_y; $j++) {
do_something($a[$j][$i]);
}
}
if you know about how RAM works you understand why
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
)
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.
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($array)."<br>";
?>
Remember that for-loops don't always need to go 'forwards'. For example, let's say I have the following code:
<?php
for ($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($i);
}
?>
In this case we could also replace the entire loop with a map, which might make your algorithm clearer (although this won't work if calculateLoopLength() == 0):
<?php
array_map('doSomething',
range(0, calculateLoopLength() - 1));
?>
You can use strtotime with for loops to loop through dates
<?php
for ($date = strtotime("2014-01-01"); $date < strtotime("2014-02-01"); $date = strtotime("+1 day", $date)) {
echo date("Y-m-d", $date)."<br />";
}
?>
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.
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.
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 $count
?>
this will be count how many "e" characters in that text (Welcome to PHP)
<?php
//this is a different way to use the 'for'
//Essa é uma maneira diferente de usar o 'for'
for($i = $x = $z = 1; $i <= 10;$i++,$x+=2,$z=&$p){
$p = $i + $x;
print "\$i = $i , \$x = $x , \$z = $z <br />";
}
?>
On the combination problem again...
It seems to me like it would make more sense to go through systematically. That would take nested for loops, where each number was put through all of it's potentials sequentially.
The following would give you all of the potential combinations of a four-digit decimal combination, printed in a comma delimited format:
<?php
for($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 $d and then do nested foreach loops as above.
- Elizabeth
For those who are having issues with needing to evaluate multiple items in expression two, please note that it cannot be chained like expressions one and three can. Although many have stated this fact, most have not stated that there is still a way to do this:
<?php
for($i = 0, $x = $nums['x_val'], $n = 15; ($i < 23 && $number != 24); $i++, $x + 5;) {
// Do Something with All Those Fun Numbers
}
?>
Also acceptable:
<?php
for($letter = ord('a'); $letter <= ord('z'); $letter++)
print chr($letter);
?>
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++;
}
?>
$val++ can be whatever operation you need to perform ITERATIONS number of times.
On my box, with no users, average run time across 100 samples with ITERATIONS = 10000000 (10 million) is:
Duff version: 7.9857 s
Obvious version: 27.608 s
The point about the speed in loops is, that the middle and the last expression are executed EVERY time it loops.
So you should try to take everything that doesn't change out of the loop.
Often you use a function to check the maximum of times it should loop. Like here:
<?php
for ($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).