Detailed explanation of extended usage examples of for loop in php

for statementIt can be said to be the most basic statement in the loop control part of PHP (and multiple languages), for The execution rules and basic usage of statements will not be discussed here. You can refer to the PHP Manualfor statement section. Its syntax is defined as follows in the PHP manual:

for (expr1; expr2; expr3)
statement

Let’s talk about several useful variations of the for statement. .

　1. Infinite Loop

The first is the well-known infinite loop (also called an "infinite loop"). Since the empty expressionnull is syntactically valid, we can leave the three expressions of the for statement empty, which will have the effect of continuously executing the for nested statement.

<?php
for (;;) {
　//放置需要不断执行的语句
}
?>

Although there are some tasks that use infinite loops, most program tasks, especially those involving PHP, will add some conditions to terminate the loop when using infinite loops.

＜?php
for (;;) {
//如果是公元2199年，则跳出循环
　if (date('Y') == '2199') {
　　break;
　}
}
?＞

　2. Use empty expressions

The next step is to use null syntax in the initialization statement expr1. The most common function of leaving expr1 blank is to complete more complex initialization work.

＜?php
if (isset($i)) {
unset($i);
　if ((int) date('') ＜ 2008) {
　　$i = 0;
　} else {
　　$i = 1;
　}
} else {
　$i =3;
}
for (;$i ＜ 10;$i++) {
　echo $i;
}
?＞

In the same way, the iteration expression expr3 may also be left blank. This can also be used to write more complex iterations, such as calling different iterations according to different conditions.

Leaving the conditional statement expr2 in the for statement blank is the infinite loop mentioned above. Of course, you can also add some more complex conditions to determine when to jump out of the loop, which will not be repeated here.

　3. Multiple loops

Using multiple loops to control multiple variables is also a feature that is often overlooked in the for statement. As in the example below, double loops are generally used in general tasks, and loops of three or more are generally of little significance.

＜?php
for ($i = 0, $j = 10;$i ＜= 10;$i++, $j--) {
　echo "$i + $j = 10\r\n";
}
?＞

The above code will output:

0 + 10 = 10
1 + 9 = 10
2 + 8 = 10
3 + 7 = 10
4 + 6 = 10
5 + 5 = 10
6 + 4 = 10
7 + 3 = 10
8 + 2 = 10
9 + 1 = 10
10 + 0 = 10

4. More complex expressions

If the three expressions of the for statement are written more complicated, you can use for optimization algorithms. You can even use a for statement without a loop body to complete some tasks. For example, to calculate accumulation or factorial:

＜?php
//计算1-5的累加结果，斌值到$j
or ($i = 1,$j = 0; $i ＜= 5;$j += $i++);
echo $j;
//计算1-5的阶乘结果，斌值到$j
for ($i = 1,$j = 1; $i ＜= 5;$j *= $i++);
echo $j;
?＞

f

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