Several 'abnormal' uses of for loop statements in PHP_PHP Tutorial

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

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 "dead loop"). Since the empty expression null is syntactically valid, we can leave the three expressions of the for statement empty, which will have the effect of continuously executing nested for statements.

＜?php
for (;;) {
//Place statements that need to be executed continuously
}
?＞
Although some tasks will use infinite loops , but most program tasks, especially the fields that PHP can involve, will add some conditions to terminate the loop when using infinite loops.

＜?php
for (;;) {
//If it is 2199 AD, break out of the loop
if (date('Y') == '2199') { break; The most common function 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, and you can also use this to write more complex Iterative formulas, such as calling different iterative formulas 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 ";
}
?＞
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 complexly, they can be used to optimize the algorithm. Use a for statement without a loop body to complete some tasks, such as calculating accumulation or factorial:

＜?php
//Calculate the cumulative result of 1-5, and the value is $j
for ( $i = 1,$j = 0; $i ＜= 5;$j += $i++);
echo $j;

//Calculate the factorial result of 1-5, the bin value is $j
for ($i = 1,$j = 1; $i ＜= 5;$j *= $i++);
echo $j;

?＞
PHP With the help of C language syntax, it will also have the characteristics of C to a certain extent. For example, the powerful for loop statement is a typical example.


http://www.bkjia.com/PHPjc/532423.html

www.bkjia.com

true
http: //www.bkjia.com/PHPjc/532423.html

TechArticle

The for statement can be said to be the most basic statement in the loop control part of PHP (and many languages) , the execution rules and basic usage of the for statement will not be discussed here. You can refer to P...