Several Useful Variations Introduction to the Usage of Loop Statements in PHP_PHP Tutorial

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
First of all, there is the infinite loop (also called "dead loop") that everyone knows. 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.
　for (;;) {
　//Place statements that need to be executed continuously
　}
　?>
　 Although some tasks will use infinite loops, most program tasks, especially It is an area that PHP can cover. When using infinite loops, some conditions for terminating the loop will be added.
　for (;;) {
　//If it is 2199 AD, break out of the loop
　if (date('Y') == '2199') {
　break;
　}
　}
　?>
　2. Use empty expressions
Next, let’s talk about using null syntax in the initialization statement expr1. The most common function of leaving expr1 blank is to complete more complex initialization. Work.

Copy code The code is as follows:

　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 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.
　

Copy code The code is as follows:

for ($i = 0, $j = 10 ;$i <= 10;$i , $j--) {
echo "$i $j = 10rn";
}
?>

above The code will output: 4 = 10
　7 3 = 10
　8 2 = 10
　9 1 = 10
　10 0 = 10
　4. A more complex expression
If the for statement is If the three expressions are written more complexly, they can be used in the optimization algorithm. You can even use a for statement without a loop body to complete some tasks. For example, calculate accumulation or factorial:



Copy code

The code is as follows:

　

//Calculate 1- The cumulative result of 5 is equal to $j for ($i = 1,$j = 0; $i <= 5;$j = $i ); echo $j; / /Calculate the factorial result of 1-5, bin value to $j

　for ($i = 1,$j = 1; $i <= 5;$j *= $i );

　echo $j ;
　?>


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



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http: //www.bkjia.com/PHPjc/324887.htmlTechArticleThe syntax definition in the PHP manual is as follows: for (expr1; expr2; expr3) statement Let’s talk about the for statement Several useful variations. 1. Infinite loop The first is the infinite loop that everyone knows (can also...