Several variations 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.

for (;;) {
 //Place statements that need to be executed continuously
}
?>

Although there are some tasks that use infinite loops, most program tasks, especially those covered by PHP , when using an infinite loop, 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 Formula

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.

if (isset($i)) {
unset($i);
if ((int) date('' ) < 2008) {
  } else { $i $i = 1

; > $i =3;
}

for (;$ i < >$i;}

?>
In the same way, the iteration expression expr3 may also be left blank and can also be used In this regard, you can 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.

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

and above The 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. You can even use a for statement without a loop body to complete some tasks. For example, calculate accumulation or factorial:


//Calculate the accumulation 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 value is $j
for ($i = 1,$j = 1; $i <= 5;$j *= $i ++);
echo $j;

?>


With the help of C language syntax, PHP 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/317210.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/317210.htmlTechArticleThe 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...