Several unconventional uses of for loop statements in PHP_PHP Tutorial

The for statement php (as the current mainstream development language) has the following syntax definition in the manual:

for (expr1; expr2; expr3)
statement
The following is said Discuss several useful variations of the for statement.

1. PHP (as the current mainstream development language) FOR infinite loop
The first is the well-known infinite loop (also called "infinite 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(as the current mainstream development language)
for (;;) {
//Place statements that need to be executed continuously
}
?＞
Although there are some tasks that use infinite loops, most program tasks, especially the areas covered by php(as the current mainstream development language), use infinite loops. Some conditions for terminating the loop will be added.

＜?php(as the current mainstream development language)
for (;;) {
//If it is 2199 AD, break out of the loop
if (date('Y') == '2199') {
break;
}
}
?＞

2.php(as the current Mainstream development languages) FOR uses 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(as the current mainstream development language)
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 iterative expression expr3 It 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. PHP (as the current mainstream development language) FOR 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(as the current mainstream development language)
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. php (as the current mainstream development language) FOR more complex expressions
If the expression is more complex, it can be used for optimization algorithms. You can even use a for statement without a loop body to complete some tasks, such as calculating accumulation or factorial:

＜?php(as Current mainstream development languages)
//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, bin value to $j
for ($i = 1,$j = 1; $ i ＜= 5;$j *= $i++);
echo $j;

?＞