What is the use of the Map of Mountains and Rivers in Dream Westward Journey 3? Several useful variations. Introduction to the use of loop statements in PHP

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 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 the areas that PHP can involve, use When looping infinitely, 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 expression
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. 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.

Copy code The code is as follows:

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

　　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 , it can be used for optimization algorithms. You can even use a for statement without a loop body to complete some tasks. For example, calculate the accumulation or factorial:

Copy the code The code is as follows:

//Calculate the accumulation result of 1-5, and the value 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;
　?>

　 PHP uses the syntax of C language and will also have C features to a certain extent, such as powerful The for loop statement is a typical example.

The above has introduced several useful variations of "Dream Westward Journey 3: What are the uses of the Mountains and Rivers Sheji Map" and introduced the use of loop statements in PHP, including the content of "Dream Westward Journey 3: What are the uses of the Mountains and Rivers Sheji Map"? I hope you are interested in PHP tutorials. Friends help.