The for loop is just a little more code, while the loop is added to it. The common tasks of a loop involved are:
Set the initial value of some counter variables.
Please check that the conditional statement is correct.
Code loop in execution.
Increment the loop counter at the end of each iteration.
The for loop allows you to define these steps with a simple line of code. It seems to have a strange form, so be sure to pay close attention to the grammar!
The syntax of the for loop is:
for (expr1; expr2; expr3)
statement
The first expression (expr1) is evaluated unconditionally before the loop starts. Worth it once.
expr2 is evaluated before each loop. If the value is TRUE, the loop continues and the nested loop statement is executed. If the value is FALSE, the loop is terminated.
expr3 is evaluated (executed) after each loop.
Each expression can be empty or include multiple expressions separated by commas. In the expression expr2, all comma-separated expressions are evaluated, but only the last result is taken. An empty expr2 means that the loop will continue indefinitely (like C, PHP considers its value to be TRUE). This may not be as useless as you think, because you often want to use a break statement to end the loop instead of using the for expression truth check.
Look at a simple example 1
Copy the code The code is as follows:
FOR ($i = 0; $i <= 2; $i++)
{
print "value is now " . $i . "
";
}
output value
value is now 0
value is now 1
value is now 2
In the first loop, $i=0, which means the expression, ($i< = 2), which is true. Therefore, when the print statement is executed, $i is incremented by 1 and becomes 1.
In the second loop, $ = 1, which means that the expression, ($i<= 2), is true. Therefore, when the print statement is executed, $i is incremented by 1 and becomes 2.
In the third iteration, $i= 2, which means that the expression, ($i<= 2), is true. Therefore, when the print statement is executed, $i is incremented and becomes 1 3.
In the fourth iteration, $i= 3, which means the expression, ($i<= 2), is false. Therefore, PHP does not execute the loop and does not execute the print statement.
Example 2
Copy code The code is as follows:
$brush_price = 5;
echo "
";
echo "| Quantity | ";
echo "Price< ;/th> |
|---|
";
for ( $counter = 10; $counter <= 100; $counter += 10) {
echo "";
echo $counter;
echo " | ";
echo $brush_price * $counter;
echo " |
";
}
echo "
";
Output value
Quantity Price
10 50
20 100
30 150
40 200
50 250
60 300
70 350
80 400
90 450
100 500
You can refer to the following examples, they all display Numbers 1 to 10:
Copy code The code is as follows:
/* example 1 */
for ($i = 1; $i <= 10; $i++) {
echo $i;
}
/* example 2 */
for ($i = 1; ; $i++) {
if ($i > 10) {
break;
}
echo $i;
}
/* example 3 */
$i = 1;
for (;;) {
if ($i > 10) {
break;
}
echo $i;
$i++;
}
/* example 4 */
for ($i = 1, $j = 0; $i < ;= 10; $j += $i, print $i, $i++);
?>
Of course, the first example looks the most normal (or the fourth one) , but users may find it convenient to use empty expressions in for loops in many situations.
PHP also supports an alternative syntax for for loops using colons.
Copy code The code is as follows:
for (expr1; expr2; expr3):
statement;
. ..
endfor;
We often need to traverse the following array:
Copy code The code is as follows:
/*
* We want to change the values of some elements in the following array during the traversal process
*/
$people = Array(
Array('name' => 'Kalle', 'salt' => 856412),
Array('name' => 'Pierre', 'salt' => 215863)
);
for($i = 0; $i < sizeof($people); ++$i)
{
$people[$i][' salt'] = rand(000000, 999999);
}
?>
The problem with the above code is that the second expression of for will cause the code to execute very slowly - because the length of the array must be calculated every time it loops. Since the length of the array always remains the same, we can use an intermediate variable to store the length of the array, and then use this variable as the second expression of the for loop. In this way, the value of the variable can be used directly during the loop without having to recalculate each time. As follows:
Copy code The code is as follows:
$people = Array(
Array('name' => 'Kalle', 'salt' => 856412),
Array('name' => 'Pierre', 'salt' => 215863)
) ;
for($i = 0, $size = sizeof($people); $i < $size; ++$i)
{
$people[$i][' salt'] = rand(000000, 999999);
}
?>
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