PHP for loop statement syntax introduction and usage examples

for loopJust a little more code with a loop added to it. The common tasks involved in a loop are:

1. Set the initial values ​​of some counters variables .
2. Please check that the conditional statement is correct.
3. Code loop in execution.
4. 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 once before the loop starts.
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.
Example 1

for ($i = 0; $i <= 2; $i++) 
{ 
print "value is now " . $i . "<br>"; 
}

Output value

value is now 0 
value is now 1 
value is now 2

In the first loop, $i=0, which means that the expression, ($i<= 2), 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

$brush_price = 5; 
echo "<table border="1" align="center">"; 
echo "<tr><th>Quantity</th>"; 
echo "<th>Price</th></tr>"; 
for ( $counter = 10; $counter <= 100; $counter += 10) { 
echo "<tr><td>"; 
echo $counter; 
echo "</td><td>"; 
echo $brush_price * $counter; 
echo "</td></tr>"; 
} 
echo "</table>";

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:

<?php 
/* 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 maybe the fourth), but the user may find that using an empty expression in a for loop is convenient in many situations.

PHP also supports an alternative syntax for for loops using colons.

for (expr1; expr2; expr3): 
statement; 
... 
endfor;

We often need to traverse the following array:

<?php 
/* 
* 我们想要在遍历的过程中改变以下数组中某些元素的值 
*/ 
$people = Array( 
Array(&#39;name&#39; => &#39;Kalle&#39;, &#39;salt&#39; => 856412), 
Array(&#39;name&#39; => &#39;Pierre&#39;, &#39;salt&#39; => 215863) 
); 
for($i = 0; $i < sizeof($people); ++$i) 
{ 
$people[$i][&#39;salt&#39;] = rand(000000, 999999); 
} 
?>

The problem with the above code is that the second expression of for will cause the code Execution is slow - because the length of the array must be calculated each time through the loop. 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:

<?php 
$people = Array( 
Array(&#39;name&#39; => &#39;Kalle&#39;, &#39;salt&#39; => 856412), 
Array(&#39;name&#39; => &#39;Pierre&#39;, &#39;salt&#39; => 215863) 
); 
for($i = 0, $size = sizeof($people); $i < $size; ++$i) 
{ 
$people[$i][&#39;salt&#39;] = rand(000000, 999999); 
} 
?>

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