How to use the while keyword in PHP and what to pay attention to

The while keyword in PHP is a statement in a loop structure, which can repeatedly execute a piece of code based on specified conditions until the conditions are not met. In this article, we will introduce how to use the while keyword and some things to pay attention to.

First, take a look at the grammatical structure of the while keyword:

while (condition) {
    // 要执行的代码块
}

In this grammatical structure, condition is an expression that will be judged as true or false. The code block in the while loop will be executed only if the condition is true. After the code block is executed, the value of condition is judged again. If it is still true, the loop body continues to be executed, and so on. Only when condition is false, the loop terminates and the program will jump out of the loop and continue executing other code.

Below we use some examples to demonstrate the use of the while keyword.

Example 1: Calculate the cumulative sum from 1 to 10

$sum = 0;
$i = 1;
while ($i <= 10) {
    $sum += $i;
    $i++;
}
echo "1到10的累加和为：" . $sum;

In this example, the $sum variable is used to save the cumulative sum, and the $i variable is used to count. The loop condition is $i

Example 2: Output the ninety-nine multiplication table

$i = 1;
while ($i <= 9) {
    $j = 1;
    while ($j <= $i) {
        echo $i . " × " . $j . " = " . ($i * $j) . "  ";
        $j++;
    }
    echo "
";
    $i++;
}

In this example, the outer while loop is used to control the number of rows of the multiplication table, and the inner while loop is used to control each row. number of columns. The loop body first outputs the values ​​of $i and $j, and then outputs their product. After the inner loop ends, the line breaks once, and then the outer loop continues to execute and outputs the multiplication table of the next line.

In addition to not limiting the number of loops, the while loop also has some matters that need attention.

First of all, you need to ensure that the condition in the loop body can become a false value within a certain number of loops, otherwise it will fall into an infinite loop and cause the program to crash. To avoid this situation, we usually modify the value of the loop condition within the loop body. For example, in the loop body in Example 1, we gradually increase the value of $i until the value of $i is greater than 10. The loop condition is not met and the loop ends.

Secondly, there need to be enough logical judgments and control statements in the loop body to ensure the effectiveness of loop execution. For example, the inner loop in Example 2 uses a conditional judgment statement. Only when the value of $j is less than or equal to $i, will the result of the multiplication table be output. This avoids outputting invalid multiplication tables.

Finally, you need to pay attention to the initialization and update of loop conditions. The initialization of the loop condition is used to set the initial state at the beginning of the loop, and the update of the loop condition is used to modify the value of the loop condition to control the progress of the loop. In Example 1, the initial value of $i is set to 1. After each loop, the value of $i will be increased by 1.

In summary, the while keyword is an important statement in the loop structure in PHP. Through reasonable loop conditions and loop body design, we can implement various complex loop logics. When using the while keyword, we need to pay attention to the setting and updating of loop conditions, as well as the processing logic in the loop body, to ensure the correct execution of the loop.

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