How to use while loop in php

There are two kinds of while loops in php: 1. While loop statement, first judge the loop condition. When the condition is satisfied, the loop body will be executed. If it is not satisfied, it will stop; the syntax "while (expression) { Loop body statement block;}". 2. The "do while" loop statement first executes the "statement block" in the loop body once, and then judges the loop condition. If it is true, the loop continues, if it is false, the loop is terminated; the syntax "do {Loop body statement block;}while(expression);".

The operating environment of this tutorial: windows7 system, PHP8 version, DELL G3 computer

The loop statement can repeatedly execute a certain condition if the conditions are met. an operation. PHP provides 4 loop control statements, namely while, do while, for and foreach loop statements. In this article we mainly introduce the while and do while loop statements.

1. while loop statement

while loop statement is awhen typeloop statement, first check the loop condition Make a judgment. When the condition is satisfied, the loop body is executed. If it is not satisfied, it stops.

Function:Repeat an operation until the specified condition is not true.

Features:Judge the expression first, and execute the corresponding statement when the expression result is true.

The general form of while loop is:

while(表达式){ //表达式为循环条件 PHP语句块; //PHP语句块为循环体 }

Statement analysis:

• First calculate the value of "expression", When the value is true, execute the "PHP statement block" in the loop body;

  Description: The calculation result of "expression" is of Boolean type (TRUE or FALSE). If it is a value of other types It will also be automatically converted to a Boolean value (because PHP is a weak language type and will automatically convert the variable into the correct data type based on the value of the variable).
  

  "Statement block" is a collection of one or more statements surrounded by{ }; if there is only one statement in the statement block, you can also omit{ }.
  

• After the execution is completed, return to the expression and calculate the value of the expression again for judgment. When the expression value is true, continue to execute the "statement block" ...This process will be repeated

• until the value of the expression is false before jumping out of the loop and executing the statement below while.

The flow chart of the while statement is as follows:

Usually the "expression" is using comparison Value calculated by operator or logical operator

Example: Calculate the value of 1 added to 100

Output result:

While loops and if statements can be nested together in multiple layers. For example, we can use a two-layer nested while loop to output a table. The code is as follows:

'; $x = 0; while ($x < 10) { echo ''; $y = 0; while ($y < 10) { echo ''.($x*10+$y).''; $y++; } echo ''; $x++; } echo ''; ?>

Note:"Infinite loop" - a loop that cannot be terminated by its own control

If the expression is always true, the loop condition is always true, Then the while loop will continue to execute and never end, becoming an "infinite loop"

After running the program, the value of the variable$iwill be output until the user forces it to close.

2. do-while loop statement

do-while loop statement is a "until type" loop Statement, it first executes the "statement block" in the loop body once, and then judges the loop condition. If it is true, the loop continues, if it is false, the loop is terminated.

Therefore: Regardless of the result of the expression, the do-while loop statement will execute the "statement block" at least once.

Characteristics of the do-while loop statement:Execute the loop body first, and then determine whether the loop condition is true.

The general form of do-while loop is:

do{ 语句块; }while(表达式);//表达式为循环条件

Description:

• and while Like loops, the calculation result of the "expression" in the do-while loop must be Boolean TRUE or FALSE. If it is a value of other types, it will be automatically converted to a Boolean value.

• The semicolon;at the end of the do-while statement cannot be omitted (it must be present). It is part of the syntax of the do while loop.

Statement analysis:

• First execute the "statement block" in the loop body once, and then determine the value of the "expression" , when the value of "expression" is true, return to re-execute the statement block in the loop body... This process will be repeated

• until the value of the expression is false, jump out Loop, at this point the loop ends and the following statements are executed.

The flow chart of the do-while loop statement is as follows:

示例：

还是之前文章的例子：计算1加到100的值

输出结果：

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