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📜  Perl-循环

📅  最后修改于: 2020-11-02 03:36:40             🧑  作者: Mango

在某些情况下，您需要多次执行一个代码块。通常，语句是按顺序执行的：函数的第一个语句首先执行，然后第二个执行，依此类推。

编程语言提供了各种控制结构，允许更复杂的执行路径。

循环语句使我们可以多次执行一个语句或一组语句，以下是大多数编程语言中循环语句的一般形式-

Perl中的循环架构

Perl编程语言提供了以下类型的循环来处理循环需求。

Sr.No. Loop Type & Description
1 while loop

Repeats a statement or group of statements while a given condition is true. It tests the condition before executing the loop body.
2 until loop

Repeats a statement or group of statements until a given condition becomes true. It tests the condition before executing the loop body.
3 for loop

Executes a sequence of statements multiple times and abbreviates the code that manages the loop variable.
4 foreach loop

The foreach loop iterates over a normal list value and sets the variable VAR to be each element of the list in turn.
5 do…while loop

Like a while statement, except that it tests the condition at the end of the loop body
6 nested loops

You can use one or more loop inside any another while, for or do..while loop.

循环控制语句

循环控制语句从其正常顺序更改执行。当执行离开作用域时，在该作用域中创建的所有自动对象都将被销毁。

Perl支持以下控制语句。单击以下链接以查看其详细信息。

Sr.No. Control Statement & Description
1 next statement

Causes the loop to skip the remainder of its body and immediately retest its condition prior to reiterating.
2 last statement

Terminates the loop statement and transfers execution to the statement immediately following the loop.
3 continue statement

A continue BLOCK, it is always executed just before the conditional is about to be evaluated again.
4 redo statement

The redo command restarts the loop block without evaluating the conditional again. The continue block, if any, is not executed.
5 goto statement

Perl supports a goto command with three forms: goto label, goto expr, and goto &name.

无限循环

如果条件永远不会为假，则循环将变为无限循环。传统上， for循环用于此目的。由于形成for循环的三个表达式都不是必需的，因此可以通过将条件表达式保留为空来进行无限循环。

#!/usr/local/bin/perl
 
for( ; ; ) {
   printf "This loop will run forever.\n";
}

您可以通过按Ctrl + C键终止上述无限循环。

当条件表达式不存在时，假定它为真。您可能有一个初始化和增量表达式，但是作为程序员，更常见的是使用for(;;)构造来表示一个无限循环。