for c循环编程 [英] for loop in c programming

问题描述

如果我们在for循环中交换循环条件和增量表达式的位置将会发生什么（i = 0; i ++; i< 5）

和

for（i = 0; i< 5; i ++）





这两者之间有什么区别。

解决方案

嘿，这是你应该知道的东西：头部用';'字符分隔的那些东西for循环是表达式，所有这三个！两个for循环编译，但这应该给出一个警告： for（i = 0; i ++; i< 5）因为最后一个表达式没有副作用（即：它没有改变任何东西）你的编译器很聪明！正如我所说，所有三个都是表达式，但第一个用于初始化，第二个用于在每次执行循环体之前进行预测试，第三个表达式在每次执行循环体之后执行。



此c代码：

  for （expr1 ; expr2; expr3）
 {
 code_in_the_body_of_for_loop 
} 

可以这样写：

 expr1  //  初始化，如i = 0;编译器应该警告这个 
  //  没有副作用（如果它没有改变任何东西）喜欢我< 5） 
 
 而（ 1 ）< span class =code-comment> //  无限循环 
 {
  if （！expr2） //  执行循环体之前的预测试 
  break ; 
 
  //  这是for循环中的代码，请注意 
  //   for循环体中的continue语句 
 < span class =code-comment> //  跳转到expr3而不是for循环的开头!!!  
 code_in_the_body_of_for_loop 
 
 expr3;  //  通常是一些改变循环计数器的表达式 
  //  如i ++或i + = 5或ptr = ptr-> parent;  
  //  如果没有副作用，编译器应该发出警告。 
} 

在你的情况下，当预测试表达式（expr2）是i ++时，for循环从不执行正文代码，因为i ++的值为零，所以它立即从你的for循环中突破！如果expr3只是 i< 5 ，你编译器也应该发出警告，因为你通常不会写一个像 i <5; 这样的语句在你的代码中，你呢？ ：D

区别在于一个是有效的C而另一个根本不会编译。你为什么要问这个问题，而不仅仅是输入它，看看会发生什么？为什么在不试图让自己看的情况下问什么东西呢？

这是for循环的正确格式。

 for（i = 0; i< 5; i ++）

如果您尝试使用其他提到的..

 for（i = 0; i ++; i< 5）

编译器不会编译它并在此处指出错误。因为它与for循环的语法冲突。

what will happen if we exchange the postion of loop condition and increment expression in "for loop" in c

for(i=0;i++;i<5)
and
for(i=0;i<5;i++)


what is the difference betwwen these two.

解决方案

Hey, here is something you should know: Those thingies separated by ';' characters in the head of the for loop are expressions, all three of them! Both for loops compile but this should give a warning: for(i=0;i++;i<5) because the last expression doesn't have a side effect (ie: it does not change anything) and your compiler is smart! As I said all three thingies are expressions but the first is used for initialization, the second is for pre-testing before every execution of the loop-body, and the third expression is executed after each execution of the loop-body.

This c code:

for (expr1; expr2; expr3)
{
   code_in_the_body_of_for_loop
}

Can be written this way:

expr1            // initialization, like i=0; the compiler should warning if this
                 // has no side effect (if it doesnt change anything like i<5)

while (1)        // infinite loop
{
    if (!expr2)  // pre testing before execution of the loop-body
        break;

                 // this is the code inside the for loop, note that
                 // a "continue" statement in the body of the  for loop
                 // jumps to expr3 and not to the beginning of the for loop!!!
    code_in_the_body_of_for_loop

    expr3;       // usually some expressions that change the loop-counter
                 // like i++ or i+=5 or ptr = ptr->parent;
                 // the compiler should warning if this has no side effect.
}

In your case when the pre-test expression (expr2) is i++ the for loop never executes the body code because the value of i++ is zero so it breaks out from your for loop immediately! You compiler should also warning if expr3 is just i<5 because you normally never write a statement like i<5; in your code, do you? :D

The difference is that one is valid C and the other is not going to compile at all. Why would you ask this, and not just type it in and see what happens ? Why ask what something does without trying it for yourself to see ?

This is the right format in case of a for loop.

for(i=0;i<5;i++)

If you try with other one you mentioned..

for(i=0;i++;i<5)

The compiler will not compile it and point error here.Because it conflicts the grammar of a for loop.

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