尽管预增量在此代码中有效，但为什么后增量运算符不起作用? [英] Why does post increment operator not work although preincrement does work in this code?

问题描述

我真的是编程新手(我是一名电子和通信工程师)，我无法弄清楚为什么一个程序有效而另一个程序无效.

I am really new to programming (I'm an electronics and comm. engineer) and I am not able to figure out why one program works and the other one doesn't.

在继续学习之前，我希望对递归函数有一个很好的了解.对此，我将不胜感激.

I'd like to get a good understanding of recursive functions before going any further with my learning. I would appreciate any help regarding this.

我知道x ++和--x之间的区别.但是在这个程序的上下文中，我认为这两个程序应该以相同的方式运行.但是他们没有.

I know the difference between the x++ and --x. But in this context of this program, I thought both of these programs should run the same way. But they don't.

void rec(int x)
{
    if(x>0)
        rec(x--);
    printf("%d",x);
}

int main()
{
    rec(4);
    _getch();

} /*This doesn't work. And shows a stackoverflow */

void rec(int x)
{
    if(x>0)
        rec(--x);
    printf("%d",x);
}

int main()
{
    rec(4);
    _getch();

} /*This gives a proper output as expected*/ 
/*The output is 00123*/

谢谢！

附言:如果这是一个琐碎或愚蠢的问题，请原谅我，但我坚持这个问题，我认为这是我可以寻求帮助的最佳地方.

P.S: Forgive me if this is a trivial or stupid question, but I am stuck on this and I thought this is the best place I can look for help.

推荐答案

void rec(int x) {
    if (x>0)
        rec(x--);
    printf("%d",x);
}

这将永远递归(或者至少直到耗尽堆栈空间为止). x--表示使用x的 current 值，然后然后减小它.

This will recurse forever (or at least until you exhaust your stack space). x-- means use the current value of xand then decrement it.

换句话说，让我们使用参数20调用rec.该参数大于零，因此它将依次使用当前值20 then 减量x再次调用rec. (但实际上是在之后调用返回.

In other words, let's call rec with the parameter 20. That's greater than zero, so it will in turn call rec again with the current value of 20 then decrement x (but effectively after the call returns.

因此，您将永远用值20调用rec，这就是为什么要耗尽堆栈的原因.

Hence you're forever calling rec with the value of 20, which is why you're blowing out your stack.

如果将x--更改为--x，则会将x 递减，然后再将其传递给函数，因此它将变为20, 19, 18, ... 1, 0，这时它将重新运行堆叠打印所有这些值.

If you change x-- to --x, it decrements x before passing it to the function, hence it will go 20, 19, 18, ... 1, 0, at which point it will run back up the stack printing all those values.

如果在if语句之前放置了printf ("before: %d\n", x)，您将看到很多20行输出到屏幕上.

If you had put a printf ("before: %d\n", x) before the if statement, you would have seen a lot of 20 lines output to the screen.

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