使用负数作为数组索引 [英] Using negative number as array index

问题描述

我沿着询问以下的输出有竞争力的问题就来了：

I came along a competitive question that asks the output of the following:

#include <stdio.h>
int main()
{
    int a[] = {0,1,2,3,4};
    int i, *ptr;
    for(ptr = a+4, i=0; i <=4; i++)
    printf("%d", ptr[-i]);
    return 0;
}

我看过这个话题：负数组索引用C 的不过是我不清楚如何-ve符号生成以相反的顺序，即该阵列。 4，3，2，1，0 。

I did read this topic: Negative array indexes in C? However it was unclear to me how the -ve symbol generates the array in the reverse order, ie. 4, 3, 2, 1, 0.

推荐答案

首先，记得在C中的前pression PTR [指数] 意味着同样的事情为 *（PTR +指数）。

First, recall that in C the expression ptr[index] means the same thing as *(ptr+index).

现在让我们来看看再次你的前pression： PTR 前设置为 A + 4 循环;然后你申请 -i 指标吧。因此，等效指针运算前pression将如下：

Now let's look at your expression again: ptr is set to a+4 before the loop; then you apply -i index to it. Therefore, the equivalent pointer arithmetic expression would be as follows:

printf("%d", *(a+4-i));

这前pression遍历数组倒退，生产，你看到的结果。

This expression iterates the array backwards, producing the results that you see.

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