(2-4 = -1)当int值分配给C中的指针吗? [英] (2 - 4 = -1) when int value assigned to pointer in C?

问题描述

我无法理解为什么在此程序2-4中给出-1，它已经将int值分配给了指针而不是地址，我知道，但是当我编译它时，编译器给出了一些警告，但是编译了程序并执行了，但是...

I am unable to get that why in this program 2 - 4 gives -1, it has assigned int values to pointers rather than addresses, I know but while I compiled it compiler gave some warnings but compiled the program and it executed but...

程序

#include<stdio.h>

int main(void) {

    int *p, *q;

    int arr[] = {1,2,3,4};

    // I know p and q are pointers and address should be assigned to them
    // but look at output, why it evaluates (p-q) to -1 while p as 2 and q as 4

    p = arr[1];
    q = arr[3];

    printf("P-Q: %d, P: %d, Q: %d", (p - q), p, q);

    return 0;
}

它给出了

P-Q: -1, P: 2, Q: 4

推荐答案

严格来说，发生的事情完全取决于您的编译器和平台...但是让我们假设我们使用的是典型的编译器，而忽略了警告.

Strictly speaking, what happens depends entirely on your compiler and platform... but let's assume we're using a typical compiler and ignoring the warnings.

让我们进一步简化您的问题:

Let's simplify your question further:

p = 2;
q = 4;

printf("P-Q: %d, P: %d, Q: %d", (p - q), p, q);

产生相同的古怪结果:

P-Q: -1, P: 2, Q: 4

正如@gsamaras指出的那样，我们正在尝试减去两个指针.让我们尝试看看这怎么会导致-1:

As @gsamaras pointed out, we're trying to subtract two pointers. Let's try and see how this might result in -1:

p - q = (2 - 4) / sizeof(int)
      = (-2)    / 4
      = -1

我建议尝试几个您自己的p和q值以查看会发生什么.

I suggest trying a couple of your own p and q values to see what happens.

p和q不同的示例:

p - q = ??
==========
0 - 0 =  0
0 - 1 = -1
0 - 2 = -1
0 - 3 = -1
0 - 4 = -1
1 - 0 =  0
1 - 1 =  0
1 - 2 = -1
1 - 3 = -1
1 - 4 = -1
2 - 0 =  0
2 - 1 =  0
2 - 2 =  0
2 - 3 = -1
2 - 4 = -1
3 - 0 =  0
3 - 1 =  0
3 - 2 =  0
3 - 3 =  0
3 - 4 = -1
4 - 0 =  1
4 - 1 =  0
4 - 2 =  0
4 - 3 =  0
4 - 4 =  0

在以下位置使用gcc -fpermissive生成:

#include <stdio.h>

int main() {
    printf("p - q = ??\n");
    printf("==========\n");

    for (int i = 0; i < 5; ++i) {
        for (int j = 0; j < 5; ++j) {
            int* p = i;
            int* q = j;

            printf("%d - %d = %2d\n", p, q, (p - q));
        }
    }

    return 0;
}

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