奇怪的XOR交换行为，同时清零数据 [英] Weird XOR swap behavior while zeroing out data

问题描述

谢谢Doug。这里是修复：

Thanks Doug. Here's the fix:

void swap(int& a, int& b) {
    if (&a == &b) // added this check to ensure the same address is not passed in
        return;

    a ^= b;
    b ^= a;
    a ^= b;
}

我正在C ++中实现快乐的乐趣，我使用整数的虚拟数据。我一直在使用XOR交换算法来交换两个值到位，但我注意到我的排序是拧紧。我改变了我的交换算法，它的工作。我添加了一些调试语句，发现XOR交换正在做一些奇怪的事情。

I am implementing quicksort for fun in C++, and I am using integers for dummy data. I had been using the XOR swapping algorithm to swap two values in place, but I noticed my sort was screwing up. I changed my swapping algorithm and it worked. I added some debugging statements, and found that the XOR swap was doing something weird.

我在交换之前和之后打印了数据，这是它打印的：

I printed the data before and after I swapped it, and this is what it printed:

...

swapping -5, -3
swapped  -3, -5

swapping -5, -5
swapped  0, 0     <---- What?

swapping -2, -4
swapped  -4, -2

...

这是我的代码：

// this might not be that great or even work as intended but it doesn't really matter for this problem
int av3index(int a[], int indx1, int indx2, int indx3) {
    if (a[indx3] <= max(a[indx1], a[indx2]) && a[indx3] >= min(a[indx1], a[indx2]))
        return indx3;

    if (a[indx2] <= max(a[indx1], a[indx3]) && a[indx2] >= min(a[indx1], a[indx3]))
        return indx2;

    if (a[indx1] <= max(a[indx2], a[indx3]) && a[indx1] >= min(a[indx2], a[indx3]))
        return indx1;
}

void swap(int& a, int& b) {
    /*
    This works
    int tmp = b;
    b = a;
    a = tmp;*/

    cout << "swapping " << a << ", " << b << endl;

    a ^= b;
    b ^= a;
    a ^= b;

    cout << "swapped  " << a << ", " << b << endl << endl;
}

void zqsort(int a[], int len) {
    if (len <= 1)
        return;

    int pivot = av3index(a, 0, len / 2, len - 1);

    swap(a[pivot], a[0]);

    int i = 1, j = len - 1;

    while (i <= j) {
        if (a[i] > a[0]) {
            while (i <= j && a[j] > a[0])
                --j;

            if (i <= j)
                swap(a[i], a[j]);
        }

        ++i;
    }

    swap(a[0], a[j]);

    zqsort(a, len / 2);
    zqsort(a + len / 2, len - len / 2);
}

int main() {
    int values[] = {5, 4, 3, 2, 1, 0, -1, -2, -3, -4, -5};

    int len = sizeof(values) / sizeof(int);

    int* arr = new int[len];

    for (int i = 0; i < len; ++i)
        arr[i] = values[i];

    zqsort(arr, len);

    cout << "sorted array:" << endl;
    for (int i = 0; i < len; ++i)
        cout << arr[i] << endl;

    cin.get();
}

我没有使用quicksort代码的任何引用，所以它可能是错误的，但我不认为这与问题密切相关。

I didn't use any references for the quicksort code so it might be wrong, but I don't think that's germane to the problem.

推荐答案

您的互换 a 和 b 是相同的位置。 XOR黑客只有在他们在不同的位置时才有效。

Your swap a and b are the same location. The XOR hack only works when they are different locations.

我想在C;这里有一个表格：

I think in C; here's a table:

           &a != &b  &a == &b
           *a   *b   *a   *b
           -5   -5   -5   -5
*a ^= *b;   0   -5    0    0
*b ^= *a;   0   -5    0    0
*a ^= *b;  -5   -5    0    0

这篇关于奇怪的XOR交换行为，同时清零数据的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！