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;
}
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.
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
...
// 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();
}
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