我不打算复制qsort算法。我正在练习编写qsort,这就是我想出来的,我对我的代码的哪一部分是错的感兴趣。请不要告诉我这是作业,因为我可以使用下面链接中的代码。
参考:http://xoax.net/comp/sci/algorithms/Lesson4.php
当这个运行时,我在控制台中得到它:
Program loaded.
run
[Switching to process 10738]
Running…
Current language: auto; currently c++
Program received signal: “EXC_ARITHMETIC”.
void myQSort(int min, int max, int* myArray)
{
// Initially find a random pivot
int pivotIndex = rand() % max;
int pivot = myArray[pivotIndex];
int i = 0 , j = max-1;
// Pointer to begining of array and one to the end
int* begin = myArray;
int* end = &myArray[max-1];
// While begin < end
while( begin < end )
{
// Find the lowest bound number to swap
while( *begin < pivot )
{
begin++;
}
while( *end > pivot )
{
// Find the highest bound number to swap
end--;
}
// Do the swap
swap(begin,end);
}
// Partition left
myQSort(0, pivotIndex-1, myArray);
// Partiion right
myQSort(pivotIndex+1,max, myArray);
}
EDIT-- 交换代码:
void swap(int* num, int* num2)
{
int temp = *num;
*num = *num2;
*num2 = temp;
}
答案 0 :(得分:3)
// sort interval [begin, end)
void myQSort(int* begin, int* end)
{
if(end - begin < 2)
return;
int* l = begin;
int* r = end - 1;
// Initially find a random pivot
int* pivot = l + rand() % (r - l + 1);
while(l != r)
{
// Find the lowest bound number to swap
while(*l < *pivot) ++l;
while(*r >= *pivot && l < r) --r;
// Do the swap
if(pivot == l) { pivot = r; }
std::swap(*l, *r);
}
// Here l == r and numbers in the interval [begin, r) are lower and in the interval [l, end) are greater or equal than the pivot
// Move pivot to the position
std::swap(*pivot, *l);
// Sort left
myQSort(begin, l);
// Sort right
myQSort(l + 1, end);
}
答案 1 :(得分:1)
您没有在任何地方使用代码中的min参数。您需要使用它设置开始和枢轴值。
答案 2 :(得分:1)
我试过制定上面的代码。但是,他们没有编译。
@Mihran:您的解决方案在算法上是正确的,但以下行会生成错误:
myQSort(min, begin - myArray, myArray);
这是因为begin
的类型为int *,myArray
的类型为long
,然后编译器会显示以下错误消息:
implicit conversion loses integer precision
这是C ++中的一个可行的解决方案:
#include <iostream>
using namespace std;
void mySwap(int& num1, int& num2){
int temp = num1;
num1 = num2;
num2 = temp;
}
void myQsort(int myArray[], int min, int max){
int pivot = myArray[(min + max) / 2];
int left = min, right = max;
while (left < right) {
while (myArray[left] < pivot) {
left++;
}
while (myArray[right] > pivot) {
right--;
}
if (left <= right) {
mySwap(myArray[left], myArray[right]);
left++;
right--;
}
}
if (min < right) {
myQsort(myArray, min, right);
}
if (left < max) {
myQsort(myArray, left, max);
}
}
int main()
{
int myArray[] = {1, 12, -5, 260, 7, 14, 3, 7, 2};
int min = 0;
int max = sizeof(myArray) / sizeof(int);
myQsort(myArray, min, max-1);
for (int i = 0; i < max; i++) {
cout<<myArray[i]<<" ";
}
return 0;
}
答案 3 :(得分:1)
这是一个明确的C ++实现,供参考:
#include <iostream>
#include <vector>
using namespace std;
int partition(std::vector<int>& arr, int low, int high) {
// set wall index
int wall_index = low;
int curr_index = low;
int pivot_elem = arr[high]; // taking last element as pivot_element
// loop through the entire received arr
for (int i = curr_index; i < high; ++i) {
// if element is less than or equal to pivot_elem
// swap the element with element on the right of the wall
// i.e swap arr[i] with arr[wall_index]
if (arr[i] <= pivot_elem) {
// swap
int temp = arr[wall_index];
arr[wall_index] = arr[i];
arr[i] = temp;
// move the wall one index to the right
wall_index++;
curr_index++;
} else {
// if the element is greater than the pivot_element
// then keep the wall at the same point and do nothing
curr_index++;
}
}
// need to swap the pivot_elem i.e arr[high] with the element right of the wall
int temp = arr[wall_index];
arr[wall_index] = arr[high];
arr[high] = temp;
return wall_index;
}
void quick_sort(std::vector<int>& arr, int low, int high) {
if (low < high) { // element with single arr always have low >= high
int split = partition(arr, low, high);
quick_sort(arr, low, split-1);
quick_sort(arr, split, high);
}
}
int main() {
std::vector<int> data = {6,13,8,4,2,7,16,3,8};
int N = data.size();
quick_sort(data, 0, N-1);
for (int i : data) {
cout << i << " ";
}
return 0;
}
答案 4 :(得分:0)
我没有在SO上看到Quicksort的干净实现,所以这是我易于理解的实现
请不要在生产代码中使用
这仅供您理解
// Swap position a with b in an array of integer numbers
void swap(int *numbers, int a, int b){
int temp = numbers[a];
numbers[a] = numbers[b];
numbers[b] = temp;
}
static int partition(int *data, int low, int high) {
int left = low, right = high, pivot = data[low];
while (left < right) {
// Everthing on the left of pivot is lower than the pivot
while ((left <= right) && data[left] <= pivot) // <= is because left is the pivot initially
left++;
// Everything on the right of the pivot is greater than the pivot
while((left <= right) && data[right] > pivot)
right--;
if (left < right)
swap(data, left, right);
}
// Put the pivot in the 'rigthful' place
swap(data, low, right);
return right;
}
// Quicksort
static void quick_sort(int *numbers, int low, int high)
{
if (high > low) {
int p_index = partition(numbers, low, high);
quick_sort(numbers, low , p_index - 1);
quick_sort(numbers, p_index + 1, high);
}
}