快速排序 C++ 代码错误分段错误 11？

Question

Code:

#include <iostream>
#include <stdlib.h>
#include <time.h>
using namespace std;
void partition(int *A, int start, int end){ //end is the position of the last number
    cout<<"start is "<<start<<" end is "<<end<<endl;
    if(end - start+1 > 1){
        int i = start + 1, j = start + 1, pivot = A[start];
        while(j <= end){
            if(A[j] <= pivot){
                swap(A[j], A[i]);
                i++;
            }
            j++;
        }
        swap(A[start], A[i-1]);
        partition(A, start, i-1);
        partition(A, i, end);
    }
    
}

void QuickSort(int *A, int n){
    partition(A, 0, n-1);
}


int main() {
    int n, method;
    string sortMethod[ ] = {
            "Insertion sort", "Selection sort", "Bubble sort", "Merge sort", "Quick sort"
    };
    int m = sizeof(sortMethod)/sizeof(string);
    srand(time(0));
    
    while (true){
        cout << "Number of test marks: ";
        cin >> n;
        if ( n==0 ) break;
        int *A = new int[n];
        for (int i=0; i < n; i++) cin>>A[i];
        //A[i] = rand()%101;

        for (int i=0; i < n; i++)cout << A[i] << " ";
        cout<<endl<<endl;


        cout << "Sorting method: \n"; 
        for (int i=0; i < m; i++) cout << i+1 << ": " << sortMethod[i] << endl;
        // cin >> method;
        method = 5;
        cout<<endl;
        int t = time(0);
        
        QuickSort(A,n);

        if (method > m) continue;       
        cout << sortMethod[method-1] << endl;
        if (n <= 100) {
            for (int i=0; i < n; i++) {
                if (i> 0 && i%10==0) cout << endl;
                cout << A[i] << " ";
            }
            cout << endl;
        }
        else {
            cout << "Sorting completed in " <<  time(0) - t  << " sec.\n";
        }
        cout << endl;
    } 
    cout << "Program ends here."<<endl;
    return 0;
}

When some of my input numbers have the same value, I get a "Segmentation fault: 11".

For example, an input of "4 7 7 3 1" would produce infinite lines of the following output: "start is 1 end is 3".

May I ask how should I improve my code? I know that Segfault 11 happens when you try to access an invalid memory, and i think that's likely what I did here, although i cant seem to identify the error. Thanks!

Answer 1

So this is what's happening here. Imagine that in a certain partition the first element (the pivot ) is also the largest.

This means that in every iteration of the while loop the value of A[j]<=pivot will be true and i will be increased.

Since both i and j are increased by 1 in every iteration there value is the same, so at the end of the loop they are both equal to end+1 .

Then you are invoking partition(A, start, i-1) which is really the same as partition(A, start, end) - meaning you have an infinite recursion which continues until you reach... well, stack overflow and the program crush with segfault.

---

The minimal fix will be to make sure that the sub partitions are strictly smaller than the original one by excluding the pivot from them. So something like this:

        partition(A, start, i-2); // i-1 is the pivot, so we don't need to include it
        partition(A, i, end);