C++ why is there a segmentation fault in my pointer selection sort?

Question

Below is my c++ code. I am trying to implement a selection sort using pointers (start and end). The code compiles, but I am getting a segmentation fault before it will sort the random generated list (currently only prints the random numbers).

Any help as to why this is and how to fix it would be greatly appreciated.

#include<stdio.h>
#include<stdlib.h>
#include <iostream>

using namespace std;

void selectionSort(int *start, int *stop) {
   for (int i = *start; i < *stop - 1; ++i) {
     int min = i;
       for (int j = i + 1; j < *stop; ++j) {
         if ((&start[0])[j] < (&start[0])[min])
            min = j;
         }
     swap((&start[0])[i], (&start[0])[min]);
   }
}


int main()
{
  int size = 10;
    int* data = new int[size];
    for (int i = 0; i < size; ++i)
   {
      data[i] = rand() % size;
    }
    for (int k = 0; k < size; k++)
    {
      cout << data[k] << " ";
    }
    cout << endl;
    selectionSort(data, data+size);
    for (int j = 0; j < size; j++)
    {
      cout << data[j+1] << " ";
    }
    return 0;
}

Answer 1

The general logic in your function is in the right direction. However, you seem to be confused between values of the elements of the array and the indexing used to access the elements of the array.

The line

for (int i = *start; i < *stop - 1; ++i)

shows the first signs of the confusion.

1. You are initializing i with the value of the first element of the array and incrementing the value in the subsequent iterations of the loop. That is not correct. Incrementing the value of the first element of the array does not make logical sense.

2. *stop causes undefined behavior since stop points to a place one past the last valid element.

You need to use int* i, int* j, and int* min to properly sort the elements. That also means updating almost the entire function accordingly. Here's an updated function that works for me.

void selectionSort(int *start, int *stop) {
   for (int* i = start; i < (stop - 1); ++i) {
      int* min = i;
      for (int* j = i + 1; j < stop; ++j) {
         if (*j < *min)
         {
            min = j;
         }
      }
      swap(*i, *min);
   }
}

Also, the following lines in main are not correct. You end up accessing the array using an out of bounds index.

for (int j = 0; j < size; j++)
{
   cout << data[j+1] << " ";
}

Replace them by

for (int k = 0; k < size; k++)
{
   cout << data[k] << " ";
}