[SOLVED] Selection sort algorithm using pointer arithmetic

Issue

I need to sort elements of array in ascending order using selection sort algorithm and pointer arithmetic.

That means the following (using pointer arithmetic):

• find the minimum element in unsorted array;
• swap the found minimum element with the first element
• repeat it until the end of array
• print sorted array

Code:

#include <stdio.h>
void swap(double **pp, double **qq) {
  double *temp = *pp;
  *pp = *qq;
  *qq = temp;
}
void sortArray(double arr[], int n) {
  double *q, *min;
  q = min = arr;
  while (min > arr + n) {
    while (q < arr + n) {
      if (*q < *min) 
        min = q;
        q++;
      }
      min++;
      swap(&min, &q);
  }
}
void writeSorted(double arr[], int n) {
  double *qq = arr;
  while (qq < arr + n) {
    printf("%g ", *qq);
    qq++;
  }
}
int main() {
  double arr[4] = {2.1, 4.23, 3.67, 1.5};
  int n = 4;
  sortArray(arr, n);
  writeSorted(arr, n);
  return 0;
}

This code prints the same unsorted array. Do you know how to fix it?

Solution

There is an error about the role of swap: you have to swap the elements, not the corresponding pointers.

Moreover, there is a confusion about definition and role of each pointer.
In particular, it is important to keep trace of the pointer to the start of next iteration.

#include <stdio.h>
void swap(double *pp, double *qq) {
  double temp = *pp;
  *pp = *qq;
  *qq = temp;
}
void sortArray(double arr[], size_t n) {
  double *start = arr;
  while (start < arr + n) {
      double *q = start + 1;
      double *min = start;
      while (q < arr + n) {
          if (*q < *min) min = q;
          q++;
      }
    swap (start, min);
    start++;
  }
}
void writeArray(double arr[], size_t n) {
  double *qq = arr;
  while (qq < arr + n) {
    printf("%g ", *qq);
    qq++;
  }
  printf ("\n");
}
int main() {
  double arr[] = {2.1, 4.23, 3.67, 1.5};
  size_t n = sizeof(arr)/sizeof(*arr);
  writeArray (arr, n);
  sortArray(arr, n);
  writeArray(arr, n);
  return 0;
}

Besides, I don’t know what are exactly your constraints for this exercise. Even by using pointers, some simplifications are possible. For example, for the print function:

void writeArray(double arr[], size_t n) {
  while (n--) {
    printf("%g ", *arr++);
  }
  printf ("\n");
}

Answered By – Damien

Answer Checked By – Marilyn (BugsFixing Volunteer)