I'm a beginner and practising C selection sort but getting different output for different values

Question

I have below code for performing selection sort:

#include<stdio.h>
    int main(){
       //selection_sort(int,int);
       int a[] = {55,932,53,98};
       int n = sizeof(a)/sizeof(a[0]);
    //    printf("%d \n",n);
       
       printf("Array before sorting: ");
       for (int i = 0; i < n; i++){
           printf("%d ", a[i]);
       }
       selection_sort(a,n);
       printf("Array after sorting: ");
       for (int k = 0; k < n; k++){
           printf("%d ", a[k]);
       }
    }
    int selection_sort(int a[], int n){
        int i,j,minidx,tmp;
        //int a[size];
        for (i = 0; i < n - 1 ; i++){
            minidx = i;
            for (j = i + 1; j < n; j++){
                if (a[minidx] > a[j]){
                        minidx = j;
                        swap_elements(&a[minidx], &a[i]);
                }
            }
        }
    }
    int swap_elements(int *a, int *b){
        int tmp = *a;
        *a = *b;
        *b = tmp;
    }

I have tried different values for the array and it is randomizing the accuracy. What am I doing I'm not supposed to do?

Answer 1

Your algorithm for selection sort is correct apart from the call to swap elements. The elements should be swapped once the minimum element has been found.

#include<stdio.h>
int swap_elements(int *a, int *b){
    int tmp = *a;
    *a = *b;
    *b = tmp;
}
int selection_sort(int a[], int n){
    int i, j, minidx;
    for (i = 0; i < n - 1 ; i++){
        minidx = i;
        for (j = i + 1; j < n; j++){
            if (a[minidx] > a[j]){
                minidx = j;
            }
        }
        swap_elements(&a[minidx], &a[i]);
    }
}

int main(){
   int a[] = {64, 25, 12, 22, 11};
   int n = sizeof(a) / sizeof(a[0]);
   
   printf("Array before sorting: ");
   for (int i = 0; i < n; i++){
       printf("%d ", a[i]);
   }
   selection_sort(a, n);
   printf("Array after sorting: ");
   for (int k = 0; k < n; k++){
       printf("%d ", a[k]);
   }
}

Try this code instead.