Undesired behaviour in MergeSort function - c

Question

I'm new to programming,I'm trying to implement a merge sort function into my program, but it's not working correctly. I went over and over the code but I can't find the problem.
If for example the user input for a 6 element array is : 3 2 4 1 6 7
The output is: 1 3 2 4 32708 32708
Can someone help me? Also, if anyone have any advice for improving my coding style would be much appreciated.Thanks.
#include <stdio.h>
#include <stdlib.h>
int main(void) {
int *a, n;
a = malloc(100 * sizeof(int)); // dynamically allocating memory for original array
if (a == NULL)
return 1;
printf("Enter n of elements in the array:");
scanf("%i", &n); // n of elements the in array
printf("Enter elements:\n");
for (int i = 0; i < n; i++) {
scanf("%i", &a[i]); // prompt user for input elements
}
int f, l, m, n1, n2; // declaring variables
f = 0; // first element
l = n - 1; // last element
m = (f + l) / 2; // mid point
n1 = m + 1 - f; // n elements l1
n2 = l - m; // n elements l2
int l1[n1]; // temp array 1
int l2[n2]; // temp array2
for (int i = 0; i < n1; i++) {
l1[i] = a[i]; // copy elements into temp l1
}
for (int j = 0; j < n2; j++) {
l2[j] = a[m + 1 + j]; // copy elements into temp l2
}
int i, j, k; // variable for arrays index
i = 0;
j = 0;
k = 0;
//sorting and copying elements in original array
while (i < n1 && j < n2) {
if (l1[i] <= l2[j]) { // if element l1 smaller or equal to l2 element
a[k] = l1[i]; // copy element l1 into original array
i++; // increment l1
} else { // if element l1 bigger than l2
a[k] = l2[j]; // copy element l2 into original array
j++; // increment l2
}
k++; // increment original array
}
// copy remaining elements (if any)
while (i < n1) {
a[k] = l1[i];
i++;
k++;
}
while (j < n2) {
a[k] = l2[i];
j++;
k++;
}
printf("Your sorted array:\n");
for (int d = 0; d < n; d++) {
printf("%i ", a[d]); // print sorted array
}
printf("\n");
free(a); // freeing original array
}

Answer 1

You need to merge recursively. You wrote only the merge part and not the recursive sort function.
More info: https://www.geeksforgeeks.org/merge-sort/
#include <stdio.h>
#include <stdlib.h>
void merge(int arr[], int l, int m, int r)
{
int i, j, k;
int n1 = m - l + 1;
int n2 = r - m;
/* create temp arrays */
int L[n1], R[n2];
/* Copy data to temp arrays L[] and R[] */
for (i = 0; i < n1; i++)
L[i] = arr[l + i];
for (j = 0; j < n2; j++)
R[j] = arr[m + 1 + j];
/* Merge the temp arrays back into arr[l..r]*/
i = 0; // Initial index of first subarray
j = 0; // Initial index of second subarray
k = l; // Initial index of merged subarray
while (i < n1 && j < n2) {
if (L[i] <= R[j]) {
arr[k] = L[i];
i++;
} else {
arr[k] = R[j];
j++;
}
k++;
}
/* Copy the remaining elements of L[], if there are any */
while (i < n1) {
arr[k] = L[i];
i++;
k++;
}
/* Copy the remaining elements of R[], if there are any */
while (j < n2) {
arr[k] = R[j];
j++;
k++;
}
}
void mergeSort(int arr[], int l, int r)
{
if (l < r) {
// Same as (l+r)/2, but avoids overflow for
// large l and h
int m = l + (r - l) / 2;
// Sort first and second halves
mergeSort(arr, l, m);
mergeSort(arr, m + 1, r);
merge(arr, l, m, r);
}
}
int main(void)
{
int *a, n;
printf("Enter n of elements in the array:");
scanf("%i", &n); //n of elements the in array
a = malloc(n * sizeof(int)); //dynamically allocating memory for original array
if (a == NULL)
return 1;
printf("Enter elements:\n");
for (int i = 0; i < n; i++) {
scanf("%i", &a[i]); //prompt user for input elements
}
mergeSort(a, 0, n - 1);
printf("Your sorted array:\n");
for (int d = 0; d < n; d++) {
printf("%i ", a[d]); //print sorted array
}
printf("\n");
free(a); //freeing original array
return 0;
}