Merge Sort using recursion in C

merge_sort.c

/**
  * Divide  : Divide the n-element array into two n/2-element
  *           sub-arrays
  * Conquer : Sort the two sub-arrays recursively using
  *           merge sort
  * Combine : Merge the two sorted subsequences to form the
  *           sorted array
  **/

#include<stdio.h>

int arr[20];       // array to be sorted

int main()
{
  int n,i;

  printf("Enter the size of array\n");  // input the elements
  scanf("%d",&n);
  printf("Enter the elements:");
  for(i=0; i<n; i++)
    scanf("%d",&arr[i]);

  merge_sort(arr,0,n-1);  // sort the array

  printf("Sorted array:");  // print sorted array
  for(i=0; i<n; i++)
    printf("%d",arr[i]);

  return 0;
}

int merge_sort(int arr[],int low,int high)
{
  int mid;
  if(low<high) {
    mid=(low+high)/2;
    // Divide and Conquer
    merge_sort(arr,low,mid);
    merge_sort(arr,mid+1,high);
    // Combine
    merge(arr,low,mid,high);
  }

  return 0;
}

int merge(int arr[],int l,int m,int h)
{
  int arr1[10],arr2[10];  // Two temporary arrays to
  hold the two arrays to be merged
  int n1,n2,i,j,k;
  n1=m-l+1;
  n2=h-m;

  for(i=0; i<n1; i++)
    arr1[i]=arr[l+i];
  for(j=0; j<n2; j++)
    arr2[j]=arr[m+j+1];

  arr1[i]=9999;  // To mark the end of each temporary array
  arr2[j]=9999;

  i=0;
  j=0;
  for(k=l; k<=h; k++) { //process of combining two sorted arrays
    if(arr1[i]<=arr2[j])
      arr[k]=arr1[i++];
    else
      arr[k]=arr2[j++];
  }

  return 0;
}

I'm getting a runtime error for thiscode

include<stdio.h>

void merge_sort(int [],int ,int );
void merge(int [], int ,int ,int );

int main()
{
int n,A[n+1],p,r,i;
printf("enter the size of the array\n");
scanf("%d",&n);
printf("enter the elements to be sorted\n");

for(i=0;i<n;i++)
{
    scanf("%d",&A[i]);
}

p=0;

r=n-1;

merge_sort(A,p,r);

printf("the sorted list  is\n");

for(i=0;i<n;i++)
{
    printf("%d\t",A[i]);
}

return 0;

}

void merge_sort(int A[],int p,int r)
{
int q;

if(p<r)
{
    q=(p+r)/2;
    merge_sort(A,p,q);
    merge_sort(A,q+1,r);
    merge(A,p,q,r);
}

}
void merge(int A[],int p, int q , int r)
{
int n1,n2,L[n1],R[n2],i,j,k;
n1=q-p+1;
n2=r-q;

for(i=0;i<n1;i++)
{
    L[i]=A[p+i];
}
L[i]=9999999;

for(j=0;j<n2;j++)
{
    R[j]=A[q+j+1];
}
R[j]=9999999;

i=0;
j=0;

for(k=p;k<=r;k++)
{
    if(L[i]<=R[i])
    {
        A[k]=L[i++];
    }

    else 
    {
        A[k]=R[j++];
    }
}

}

#include<stdio.h>

int arr[20]; // array to be sorted

int merge(int arr[],int l,int m,int h)
{
int arr1[10],arr2[10]; // Two temporary arrays to hold the two arrays to be merged

int n1,n2,i,j,k;
n1=m-l+1;
n2=h-m;

for(i=0; i<n1; i++)
arr1[i]=arr[l+i];
for(j=0; j<n2; j++)
arr2[j]=arr[m+j+1];

arr1[i]=9999; // To mark the end of each temporary array
arr2[j]=9999;

i=0;
j=0;
for(k=l; k<=h; k++) { //process of combining two sorted arrays
if(arr1[i]<=arr2[j])
arr[k]=arr1[i++];
else
arr[k]=arr2[j++];
}
return 0;
}

int merge_sort(int arr[],int low,int high)
{
int mid;
if(low<high) {
mid=(low+high)/2;
// Divide and Conquer
merge_sort(arr,low,mid);
merge_sort(arr,mid+1,high);
// Combine
merge(arr,low,mid,high);
}

return 0;
}

int main()
{
int n,i;

printf("Enter the size of array\n"); // input the elements
scanf("%d",&n);
printf("Enter the elements:");
for(i=0; i<n; i++)
scanf("%d",&arr[i]);

merge_sort(arr,0,n-1); // sort the array

printf("Sorted array:"); // print sorted array
for(i=0; i<n; i++)
printf("%d ",arr[i]);

return 0;
}

`

Hey can someone explain me what's happening at these 2 lines?
merge_sort(arr,low,mid); /This line calling function int merge_sort(int arr[],int low,int high) again? And passing value of high as same as new value of mid?/
merge_sort(arr,mid+1,high);/* Similarly,here this line calling function int merge_sort(int arr[],int low,int high) again? And passing value of low same as mid +1 ? */
What is the right explanation?

can you change this code to descending version?

Helped me a lot. Thanks.

https://gist.github.com/itrare/7602c8dc845ec8dd41fc42b0a5d1680d check this...

1. implement insertion sort,
2. generated 1 M random input
3. write the input in a file

if possible solve this problem