Merge Sort-Segmentation fault - c

Question

I have written a program in C for the Merge Sort but I get a Segmentation Fault and I don't know why? Could you give me a hint what could be wrong?
That's the code:
#include <stdio.h>
#include <math.h>
void Merge(int A[],int p,int q,int r){
int n1,n2,i,j,k;
n1=q-p+1;
n2=r-q;
int L[n1],R[n2];
for (i=0; i<n1; i++) L[i]=A[p+i-1];
for (j=0; j<n2; j++) R[j]=A[q+j];
i=0;
j=0;
for (k=p; k<r; k++){
if (L[i]<=R[j]){
A[k]=L[i];
i=i+1;
}
else{
A[k]=R[j];
j=j+1;
}
}
}
void Sort(int A[],int p,int r){
int q;
if (p<r){
q=floor((p+r)/2);
Sort(A,p,q);
Sort(A,q+1,r);
Merge(A,p,q,r);
}
}
int main()
{
int n,i,p,r;
printf("Give a value for n: \n");
scanf("%d",&n);
int A[n];
for (i=0; i<n; i++){
printf ("Give %d th value of the array: \n",i+1 );
scanf("%d",&A[i]);
}
printf("Give me a value for p:\n");
scanf("%d",&p);
printf("Give me a value for r:\n");
scanf("%d",&r);
Sort(A,p,r);
printf("p=%d, r=%d \n \n", p,r);
for (i=0; i<n; i++) printf("%d", A[i]);
return 0;
}
That's what I got,giving inputs at the terminal:
Give a value for n:
8
Give 1 th value of the array:
1
Give 2 th value of the array:
6
Give 3 th value of the array:
5
Give 4 th value of the array:
3
Give 5 th value of the array:
5
Give 6 th value of the array:
4
Give 7 th value of the array:
2
Give 8 th value of the array:
7
Give me a value for p:
2
Give me a value for r:
6
Segmentation fault (core dumped)
Have I done maybe something wrong at the function Sort?

Answer 1

A probable culprit is these two lines:
int n1,n2,i,j,k;
int L[n1],R[n2];
In the first you declare n1 and n2, but don't initialize them. That means their value is indeterminate and that you should not use those variable until you have initialized them.
However, in the next line you use them anyway and that causes undefined behavior which means your whole program is ill-formed and can not be relied upon to work properly.

Answer 2

Your n1 and n2 variables in function Merge are undefined prior to you instantiating variables L[] and R[].
Try this:
int n1, n2, i, j, k;
n1 = q - p + 1;
n2 = r - q;
// Now you can instantiate your arrays
int L[n1], R[n2];

Answer 3

#include<stdio.h>
void mergesort(int a[],int beg,int end)
{
int mid;
if(beg<end)
mid = (beg+end)/2;
mergesort(a,beg,mid);
mergesort(a,mid+1,end);
merge(a,beg,mid,end);
}
void merge(int a[],int beg,int mid,int end)
{
int i=beg,j=mid+1,index=beg,temp[100],k;
while((i<=mid) && (j<=end))
{
if(a[i]<a[j])
{
temp[index]=a[i];
i=i+1;
}
else
{
temp[index]=a[j];
j++;
}
index++;
}
if(i>mid)
{
while(j<=end)
{
temp[index]=a[j];
j++;
index++;
}
}
else
{
while(i<=mid)
{
temp[index]=a[i];
i++;
index++;
}
}
for(k=beg;k<index;k++)
{
a[k]=temp[k];
}
}
int main()
{
int a[100],n,i,j;
printf("enter size of array");
scanf("%d",&n);
printf("enter elements");
for(i=0;i<n;i++)
{
scanf("%d",&a[i]);
}
mergesort(a,0,n-1);
for(i=0;i<n;i++)
{
printf("5d",a[i]);
}
return 0;
}