Mergesort output changes when printf is used - c

Question

While trying to craft up a mergesort in C as an exercise, I am encountering a weird issue where the output of the algorithm changes with the presence of a printf. Let me elaborate it further. I have a "MergeSort.h" file in which I have my mergesort algorithm as follows:
#ifndef _MSORT_H_
#define _MSORT_H_
void merge(int a[],int lo,int mi,int hi){
int n1 = (mi - lo) +1;
int n2 = hi - mi;
int left[n1];
int right[n2];
int i;
int j;
int k;
for(i = 0;i < n1;i++){
left[i] = a[lo+i];
}
for(j = 0;j < n2;j++){
right[j] = a[mi+j+1];
}
i = 0;
j = 0;
for(k = lo;k <= hi;k++){
if(left[i] < right[j]){
a[k] = left[i];
i = i + 1;
}else{
a[k] = right[j];
j = j+1;
}
}
}
void msorthelper(int a[],int p,int r){
if(p < r){
int mid = (p + r)/2;
msorthelper(a,0,mid);
msorthelper(a,mid+1,r);
merge(a,p,mid,r);
}
}
void msortex(int a[],int size){
msorthelper(a,0,size-1);
int counter;
for(counter = 0;counter < size;counter++){
printf(" %d ",a[counter]);
}
}
#endif
I have my corresponding sort tester in sorttester.c:
#include <stdio.h>
#include "mergesort.h"
int main(){
int out[] = {8,1,6};
msortex(out,3);
}
The output from running the sorttester is as follows:
1 0 0
Now here's the interesting part, when I put any printf statement to the beginning of the merge, the correct output is generated. Here's an example:
#ifndef _MSORT_H_
#define _MSORT_H_
void merge(int a[],int lo,int mi,int hi){
printf("Hello\n");
int n1 = (mi - lo) +1;
int n2 = hi - mi;
int left[n1];
int right[n2];
.................Rest of the code.....
The output now is:
HelloHello 1 6 8
This is the correct sorted order for the array 8 6 1.
Can somebody tell me what I might be doing wrong here that is causing the output to be drastically by the presence of a printf?
Thanks

Answer 1

Have a look at your merging part of code:
for(k = lo;k <= hi;k++){
if(left[i] < right[j]){
a[k] = left[i];
i = i + 1;
}else{
a[k] = right[j];
j = j+1;
}
}
Lets assume that
left[] = {1, 2}
right[] = {3, 4, 5}
After 2 iterations of your merging loop (k) the value of "i" will be 2. From now on, you will try to compare
left[2] < right[j]
Which is invalid, as left[2] will refer to some random memory (left array size is only 2, so element of index 2 doesnt exist, only 0 and 1)
So, if you add guards for the i and j values, for example by changing first if condition to:
if(i != n1 && (j == n2 || left[i] < right[j])){
You should be fine.
Anyway, I should also tell you that you shouldnt declare array sizes with values that are not const, like:
int left[n1];
int right[n2];
Its actually forbidden by standard. You should either allocate it dynamically, use vectors(if c++) or jus declare them global with large enough size (maximum n value)