mergesort C implementation - c

Question

i wrote this code in C language on Xcode following the algorithm of mergesort.
The problem is that sometimes i get EXC_BAD_ACCESS and i can't manage where the error is!
The merge algorithm should work (i tried it outside the mergesort function and works!). Thank you for your help and patience!
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#define DIM 6
void mymerge (int v[], int i1,int i2, int last); //mergesort core: merge two ordinated arrays in one bigger ordinated array
void mymergesort (int v[], int lower, int upper);//mergesort
void printv (int v[],int lower, int upper);
int main () {
int i;
srand((unsigned int)time(NULL));
int v[DIM];
for (i=0; i<DIM; i++)
v[i]=rand()%15;
printv(v, 0, DIM-1);
getc(stdin);
mymergesort(v, 0, DIM-1);
printv(v, 0, DIM-1);
}
void printv (int v[],int lower, int upper){
int i;
for (i=lower; i<=upper; i++)
printf("%d\t",v[i]);
}
void mymergesort (int v[], int lower, int upper){
int mid=(upper+lower)/2;
if (upper<lower) {
mymergesort(v, lower, mid);
mymergesort(v, mid+1, upper);
mymerge(v,lower,mid+1,upper);
}
}
void mymerge (int v[], int i1,int i2, int last){
int i=i1,j=i2,k=i1,*vout;
vout=(int*)malloc((last-i1+1)*sizeof(int));
while (i<i2 && j<=last) {
if (v[i]<=v[j]) {
vout[k++]=v[i++];
}else {
vout[k++]=v[j++];
}
}
for (;i<i2;i++) vout[k++]=v[i];
for (;j<=last;j++) vout[k++]=v[j];
for (k=i1; k<=last; k++) v[k]=vout[k];
free(vout);
}
EDIT:
thank you very much! but i think think there is another problem, when I try to sort a bigger array (200 elements), the program doesn't work (i get a malloc error: incorrect checksum for freed object - object was probably modified after being freed). But if I run it from the xCode debugger everything works fine

Answer 1

This: vout=(int*)malloc((last-i1)*sizeof(int)); is wrong.
First, the number of elements you want is last-i1+1, not last-i1 - classic off-by-1. This kind of error is one of the reasons why the convention in C code is to make lower bounds inclusive and upper bounds exclusive - less +1 and -1 you need to do, less opportunity to screw up.
The more serious error is that you index vout starting from i1. If you do it this way, you need to allocate last+1 element for vout, and you never use the first i1 (index 0 .. i1-1).
Fix: First, allocate last-i1+1 elements. Second, initialize k to 0 at the beginning, not i1. Third, change the final copy to be
for (k=i1; k<=last; k++) v[k] = vout[k-i1];

Answer 2

You have two problems. The first is that your calculation of the midpoint is incorrect - you use (upper - lower)/ 2, but this is not guaranteed to lie between lower and upper. What you actually want is lower + (upper - lower) / 2. It's also not necessary to do any work if there's only 1 number in the interval to be sorted - so the mymergesort() function should look like:
void mymergesort (int v[], int lower, int upper)
{
if (upper > lower) {
int mid = lower + (upper - lower)/2;
mymergesort(v, lower, mid);
mymergesort(v, mid+1, upper);
mymerge(v,lower,mid+1,upper);
}
}
The second problem is the one in the mymerge() function already pointed out by Fabian Giesen.

Answer 3

#include<stdio.h>
#include<stdlib.h>
void merge(int *a, int n1, int *b, int n2, int *arr)
{
int i=0, j=0, n=0;
while(i<n1 && j<n2)
{
if (a[i] < b[j])
{
arr[n++] = a[i];
i++;
}
else
{
arr[n++] = b[j];
j++;
}
}
while( i < n1)
arr[n++] = a[i++];
while( j < n2)
arr[n++] = b[j++];
}
void merge_sort(int *a, int n)
{
int left[n/2], right[n-n/2],i=0;
if (n<=1)
return ;
while(i<n/2)
left[i] = a[i++];
while(i<n)
right[i - n/2] = a[i++];
merge_sort( left, n/2 );
merge_sort( right, n-n/2);
merge(left, n/2, right, n-n/2, a);
}
void main()
{
int a[] = { 6, 5, 3, 1,9, 8, 7, 2, 4},i;
merge_sort(a,sizeof(a)/sizeof(a[0]));
for(i=0;i<9;i++)
printf("--%d",a[i]);
printf("\n");
}
-- s.k

Answer 4

#include<stdio.h>
#include<conio.h>
#define max 20
/*** function for merging the adjecent subarrays in sorted order ***/
void merge(int A[max],int n,int low,int high, int mid)
{
int i=low,j=mid+1,k,temp;
while((i<=j)&&(j<=high))
{
if(A[i]>A[j]) /** if element of the second half is greater then exchg and shift **/
{
temp=A[j];
for(k=j;k>i;k--) /** shifting the elements **/
{
A[k]=A[k-1];
}
A[i]=temp;
j++;
}
i++;
}
}
/******* iterative function for merge sort ********/
void merge_sort(int A[max],int n,int low,int high)
{
int mid;
if(low<high) /** terminating condition **/
{
mid=(high+low)/2; /** calculating the mid point ***/
merge_sort(A,n,low,mid); /*** recursive call for left half of the array ***/
merge_sort(A,n,mid+1,high); /*** recursive call for right half of the array ***/
merge(A,n,low,high,mid); /** merging the both parts of the array **/
}
}
/******* begening of the main function **********/
int main()
{
int A[max],n,i;
/** reading the inputs fro users **/
printf("\n enter the size of the array\n");
scanf("%d",&n);
printf("\n enter the array \n");
for(i=0;i<n;i++)
{
scanf("%d",&A[i]);
}
/*** calling merge sort ***/
merge_sort(A,n,0,n-1);
/** printing the sorted array **/
for(i=0;i<10;i++)
{
printf("\n\t%d",A[i]);
}
getch();
return 0;
}