Array size problem while testing mergesort - c

Question

I have to test Mergesort with different sizes of data. The problem I encounter is that if the array of ints is around the size of 5000000 my code is crashing but without giving an error. I calculated the amount of space the array would take and if I'm not wrong this would be about 0,02 GB and my RAM is 8GB. I also searched if there is a max size for an array and didnt find anything useful. So where is the problem?
/* C program for Merge Sort */
#include<stdlib.h>
#include<stdio.h>
#include <time.h>
#define MAX 2000000
int arr[MAX];
// Merges two subarrays of arr[].
// First subarray is arr[l..m]
// Second subarray is arr[m+1..r]
void merge(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++;
}
}
/* l is for left index and r is right index of the
sub-array of arr to be sorted */
void mergeSort(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(l, m);
mergeSort(m+1, r);
merge(l, m, r);
}
}
/* UTILITY FUNCTIONS */
/* Function to print an array */
void printArray()
{
int i;
for (i=0; i < MAX; i++)
printf("%d ", arr[i]);
printf("\n");
}
/* Driver program to test above functions */
int main()
{
srand(time(NULL));
for (int var = 0; var < MAX; ++var) {
arr[var] = rand() % 100;
}
clock_t t1, t2;
//printf("Given array is \n");
// printArray();
t1 = clock();
mergeSort(0, MAX - 1);
t2 = clock();
printf("\nSorted array is \n");
printArray();
float timetaken = t2 - t1;
timetaken /= CLOCKS_PER_SEC;
printf("Time spend: %f\n", timetaken);
return 0;
}

Answer 1

in
void merge(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];
...
Probably L and R are too large to be placed in the stack, the size of the stack is limited, do ulimit-a to see the limit.
If this is the reason you will have to allocate them in the heap (malloc), and don't forget after to free them ;-)
When you need a big array whose size if a constant (this is not the case here) and the function using it is not directly nor indirectly recursive, you can use a static var, I mean for example : void foo() { ... static int v[4096]; ... } rather than void foo() { ... int v[4096]; ... }