rombi rombi - 3 months ago 25
C++ Question

Segmentation fault on large size array with heap allocation

The following code gives me a segmentation fault when run on a 4Gb machine even after i dynamically allocate the space to the array holding 10 million entries. It works fine with 1 million entries i.e. n = 1000000. The following code sorts integer values along with their index value using radix sort. What should i do to make this program work for 10 million entries.?

int main()
{
int n=10000000; // 10 million entries
int *arr=new int [n]; // declare heap memory for array
int *arri=new int [n]; // declare heap memory for array index

for(int i=0;i<n;i++) // initialize array with random number from 0-100
{
arr[i]=((rand()%100)+0);
}

for(i=0;i<n;i++) // initialize index position for array
{
arri[i]=i;
}

radixsort(arr, n ,arri);
return 0;
}

// The main function to that sorts arr[] of size n using Radix Sort
void radixsort(int *arr, int n,int *arri)
{ int m=99; //getMax(arr,n,arri);

// Find the maximum number to know number of digits


// Do counting sort for every digit. Note that instead
// of passing digit number, exp is passed. exp is 10^i
// where i is current digit number
for (int exp = 1; m/exp > 0; exp *= 10)
countSort(arr, n, exp,arri);


}

void countSort(int *arr, int n, int exp,int *arri)
{
int output[n],output1[n]; // output array
int i, count[10] = {0};

// Store count of occurrences in count[]
for (i = 0; i < n; i++)
{
count[ (arr[i]/exp)%10 ]++;

}

// Change count[i] so that count[i] now contains actual
// position of this digit in output[]
for (i = 1; i < 10; i++)
count[i] += count[i - 1];

// Build the output array
for (i = n - 1; i >= 0; i--)
{

output[count[ (arr[i]/exp)%10 ] - 1] = arr[i];
output1[count[ (arr[i]/exp)%10 ] - 1] = arri[i];
count[ (arr[i]/exp)%10 ]--;
}

// Copy the output array to arr[], so that arr[] now
// contains sorted numbers according to current digit
for (i = 0; i < n; i++)
{
arr[i] = output[i];
arri[i]=output1[i];
}

}

Answer

The problem is in countSort. The output and output1 arrays are local arrays, not dynamically allocated, and they're too big for local variables. You're also using the C feature of variable-length arrays, which aren't part of standard C++. Change them to:

int *output = new int[n];
int *output1 = new int[n];

and add

delete[] output;
delete[] output1;

at the end of the function.