CODEWITHSUNDEEP
c#
Ankur Gupta
Ankur Gupta

Reputation: 65

C#: Out of memory Exception

I have written a console application

   Int64 sum = 0;
        int T = Convert.ToInt32(Console.ReadLine());
        Int64[] input = new Int64[T];
        for (int i = 0; i < T; i++)
        {
            input[i] = Convert.ToInt32(Console.ReadLine());
        }
        for (int i = 0; i < T; i++)
        {
            int[,] Matrix = new int[input[i], input[i]];
            sum = 0;
            for (int j = 0; j < input[i]; j++)
            {
                for (int k = 0; k < input[i]; k++)
                {

                    Matrix[j, k] = Math.Abs(j - k);
                    sum += Matrix[j, k];
                }
            }
            Console.WriteLine(sum);
        }

When I gave input as

2
1
999999

It gave Out of memory exception. Can you please help.

Upvotes: 0

Views: 166

Answers (2)

Jens Meinecke
Jens Meinecke

Reputation: 2940

Of course instead of doing the full squared set of calculations, you can calculate the sum thus:

for (int i = 0; i < T; i++)
{
    sum = 0;

    for (int j = 1, term = 0; j < input[i]; j++)
    {
        term += j;
        sum += term * 2;
    }

    Console.WriteLine(sum);
}

So now the calculation is O(n) instead of O(n^2)

And if you need to know what the value in Matrix[x,y] would have been, you can calculate it by the simple expression Math.Abs(x - y) thus there is no need to store that value.

Upvotes: 0

Jens Meinecke
Jens Meinecke

Reputation: 2940

Look at what you are allocating:

input[] is allocated as 2 elements (16 bytes) - no worries

But then you enter values: 1 and 999999 and in the first iteration of the loop attempt to allocate

Matrix[1,1] = 4 bytes - again no worries,

but the second time round you try to allocate

Matrix[999999, 999999]

which is 4 * 10e12 bytes and certainly beyond the capacity of your computer even with swap space on the disk.

I suspect that this is not what you really want to allocate (you'd never be able to fill or manipulate that many elements anyway...)

If you are merely trying to do the calculations as per your original code, there is not need to allocate or use the array, as you only ever store one value and immediately use that value and then never again.

Int64 sum = 0;
int T = Convert.ToInt32(Console.ReadLine());

Int64[] input = new Int64[T];

for (int i = 0; i < T; i++)
    input[i] = Convert.ToInt32(Console.ReadLine());

for (int i = 0; i < T; i++)
{
    // int[,] Matrix = new int[input[i], input[i]];
    sum = 0;

    for (int j = 0; j < input[i]; j++)
        for (int k = 0; k < input[i]; k++)
            {
                //Matrix[j, k] = Math.Abs(j - k);
                //sum += Matrix[j, k];
                sum += Math.Abs(j - k);
            }
    Console.WriteLine(sum);
}

But now beware - a trillion sums is going to take forever to calculate - it won't bomb out, but you might like to take a vacation, get married and have kids before you can expect a result.

Upvotes: 2

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