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c#matrix
harsh
harsh

Reputation: 109

Multiplying two matrices but getting the wrong answer

This is my matrix code. I am multiplying two matrices. One of the matrices is scalar (meaning diagonal elements are the same), but when I run this code, I am getting the wrong answer.

static void Main(string[] args)
{
    int[,] matrix1 = new int[3, 3];
    int[,] matrix2 = new int[3, 3];
    int[,] result = new int[3, 3];

    for (int i = 0; i < 3; i++)
    {
        for (int j = 0; j < 3; j++)
        {
            Console.WriteLine("Enter 1st Matrix: ");
            matrix1[i, j] = Convert.ToInt32(Console.ReadLine());
        }
    }

    Console.ReadLine();

    for (int k = 0; k < 3; k++)
    {
        for (int l = 0; l < 3; l++)
        {
            Console.WriteLine("Enter 2nd Matrix: ");
            matrix2[k, l] = Convert.ToInt32(Console.ReadLine());
        }
    }

    Console.WriteLine();
    Console.WriteLine("Matrix 1: ");

    for (int i = 0; i < 3; i++)
    {
        for (int j = 0; j < 3; j++)
        {
            Console.Write(matrix1[i, j] + " ");
        }
        Console.WriteLine();
    }

    Console.WriteLine();
    Console.WriteLine("Matrix 2: ");

    for (int k = 0; k < 3; k++)
    {
        for (int l = 0; l < 3; l++)
        {
            Console.Write(matrix2[k, l] + " ");
        }
        Console.WriteLine();
    }

    Console.WriteLine("Matrix 1 * Matrix 2: ");

    for (int i = 0; i < 3; i++)
    {
        for (int j = 0; j < 3; j++)
        {
            result[i, j] = result[i, j] + matrix1[i, j] * matrix2[i, j];
            Console.Write(result[i, j] + " ");
        }
        Console.WriteLine();
    }

    Console.ReadLine();
    Console.ReadLine();
}

Upvotes: 1

Views: 1188

Answers (1)

penartur
penartur

Reputation: 9912

You're not multiplying the matrices, you're multiplying their values. See https://en.wikipedia.org/wiki/Matrix_multiplication

E.g. {{1, 1}, {0, 0}} x {{1, 0}, {1, 0}} should result in {{2, 0}, {0, 0}}, not {{1, 0}, {0, 0}}.

Here is the right code for matrix multiplication (note that it has the complexity of O(n^3), not O(n^2)):

        for (int i = 0; i < 3; i++)
        {
            for (int j = 0; j < 3; j++)
            {
                result[i, j] = 0;
                for(int k = 0; k < 3; k++)
                {
                    result[i, j] = result[i, j] + matrix1[i, k] * matrix2[k, j];
                }
                Console.Write(result[i, j] + " ");
            }
            Console.WriteLine();
        }

Upvotes: 4

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