C#•2y ago

❔ Matrix Multiplication

Hi there, I need help understanding the following source code, I have added my questions in the comments.

namespace SolH
{
    class H7
    {
        static int[,] MatrixMultiply(int[,] A, int[,] B)
        {
            int[,] R = new int[A.GetLength(0), B.GetLength(1)];

            for (int i = 0; i < A.GetLength(0); i++)
            {
                for (int j = 0; j < B.GetLength(1); j++)
                {
                    for (int k = 0; k < A.GetLength(1); k++) // why are we looping through A's columns?
                    {
                        R[i, j] = R[i, j] + A[i, k] * B[k, j]; //why do we need to add R[i, j]? Isnt it an empty array after we initialize it? 
                    }
                }
            }
            return R;
        }

namespace SolH
{
    class H7
    {
        static int[,] MatrixMultiply(int[,] A, int[,] B)
        {
            int[,] R = new int[A.GetLength(0), B.GetLength(1)];

            for (int i = 0; i < A.GetLength(0); i++)
            {
                for (int j = 0; j < B.GetLength(1); j++)
                {
                    for (int k = 0; k < A.GetLength(1); k++) // why are we looping through A's columns?
                    {
                        R[i, j] = R[i, j] + A[i, k] * B[k, j]; //why do we need to add R[i, j]? Isnt it an empty array after we initialize it? 
                    }
                }
            }
            return R;
        }

Here is the question: Write a static method MatrixMultiply(int[,] A, int [,] B) that will perform a matrix multiplication and return another 2 dimensional array.

4 Replies

HimmDawg•2y ago

// why are we looping through A's columns?

Because the first loop here

for (int i = 0; i < A.GetLength(0); i++)

for (int i = 0; i < A.GetLength(0); i++)

loops over the rows. If we then loop over the columns, then we can get the values in each row. Since B has a much rows as A has columns, this will work out to build the sum for the respective cell in C

// Isnt it an empty array after we initialize it?

No. It's just a bunch of integers. the default value for int is 0

.logik.•2y ago

You also need to add R[i, j] because the way you create x_12 (using the wiki image) is starting with x_12=0 x_12 = x_12 + a_11b_12 And then x_12 = x_12 + a_12b_22 Which gives you the proper cross product

moshimoshiOP•2y ago

Thank you!!! I get it now!!! Thank you for breaking it down so clearly 🙂

Accord•2y ago

Was this issue resolved? If so, run /close - otherwise I will mark this as stale and this post will be archived until there is new activity.

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❔ Matrix Multiplication