How to make matrix multiplication by parts in C++

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I need help, i can't imagine a C++ code for this.
I have to resolve the matrix mult. A*B=C and A is a 256x128 elements and B is a 128x256 elements, so C is a 256x256 elements.
But to solve this i need to create "sections" of A and B matrix (for example, Asection=4x128 and Bsection 128x4 and that mult. will give a Csection=4x4 matrix) and then mult. Asection with the next section of B (Bsection finished on 4 column, so the next section is 128x4 but starts in column 5). And this new mult. will give next Csection, etc.
This link has a representation of the problem

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THE BIG DEAL is that actually A B and C aren't "matrix", they're Vectors so A=[0,1,2....127(here finishes the first row),128,129....255 (2nd row, etc till 32768 elements)], the same for B and C. (pd: i know that vectors are matrix, but you know what i mean).

I did a code but just for a simply matrix mult, A has 2048 elemets and B too (64x32 and 32x64). This is the structure of the mult in my original problem, and could be the first "section" of C but i don't know how to put the others C sections.

What I have tried:

for (int i = 0; i < 64; i++)
{
for (int j = 0; j < 64; j++)
{
for (int k = 0; k < 32; k++)
{
C[i * 64 + j] += A[i * 32 + k] * B[j * 32 + k];

}
}
}

Posted 15-Aug-20 12:13pm

Member 14915155

Updated 16-Aug-20 3:43am

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[no name] 16-Aug-20 6:07am

For me your code looks fine. The "sections" of 'C' are calculated by 'i*64' while you are doing this: 'C[i*64+j]'. Or what am I missing?

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User 11060979 · Answer 1 · 2020-08-16T03:43:00

Preface
Of course in c++ there are much more elegant solutions possible. This in case you implement a class Matrix (and Vector). But that was not your question.

Implementation trial according to your question
Here an implementation completely untested, but I think because of some meaningfull namings more easy to understand and hopefully correct. Another thing: I'm assuming the matrices hold double values. Change the type according to your needs.

C++

void MatrixMul(double A[], int ARows, int ACols,
               double B[], int BRows, int BCols,
               double C[])
{
    // Assertion test. For matrix multiplication the left hand column count needs to match the right hand row count.  
    if (ACols != BRows)
    {
      throw(*new Exception("Dimension Error"));
    }

   int CRows= ARows; // Result Rows
   int CCols= BCols; // Result Cols

   for(int CRowIx= 0; CRowIx < CRows; CRowIx++)
   {
      for(int CColIx= 0; CColIx < CCols; CColIx++)
      {
         double Sum= 0.0; // Assuming the matrices hold double values. 
         for(int k= 0; k < ACols; k++)
         {
            // sum+= B[k][CColIx] * A[CRowIx][k];
            Sum+= B[k * BCols + CColIx] * A[CRowIx * ACols + k];
         }
         // C[CRowIx][CColIx]= sum;
         C[CRowIx * CCols + CColIx]= Sum;
     }
   }
}

Notes
Of course more index range checking are necessary to make it 'access violation" safe.

I hope it helps.

Richard MacCutchan · Answer 2 · 2020-08-15T23:02:00

You can create matrix objects on memory just by using 2 dimensional arrays. So for your example you would code:

C++

#define A_ROWS   256
#define A_COLS   128
#define B_ROWS   A_COLS
#define B_COLS   A_ROWS

int A[A_ROWS][A_COLS];
int B[B_ROWS][B_COLS];

Now by using different values for the number of rows and columns you can address any subset of the two matrices.

How to make matrix multiplication by parts in C++

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How to make matrix multiplication by parts in C++

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