Recursive matrix multiplication

Question

I am reading Introduction to Algorithms by CLRS. Book shows pseudocode for simple divide and conquer matrix multiplication:

n = A.rows
let c be a new n x n matrix
if n == 1
    c11 = a11 * b11
else partition A, B, and C
    C11 = SquareMatrixMultiplyRecursive(A11, B11)
        + SquareMatrixMultiplyRecursive(A12, B21)
    //...
return C

Where for example, A11 is submatrix of A of size n/2 x n/2. Author also hints that I should use index calculations instead of creating new matrices to represent submatrices, so I did this:

#include <iostream>
#include <vector>

template<class T>
struct Matrix
{
    Matrix(size_t r, size_t c)
    {
        Data.resize(c, std::vector<T>(r, 0));
    }    

    void SetSubMatrix(const int r, const int c, const int n, const Matrix<T>& A, const Matrix<T>& B)
    {
        for(int _c=c; _c<n; ++_c)
        {
            for(int _r=r; _r<n; ++_r)
            {
                Data[_c][_r] = A.Data[_c][_r] + B.Data[_c][_r];
            }
        }
    }

    static Matrix<T> SquareMultiplyRecursive(Matrix<T>& A, Matrix<T>& B, int ar, int ac, int br, int bc, int n)
    {
        Matrix<T> C(n, n);

        if(n == 1)
        {
            C.Data[0][0] = A.Data[ac][ar] * B.Data[bc][br];
        }
        else
        {
            C.SetSubMatrix(0, 0, n / 2,
                           SquareMultiplyRecursive(A, B, ar, ac, br, bc, n / 2),
                           SquareMultiplyRecursive(A, B, ar, ac + (n / 2), br + (n / 2), bc, n / 2));

            C.SetSubMatrix(0, n / 2, n / 2,
                           SquareMultiplyRecursive(A, B, ar, ac, br, bc + (n / 2), n / 2),
                           SquareMultiplyRecursive(A, B, ar, ac + (n / 2), br + (n / 2), bc + (n / 2), n / 2));

            C.SetSubMatrix(n / 2, 0, n / 2,
                           SquareMultiplyRecursive(A, B, ar + (n / 2), ac, br, bc, n / 2),
                           SquareMultiplyRecursive(A, B, ar + (n / 2), ac + (n / 2), br + (n / 2), bc, n / 2));

            C.SetSubMatrix(n / 2, n / 2, n / 2,
                           SquareMultiplyRecursive(A, B, ar + (n / 2), ac, br, bc + (n / 2), n / 2),
                           SquareMultiplyRecursive(A, B, ar + (n / 2), ac + (n / 2), br + (n / 2), bc + (n / 2), n / 2));
        }

        return C;
    }

    void Print()
    {
        for(int c=0; c<Data.size(); ++c)
        {
            for(int r=0; r<Data[0].size(); ++r)
            {
                std::cout << Data[c][r] << " ";
            }
            std::cout << "\n";
        }
        std::cout << "\n";
    }

    std::vector<std::vector<T> > Data;
};

int main()
{
    Matrix<int> A(2, 2);
    Matrix<int> B(2, 2);
    A.Data[0][0] = 2;
    A.Data[0][1] = 1;
    A.Data[1][0] = 1;
    A.Data[1][1] = 2;

    B.Data[0][0] = 2;
    B.Data[0][1] = 1;
    B.Data[1][0] = 1;
    B.Data[1][1] = 2;

    A.Print();
    B.Print();

    Matrix<int> C(Matrix<int>::SquareMultiplyRecursive(A, B, 0, 0, 0, 0, 2));

    C.Print();
}

It gives me incorrect results, tho I am not sure what I'm doing wrong...

Answer 1

// Recursive naive matrix multiplication in C, not strassen.
// 2013-Feb-15 Fri 12:28 moshahmed/at/gmail

#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define M 2
#define N (1<<M)

typedef int mat[N][N]; // mat[2**M,2**M]  for divide and conquer mult.
typedef struct { int ra, rb, ca, cb; } corners; // for tracking rows and columns.

// set A[a] = k
void set(mat A, corners a, int k){
  int i,j;
  for(i=a.ra;i<a.rb;i++)
    for(j=a.ca;j<a.cb;j++)
      A[i][j] = k;
}

// set A[a] = [random(l..h)].
void randk(mat A, corners a, int l, int h){
  int i,j;
  for(i=a.ra;i<a.rb;i++)
    for(j=a.ca;j<a.cb;j++)
      A[i][j] = l + rand()% (h-l);
}

// Print A[a]
void print(mat A, corners a, char *name) {
  int i,j;
  printf("%s = {\n",name);
  for(i=a.ra;i<a.rb;i++){
    for(j=a.ca;j<a.cb;j++)
      printf("%4d, ", A[i][j]);
    printf("\n");
  }
  printf("}\n");
}

// Return 1/4 of the matrix: top/bottom , left/right.
void find_corners(corners a, int i, int j, corners *b) {
  int rm = a.ra + (a.rb - a.ra)/2 ;
  int cm = a.ca + (a.cb - a.ca)/2 ;
  *b = a;
  if (i==0)  b->rb = rm;     // top rows
  else       b->ra = rm;     // bot rows
  if (j==0)  b->cb = cm;     // left cols
  else       b->ca = cm;     // right cols
}

// Naive Multiply: A[a] * B[b] => C[c], recursively.
void mul(mat A, mat B, mat C, corners a, corners b, corners c) {
  corners aii[2][2], bii[2][2], cii[2][2];
  int i, j, m, n, p;

  // Check: A[m n] * B[n p] = C[m p]
  m = a.rb - a.ra; assert(m==(c.rb-c.ra));
  n = a.cb - a.ca; assert(n==(b.rb-b.ra));
  p = b.cb - b.ca; assert(p==(c.cb-c.ca));
  assert(m>0);

  if (n==1) {
    C[c.ra][c.ca] += A[a.ra][a.ca] * B[b.ra][b.ca];
    return;
  }

  // Create the smaller matrices:
  for(i=0;i<2;i++) {
  for(j=0;j<2;j++) {
        find_corners(a, i, j, &aii[i][j]);
        find_corners(b, i, j, &bii[i][j]);
        find_corners(c, i, j, &cii[i][j]);
      }
  }

  // Now do the 8 sub matrix multiplications.
  // C00 = A00*B00 + A01*B10
  // C01 = A00*B01 + A01*B11
  // C10 = A10*B00 + A11*B10
  // C11 = A10*B01 + A11*B11

  mul( A, B, C, aii[0][0], bii[0][0], cii[0][0] );
  mul( A, B, C, aii[0][1], bii[1][0], cii[0][0] );

  mul( A, B, C, aii[0][0], bii[0][1], cii[0][1] );
  mul( A, B, C, aii[0][1], bii[1][1], cii[0][1] );

  mul( A, B, C, aii[1][0], bii[0][0], cii[1][0] );
  mul( A, B, C, aii[1][1], bii[1][0], cii[1][0] );

  mul( A, B, C, aii[1][0], bii[0][1], cii[1][1] );
  mul( A, B, C, aii[1][1], bii[1][1], cii[1][1] );

}

int main() {
  mat A, B, C;
  corners ai = {0,N,0,N};
  corners bi = {0,N,0,N};
  corners ci = {0,N,0,N};
  //set(A,ai,2);
  //set(B,bi,2);
  srand(time(0));
  randk(A,ai, 0, 2);
  randk(B,bi, 0, 2);
  set(C,ci,0); // set to zero before mult.
  print(A, ai, "A");
  print(B, bi, "B");
  mul(A,B,C, ai, bi, ci);
  print(C, ci, "C");
  return 0;
}