multiply 2d arrays as 1d

[glao]

If I want to multiple 2d arrays as 1d how should I do it?

I mean ,I have a 2d array and I map it as 1d:

for(int i=0;i<rows*cols;i++)
   A[i]=...

I know how to multiply 2 arrays as 2d ,but representing them as 1d?

[Stuugie]

Where is your 2D array in the code you provided?

[glao]

It doesn't matter.
I just try to multiply 2 arrays.

For example let's say we have A matrix (2x2) and X matrix (2x1)and we want to compute result matrix(2x1):

int N=2;
int R=1;
for (int i=0;i<N;i++){
   for (int j=0;j<R;j++){
     for (int k=0;k<R;k++){
      result[i*R+j]+=A[i+k*N]*X[i*R+j];
   } 
  }
   }

This doesn't work right.I can't figure how to deal with A matrix I think.

[David W]

Is what you are trying to do is to transform a vector?

i.e. M1*v1 => v2
i.e. Matrix1 times vector1 = vector2 ?

[glao]

No,I am trying to multiply 2 2d arrays but dealing with them as 1d.

I also tried:

result[i*R+j]+=A[i*R+k]*X[k*R+j];

but still the same..

[David W]

Can you show your map of a 'n by m' Matrix to a vector of length n*m ?

And then do regular matrix multipliction ( with appropriate size checks) ...

Then ... M1*M2 => M3

should map to ...

vm1*vm2 => vm3

[glao]

for (int i=0;i<n;i++){
    for (int j=0;j<m;j++){
      a[i*m+j]=0;
      }

     }

[David W]

That is not a map of M -> v ...

v is all set to zero there,

( but it shows the general idea,
if zero is replaced by M[i][j] )

Now show your code for ...

M1*M2 => M3

with row & column checking

and the appropriate size for M3

[glao]

I showed my code,above..row is N and column is N..

[Slavi]

are you trying to multiply each element of the matrix with a corresponding element in the vector? as in element 1 in the matrix * element 1 in the vector?

[glao]

I am trying to multiply 2 2d arrays (just for this example the number of rows is 2(N) and number of columns is 1(R).

The A matrix is (NN) and X is (NR) , so the result is (N*R).

Normally,I would do :  result[i][j]=A[i][k]*X[k][j]

[glao]

Ok ,it goes like:

 for (int i=0;i<rows;i++){
   for (int j=0;j<cols;j++){
     result[i*cols+j]=0;
     for (int k=0;k<rows;k++){
    result[i*cols+j]+=A[i*rows+k]*B[k*cols+j];

   }

   }
}