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Matrix Multiplication using Multi-Dimensional Arrays

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vishesh

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Re: Matrix Multiplication using Multi-Dimensional Arrays

#11

Apr 5th, 2009

First of all replace M[i][j] = m1[i][k]*m2[k][j]; with M[i][j] += m1[i][k]*m2[k][j]; . See the algorithm in above post, for explanation. Also dont forget to set M[i][j] value to 0 before running third loop.

 Help with Code Tags 
 C++ Syntax (Toggle Plain Text) 
infile >> m1[i][j] >> m2[i][j];
M[i][j] += m1[i][k]*m2[k][j];
infile >> m1[i][j] >> m2[i][j];
M[i][j] += m1[i][k]*m2[k][j];

Here in second line you are multiplying two array members which haven't got their value yet. You're taking values for different cells. Better take inputs earlier than start multiplying matrices or take it like infile >> m1[i][k] >> m2[k][j] but wouldnt it be a bit awkward

Last edited by vishesh; Apr 5th, 2009 at 5:18 am.

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ARYT

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Re: Matrix Multiplication using Multi-Dimensional Arrays

#12

Apr 5th, 2009

Well, thanks but I think k shouldn't have a value. It’s basically 0 and later it's used for increment only.
I've also added that += and your infile method, but no difference.

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vishesh

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Re: Matrix Multiplication using Multi-Dimensional Arrays

#13

Apr 5th, 2009

Oops...I just missed a point in previous post. Anyway the loop gotta look like this.

 Help with Code Tags 
 cpp Syntax (Toggle Plain Text) 
//take matrices values before starting
for (int i=0; i<6; i++)
{ 
  for (int j=0; j<6; j++)
   {
     M[i][j] = 0;
     for (int k=0; k<6; k++)
     {
        M[i][j] += m1[i][k] * m2[k][j];
     }
     cout << M[i][j] << " ";  // this is what i missed to tell
   } 
   cout<<endl;
}
//take matrices values before starting
for (int i=0; i<6; i++)
{ 
  for (int j=0; j<6; j++)
   {
     M[i][j] = 0;
     for (int k=0; k<6; k++)
     {
        M[i][j] += m1[i][k] * m2[k][j];
     }
     cout << M[i][j] << " ";  // this is what i missed to tell
   } 
   cout<<endl;
}