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Nov 9th, 2009

Matrix multiplication

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Hey, so I have to make a program that has to multiply two 3x3 matrices (among other things). I used classes to do the project, but I just can't seem to get this multiplication to work. Here is my code so far:

C++ Syntax (Toggle Plain Text)
void matrix::mult(const matrix& u)
{
    double c[9];
    double a[9];
    for (int i=0; i<rows; i++)
    {
        for (int j=0; j<columns; j++)
        {
            for(int r=0; r<columns; r++)
            {
                c[i*columns+j]+=a[i*columns+r]*u.data[r*columns+j];
            }
        }
    }
}
void matrix::mult(const matrix& u)
{
    double c[9];
    double a[9];
    for (int i=0; i<rows; i++)
    {
        for (int j=0; j<columns; j++)
        {
            for(int r=0; r<columns; r++)
            {
                c[i*columns+j]+=a[i*columns+r]*u.data[r*columns+j];
            }
        }
    }
}

Any advice would be great, and please let me know if there's anything I should clarify. Thank You.

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uncbball

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Nov 9th, 2009

Re: Matrix multiplication

First thing that comes to mind is that a[9] and c[9] are not initialized.

Next you are using variables columns and row. So check that this->rows == u.columns. Then set the data in this. as c[] is just ignored.

Other than that it looks ok.

Last edited by StuXYZ; Nov 9th, 2009 at 11:35 pm.

StuXYZ

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Nov 10th, 2009

Re: Matrix multiplication

Click to Expand / Collapse Quote originally posted by StuXYZ ...

Next you are using variables columns and row. So check that this->rows == u.columns. Then set the data in this. as c[] is just ignored.

Hey, thanks for replying. I'm not quite sure what you mean with the above though. And if it matters, rows and columns aren't actually variables, they're constants.

uncbball

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Nov 10th, 2009

Re: Matrix multiplication

Why do you have 3 loops for a 1d vector?

If you want to emulate it as a 2d then all you need is 2 for loops.

C++ Syntax (Toggle Plain Text)
int A[4] = {1,2,3,4};
int B[4] = {1,2,3,4};
int R[4] = {0};
 
for(int i = 0; i < 4; i++)
{
     for(int j = 0; j < 4; j++)
         R[ j + 4*i] = A[j + 4*i] * B[j + 4*i];
}
int A[4] = {1,2,3,4};
int B[4] = {1,2,3,4};
int R[4] = {0};

for(int i = 0; i < 4; i++)
{
     for(int j = 0; j < 4; j++)
         R[ j + 4*i] = A[j + 4*i] * B[j + 4*i];
}

Its better to make a function that converts (j+4*i) into a 2d index so it becomes more readable.

firstPerson

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Nov 10th, 2009

Re: Matrix multiplication

Hi.
Try this. Naive, but should give you some idea.
The way you architect the classes entirely depends on you.

C++ Syntax (Toggle Plain Text)
#include<iostream>
using namespace std;
const int SIZE = 3;
 
class  Matrix{
	public:
		int matrix[SIZE][SIZE];
		void read_matrix();
};
 
class MatOps{
	public:
		Matrix mul_matrix(Matrix &, Matrix &);
};
 
void Matrix :: read_matrix(){
	cout<<"Enter matrix(3 x 3)"<<endl;
	int i = 0;
	int j = 0;
	for(i = 0; i < SIZE; i++){
		for(j = 0; j < SIZE; j++){
			cin>>matrix[i][j];
		}	
	}//for ends
}//read_matrix() ends
 
 
Matrix MatOps :: mul_matrix(Matrix & a, Matrix & b){
	Matrix result;
	int i = 0;
	int j = 0;
	int k = 0;
 
	for(i = 0; i < SIZE; i++){
		for(j = 0; j < SIZE; j++){
			result.matrix[i][j] = 0;
			for(k = 0; k < SIZE; k++){
				result.matrix[i][j] += a.matrix[i][k] * b.matrix[k][j];
			}
		}	
	}//for ends
 
	return result;
}//read_matrix() ends
 
 
int main(){
	Matrix a, b;
	a.read_matrix();
 
	b.read_matrix();
 
	MatOps obj;
	Matrix result = obj.mul_matrix(a, b);
 
	for(int i = 0; i < SIZE; i++){
		for(int j = 0; j < SIZE; j++){
			cout<<result.matrix[i][j]<<" ";
		}
		cout<<endl;
	}	
 
	return 0;
}//main() ends
#include<iostream>
using namespace std;
const int SIZE = 3;

class  Matrix{
	public:
		int matrix[SIZE][SIZE];
		void read_matrix();
};

class MatOps{
	public:
		Matrix mul_matrix(Matrix &, Matrix &);
};

void Matrix :: read_matrix(){
	cout<<"Enter matrix(3 x 3)"<<endl;
	int i = 0;
	int j = 0;
	for(i = 0; i < SIZE; i++){
		for(j = 0; j < SIZE; j++){
			cin>>matrix[i][j];
		}	
	}//for ends
}//read_matrix() ends


Matrix MatOps :: mul_matrix(Matrix & a, Matrix & b){
	Matrix result;
	int i = 0;
	int j = 0;
	int k = 0;
	
	for(i = 0; i < SIZE; i++){
		for(j = 0; j < SIZE; j++){
			result.matrix[i][j] = 0;
			for(k = 0; k < SIZE; k++){
				result.matrix[i][j] += a.matrix[i][k] * b.matrix[k][j];
			}
		}	
	}//for ends
	
	return result;
}//read_matrix() ends


int main(){
	Matrix a, b;
	a.read_matrix();
	
	b.read_matrix();
	
	MatOps obj;
	Matrix result = obj.mul_matrix(a, b);
	
	for(int i = 0; i < SIZE; i++){
		for(int j = 0; j < SIZE; j++){
			cout<<result.matrix[i][j]<<" ";
		}
		cout<<endl;
	}	
	
	return 0;
}//main() ends

Cheers.

abhi_elementx

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