DaniWeb Forum Index > Software Development > C++

C++ RSS

Matrix multiplication

Please support our C++ advertiser: Intel Parallel Studio Home

•

Join Date: Nov 2009

Posts: 2

Reputation: uncbball is an unknown quantity at this point

Solved Threads: 0

uncbball

Offline

Newbie Poster

Matrix multiplication

25 Days Ago

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:

 Help with Code Tags 
 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.

•

Join Date: Nov 2008

Posts: 394

Reputation: StuXYZ is a glorious beacon of light

Solved Threads: 72

StuXYZ

Offline

Posting Whiz

25 Days Ago

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; 25 Days Ago at 11:35 pm.

experience is the most expensive way to learn anything

•

Join Date: Nov 2009

Posts: 2

Reputation: uncbball is an unknown quantity at this point

Solved Threads: 0

uncbball

Offline

Newbie Poster

25 Days Ago

•

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.

•

Join Date: Dec 2008

Posts: 1,261

Reputation: firstPerson is just really nice

Solved Threads: 157

firstPerson firstPerson is online now

Online

Nearly a Posting Virtuoso

24 Days Ago

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.

 Help with Code Tags 
 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.

Help with Code Tags

(Toggle Plain Text)

1) What word becomes shorter if you add a letter to it? [ Solved by : niek_e, Paul Thompson]
2) What does this sequence  equal to :  (.5u - .5a)(.5u-.5b)(.5u-.5c) ...[*]
      [*solved by : murtan]
3) What is the 123456789 prime numer?

•

Join Date: Dec 2007

Posts: 113

Reputation: abhi_elementx is an unknown quantity at this point

Solved Threads: 6

abhi_elementx abhi_elementx is offline

Offline

Junior Poster

24 Days Ago

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

 Help with Code Tags 
 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