Forming a matrix multiplication program

[mshoppas]

Hey guys, I just started Linear Algebra for my programming degree and to full understand everything I want to put it into code and make my own "matrix calculator." I have all the theory but I am having issues keeping the matrix variable and moving it around the class' functions to create it and output it. Once I know I have it saved I think I can get the addition/multiplication to work. Thanks! Here is the code I have so far:

#include <string>
#include <iostream>

using namespace std;

class matrix{
    
private:
  
    int q = 1, s = 1, a = 1, b = 1;
    int i=0, j=0;
    int *r, *c, *w, *x;
    int rows[2], columns[2];
    
    int matrix[2][2];

public:
    
    void getInfo1(int matrix[2][2]);
    void getInfo2();
    void output1();
    void output2();
    
};

int main()
{

    matrix m1, m2;


    return 0;
}



void matrix::getInfo1(int matrix[2][2])
{
    cout<<"How many rows is your first matrix?"<<endl;
    cin>>rows[0];
    cout<<"How many columns is your first matrix?"<<endl;
    cin>>columns[0];
    
    //assigns rows and columns to pointers
    
    r = &rows[0];
    c = &columns[0];
    
    //Creates an array of matrices
    
    int matrix1[*r][*c];
    
    //assigns value for the matrix1 definied by rxc
    
    for (i=0; i<rows[0]; i++) {
        
        for (j=0; j<columns[0]; j++) {
            
            cout<<"What is your "<<q<<" "<<s<<" term?"<<endl;
            cin>>matrix1[i][j];
            
            s++;
            
        }//end column (j) for loop
        
        s=1;
        q++;
        
    }//end row (i) for loop
}

/*
void matrix::getInfo2()
{
    cout<<"How many rows is your second matrix?"<<endl;
    cin>>rows[1];
    cout<<"How many columns is your second matrix?"<<endl;
    cin>>columns[1];
    
    w = &rows[1];
    x = &columns[1];
    
    int matrix2[*w][*x];
    
    //assings values for the matrix2 definied by w x
    for (i=0; i<rows[1]; i++) {
        
        for (j=0; j<columns[1]; j++) {
            
            cout<<"What is your "<<a<<" "<<b<<" term?"<<endl;
            cin>>matrix2[i][j];
            
            a++;
            
        }//end column (j) for loop
        
        a=1;
        b++;
        
    }//end row (i) for loop
  
}*/

void matrix::output1()
{
    //outputs the matrix1 of numbers
    for (i=0; i<rows[0]; i++) {
        
        for (j=0; j!=columns[0]; j++) {
            
            cout<<matrix1[i][j];
            cout<<"  ";
            
        }//end column (j) output for loop
        
        cout<<endl;
        
    }//end row (i) output for loop
}

/*
void matrix::output2()
{
    //outputs the matrix2 of numbers
    
    for (i=0; i<rows[1]; i++) {
        
        for (j=0; j!=columns[1]; j++) {
            
            cout<<matrix2[i][j];
            cout<<"  ";
            
        }//end column (j) output for loop
        
        cout<<endl;
        
    }//end row (i) output for loop
}
*/

[tath]

You can create static array for matrix like:

#define MAX 20

class matrix
{
private:
values[MAX][MAX];
};

and when getInfo1 is executed you can show boundaries to user and check his input.

More professional would be creating dynamic array, ether of using pointer to pointer array ( **p) and new or vector class from std library (i wrote example in this topic http://www.cplusplus.com/forum/general/107678/ )

[mshoppas]

SWEET! But what do you mean by check boundaries? I can define MAX 20 and set my array to that and use it in the functions correct? But then how would I nullify all the other parts if its a 4X4 and not a 20X20? Or do I copy what I from that array into an array of a distinct size? I tried using pointers but I do not know how to call it then because I do not know what the actual size would be. Thank you so much for your help!

[tath]

this is what i meant:

	unsigned row_size;
	unsigned col_size;

	unsigned row;
	unsigned col;

	int matrix[MAX_SIZE][MAX_SIZE];
	
	do
	{
	cout << "enter number of columns: ";
	cin >> col_size;
	} while (col_size > MAX_SIZE);

	do
	{
	cout << "enter number of rows: ";
	cin >> row_size;
	} while (row_size > MAX_SIZE); 
	
	//now that you know matrix is of size [col_size][row_size]
	cout << "enter values:" << endl;

	for (row=0; row < row_size; row++)
		for (col=0; col < col_size; col++)
		{
			cout << "[" << row << "][" << col << "] = ";
			cin >> matrix[row][col];
		}

	cout << endl << "values entered:" << endl;
	for (row=0; row < row_size; row++)
	{
		for (col=0; col < col_size; col++)
		{
			cout << matrix[row][col] << " ";
		}
		cout << endl;
	}

	cout << endl;

but as i said before, this is static memory solution (C-like) and user can't enter matrix size bigger than defined value. If you would like to do this dynamically (user can enter any matrix size he wants):

	unsigned row_size;
	unsigned col_size;

	unsigned row;
	unsigned col;

	int **matrix;
	
	do
	{
	cout << "enter number of columns: ";
	cin >> col_size;
	} while (col_size <= 0);

	do
	{
	cout << "enter number of rows: ";
	cin >> row_size;
	} while (row_size <= 0); 
	
	//now that you know matrix is of size [col_size][row_size]
	// create it:
	matrix = new int*[col_size]; // create columns

	for (col=0; col < col_size; col++) //create rows
		matrix[col] = new int[row_size];

	cout << "enter values:" << endl;

	for (row=0; row < row_size; row++)
		for (col=0; col < col_size; col++)
		{
			cout << "[" << row << "][" << col << "] = ";
			cin >> matrix[col][row];
		}

	cout << endl << "values entered:" << endl;
	
	for (row=0; row < row_size; row++)
	{
		for (col=0; col < col_size; col++)
		{
			cout << matrix[col][row] << " ";
		}
		cout << endl;
	}

	cout << endl;


	for (col=0; col < col_size; col++)		
	{
		delete [] matrix[col]; //delete rows
	}
	delete [] matrix; //delete cols 

or you can use http://www.cplusplus.com/forum/general/107678/ ;)

[keskiverto]

@tath: Normal delete does not match the array-form of new; use array delete.

You do use column-major matrix. C/C++ style is to use row-major. That is not an error, but has to be documented.

Furthermore, you could allocate a single block for data rather than col_size separate blocks.

PS. Prefer a const to macro.

[tath]

@keskiverto thanks for your input - i always enjoy getting more info.

You do use column-major matrix

i wanted 2nd example to be similar to 1st one, to clearly see the difference.

use array delete

overlooked. Edited.