Pass a 2D array to a function for a n*n matrix

Question

I have coded this program to get two matrices and in a different function, it calculates the addition of the two matrices, but when I want to get the two arrays as input arguments in my "sum" function it generates errors and it doesn't work.

My efforts include reviewing this question at Stackoverflow : Passing a 2D array to a C++ function It helps if only I set A and B arrays with numerical dimensions like : A[2][2] not A[ar][ac]. I want the function to work for n*n matrices not up to a limited number.

#include <iostream>
#include <iomanip>
using namespace std;

template <size_t row, size_t column, size_t row2, size_t column2 >
void sum(int(&array)[row][column], int(&array2)[row2][column2]);

int main()
{
    int ar, ac, br, bc;
    
    cout << "Welcome to my matrix calculator. \nPlease enter the values of you matrices.";
    cout << "\n\n\tFor your FIRST matrix;";
    cout << "\n\tNumber of \'ROWS\' : ";
    cin >> ar;
    cout << "\tNumber of \'COLUMNS\' : ";
    cin >> ac;
    
    int A[ar][ac];
    
    cout << "\n\tPlease enter the values of your matrix." << endl << endl;
    
    for (int i=0; i<ar; i++)
    {
        for (int j=0; j<ac; j++)
        {
            int m = i;
            m++;
            int n = j;
            n++;
            
            cout << "\tNumber[" << m << "][" << n << "] : ";
            cin >> A[i][j];
        }
        cout << endl;
    }
    
    
    cout << "\n\tFor your SECOND matrix;";
    cout << "\n\tNumber of \'ROWS\' : ";
    cin >> br;
    cout << "\tNumber of \'COLUMNS\' : ";
    cin >> bc;
    
    int B[br][bc];
    
    cout << "\n\tPlease enter the values of your matrix." << endl << endl;
    
    for (int i=0; i<br; i++)
    {
        for (int j=0; j<bc; j++)
        {
            int m = i;
            m++;
            int n = j;
            n++;
            
            cout << "\tNumber[" << m << "][" << n << "] : ";
            cin >> B[i][j];
        }
        cout << endl;
    }
    

    cout << "\nYour FIRST matrix is : " << endl;
    
    for (int i=0; i<ar; i++)
    {
        cout << endl  << "\t" << "|";
        for (int j=0; j<ac; j++)
        {
            cout << setw(4) << A[i][j];
        }
        cout << "  |";
    }
    
    cout << "\n\nYour SECOND matrix is : " << endl;
    
    for (int i=0; i<br; i++)
    {
        cout << endl  << "\t" << "|";
        for (int j=0; j<bc; j++)
        {
            cout << setw(4) << B[i][j];
        }
        cout << "  |";
    }
    
    sum(A, B);
}


template <size_t row, size_t column, size_t row2, size_t column2 >
void sum(int(&array)[row][column], int(&array2)[row2][column2])
{
    if(row==row2 && column==column2)
    {
        cout << "\n\n\tThe addition of your matrices is = ";
        int add[row][column];
        
        for(int i=0; i<row; i++)
        {
            for(int j; j<column; j++)
            {
                add[i][j] = array[i][j] + array2[i][j];
            }
        }
        
        for(int i=0; i<row; i++)
        {
            cout << endl  << "\t" << "|";
            for(int j=0; j<column; j++)
            {
                cout << setw(4) << add[i][j];
            }
            cout << "  |";
        }
    }
    
    else
    {
        cout<< "Matrixes with different rows and columns can \'not\' be added together.";
    }
}

I did try whatever was on the internet but none of work for n*n matrices, could you please help? Thanks in advance!

This is impossible in Standard C++ and extremely hard in all of the Variable Length Array allowing extended C++ implementations that I know of. Use [something like this instead](https://stackoverflow.com/a/36123944/4581301). — user4581301, Dec 16 '22 at 16:08
IMO The best way is to model your matrices as what they are matrix classes and pass those around. E.g. make your 2D vector a member of a class called Matrix and use a 2d array inside it e.g. std::vector> values. But if you can just use a matrix/math library, then you can reuse tested code and focus at your real problem. — Pepijn Kramer, Dec 16 '22 at 16:08
Bah! `vector` of `vector`s have severe performance problems due to poor spatial locality. — user4581301, Dec 16 '22 at 16:09
@πάνταῥεῖ I have already declared and initialized ar and ac. — Anonymous, Dec 16 '22 at 16:13
@Anonymous it's not standard c++ though: https://stackoverflow.com/questions/1887097/why-arent-variable-length-arrays-part-of-the-c-standard?r=Saves_AllUserSaves — πάντα ῥεῖ, Dec 16 '22 at 16:14
*I did try whatever was on the internet* -- `int A[ar][ac];` -- And by using the internet as the way to learn C++ instead of using good C++ books, you ran right into the invalid use of arrays. This is the danger of going to websites that use this invalid code. Every *good* C++ *book* would never show arrays declared this way -- why? -- because it isn't valid C++. — PaulMcKenzie, Dec 16 '22 at 16:22
@PaulMcKenzie Thanks for your tip, but I learned C++ by book, searched on websites to solve my current problen, but I have seen codes like this : 'int size=10; int Array[size]; That's why I coded so. — Anonymous, Dec 16 '22 at 16:27
@Anonymous *but I have seen codes like this : 'int size=10; int Array[size];* -- Where did you see such code? That's my whole point -- you got this code from a dodgy and crap websites that supposedly "teach C++". Look in the C++ books you say you are using -- do any of them show arrays being declared this way? — PaulMcKenzie, Dec 16 '22 at 16:29
@PaulMcKenzie I wrote the whole code on my own, not using any website, looked for a way to get a 2D array in a function on the net, could you be as kind as to say if I don't declare A like: A[n1][n2] how should I declare it to get as much numbers as wanted? I don't insist my code is right, but I didn't know any other way. Thanks. — Anonymous, Dec 16 '22 at 16:33
Dynamic arrays are done by using `std::vector`. This was mentioned already in the previous comments. — PaulMcKenzie, Dec 16 '22 at 16:34

Lou Franco · Accepted Answer · 2022-12-16T16:29:37.547

1

You could just interpret a 1-dimensional array as two-dimensional (one row after another)

// To allocate the array, multiply the number of rows by the number of columns
int *A = new int[ar * ac];
int *B = new int[br * bc];

// To access [i][j], multiply i by the number of columns, and add j
A[(i*ac) + j] = x;

sum(A, ar, ac, B, br, bc);

// don't forget to delete the arrays (because you new'd them)
delete[] A;
delete[] B;

// ....

void sum (int *array1, size_t r1, size_t c1, int *array2, size_t r2, size_t c2) {
   // check for compatibility, nested loop and then ....  
   add[c1 * i + j] = array1[c1 * i + j] + array2[c1 * i + j];
   // ....
}

Leaving out a lot of code, but hopefully you get the idea.

This is how to do it with just regular int arrays. To do complex matrix stuff, you should be building up a custom class that hides the details and makes it easier to just think of the objects as 2-d matrices.

edited Dec 16 '22 at 16:29

answered Dec 16 '22 at 16:14

Lou Franco

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1

That's way easier to do simply using `std::vector` – πάντα ῥεῖ Dec 16 '22 at 16:15
Give an answer! – Lou Franco Dec 16 '22 at 16:16
3

Don't forget to delete the arrays. – Jim Rhodes Dec 16 '22 at 16:17
Yes, there are lots of details left out -- it's just to give the idea. – Lou Franco Dec 16 '22 at 16:18
Thank you, but didn't get the idea :( I'm new to C++ unfortunately. – Anonymous Dec 16 '22 at 16:24
1

If I have a 3x3 matrix, that could just be an array of 9 values (3 * 3). The first three values are in indices 0, 1, 2. The next row is in 3, 4, 5. And the third row is in 6, 7, 8. – Lou Franco Dec 16 '22 at 16:26
1

To allocate the array, you multiply the number of rows by the number of columns. To access `[i][j]`, you multiply `i` by the number of columns in the matrix and add `j`. – Lou Franco Dec 16 '22 at 16:26
Thank you very much, but it generates an error about "add" in the last line of your sample. – Anonymous Dec 16 '22 at 16:44
1

I am not putting in all of the code -- you need to use the idea to write your full sum function with the if and the loops. You had `int add[row][column];` -- but that doesn't work -- so rewrite that line using what I have explained. – Lou Franco Dec 16 '22 at 17:03

Pass a 2D array to a function for a n*n matrix

1 Answers1