Creating and filling a matrix by input in C++

Question

I'm trying to write a program in C++ which is able to multiply matrices. Unfortunately, I'm not able to create a matrice out of 2 vectors. The goal: I type in the number of rows and the number of columns. Then there should be a matrix created with those dimensions. Then I can fill (for example rows = 2 cols = 2 => Matrix = 2 x 2) the matrix by typing in the numbers. I tried it with this 2 codes: (second one is in a header file)

#include <iostream>
#include "Matrix_functions.hpp"

using namespace std;

int main ()
{
    //matrices and dimensions
    int rows1, cols1, rows2, cols2;
    int **matrix1, **matrix2, **result = 0;

    cout << "Enter matrix dimensions" << "\n" << endl;
    cin >> rows1 >> cols1 >> rows2 >> cols2;

    cout << "Enter a matrix" << "\n" << endl;

    matrix1 = new int*[rows1];
    matrix2 = new int*[rows2];

    // Read values from the command line into a matrix
    read_matrix(matrix1, rows1, cols1);
    read_matrix(matrix2, rows2, cols2);

    // Multiply matrix1 one and matrix2, and put the result in matrix result
    //multiply_matrix(matrix1, rows1, cols1, matrix2, rows2, cols2, result);

    //print_matrix(result, rows1, cols2);

    //TODO: free memory holding the matrices

    return 0;
}

This is the main code. Now the header file with the read_matrix function:

#ifndef MATRIX_FUNCTIONS_H_INCLUDED
#define MATRIX_FUNCTIONS_H_INCLUDED

void read_matrix(int** matrix, int rows, int cols)
{
    for(int i = 0; i < rows; ++i)
        matrix[i] = new int[cols];

}

//int print_matrix(int result, int rows1, int cols1)
//{
//    return 0;
//}

//int multiply_matrix(int matrix2, int rows2, int cols2, int matrix3, int rows3, int cols3, int result2)
//{
//    return result2;
//}

#endif // MATRIX_FUNCTIONS_H_INCLUDED

The first part works. I can fill in the dimensions. But then it prints: Enter a matrix, and the program quits. Why am I not able to fill in the numbers of the matrix?

I hope someone is able to help me. If there are things unclear, let me know.

Thanks in advance:D (dont pay attention to most of the comments; those are for the rest of the multiplying code)

To start with, don't use pointers. Use [`std::vector`](http://en.cppreference.com/w/cpp/container/vector). — Some programmer dude, Sep 15 '13 at 13:58
To continue, the program quits because you do not actually have any code to read input from the user for the matrix values. — Some programmer dude, Sep 15 '13 at 13:59
Use a 1d-approch (`std::vector` and only one) and see http://stackoverflow.com/questions/17259877/1d-or-2d-array-whats-faster/17260533#17260533 why. — Pixelchemist, Sep 15 '13 at 14:02
To end, don't put non-inline or non-templated code in header files. *Declare* the functions in the header file, but *define* them in a source file. — Some programmer dude, Sep 15 '13 at 14:02

score 1 · Accepted Answer · edited Aug 17 '19 at 18:02

1

You forgot to cin the numbers, try this:

void read_matrix(int** matrix, int rows, int cols)
{
    for (int i = 0; i < rows; ++i) {
        matrix[i] = new int[cols];
        for (int j = 0; j < cols; ++j) {
            cin >> matrix[i][j];
        }
    }
}

edited Aug 17 '19 at 18:02

Maor Refaeli

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answered Sep 15 '13 at 14:02

frogatto

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Creating and filling a matrix by input in C++

1 Answers1