Why do I have errors about variables that I pass to a function to compute the multiplication of 2 matrices?

Shishykish (140)
Hello,

I am trying to calculate the multiplication of 2 matrices. I have firstly dynamically created the matrices following user input of their sizes. Then when I pass the pointer itself to a function I have made, I have a variety of errors,

ays

argument of type "double **" is incompatible with parameter of type "double *"
void Multiply function, in relation to the way I have indexed the arr

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#include <iostream>
#include <cassert>

using namespace std;

void Multiply(double*[], double*[], double*[], int, int, int, int);
double** MakeMatrix(int, int);
bool MatrixMultiplicationCheck(int, int);
void DataEntryMatrix(double**, int, int);
void FreeMatrixMemory(int numRows, double** matrix);

double** MakeMatrix(int numRows, int numCols)
{
    double** matrix;
    matrix = new double* [numRows]; 
    for (int i = 0; i < numRows; i++) 
    {
        matrix[i] = new double[numCols];
    } 
    return matrix;
}

void FreeMatrixMemory(int numRows, double** matrix)
{
    for (int i = 0; i < numRows; i++) 
    {
        delete[] matrix[i];
    } 
    delete[] matrix;
}

void DataEntryMatrix(double** x, int numRows, int numCols)
{
    double temp;

    for (int i = 0; i < numRows; i++)
    {
        for (int j = 0; j < numCols; j++)
        {
            cout << "What is number you would like to enter?" << endl;
            cin >> temp;
            x[i][j] = temp;
        }
    }
    cout << "The elements of your matirix are " << endl;
    cout << "{ ";

    for (int i = 0; i < numRows; i++)
    {
        for (int j = 0; j < numCols; j++)
        {
            cout << x[i][j] << ", ";
        }
    }
    cout << "}" << endl;
}

bool MatrixMultiplicationCheck(int columns1, int rows2)
{
    assert(columns1 != rows2);
    {
        cout << "Matrices cannot be mutiplied" << endl;
        return false;
    }

    return true;
}

void Multiply(double* x[], double* y[], double* z[], int numRows1, int numCols1, int numRows2, int numCols2)
{
    for (int i = 0; i < numCols1; i++)
    {
        for (int j = 0; j < numCols1; j++)
        {
            z[i][j] = 0;
            for (int k = 0; k < numCols1; k++)
            {
                z[i][j] += x[i][k] * y[k][j];
            }

        }
    }
}

int main()
{

    int i,j,k,l = 0;

    cout << "How many rows in your first matrix?" << endl;
    cin >> i;
    cout << "How many columns in your first matrix?" << endl;
    cin >> j;

    cout << "How many rows in your 2nd matrix?" << endl;
    cin >> k;
    cout << "How many columns in your 2nd matrix?" << endl;
    cin >> l;

    cout << "Matrix A has rows = " << i << " and columns = " << j << endl;
    cout << "Matrix B has rows = " << k << " and columns = " << l << endl;

    MatrixMultiplicationCheck(j, k);

    double** A;
    double** B;
    double** C;

    A = MakeMatrix(i, j);
    B = MakeMatrix(k, l);
    C = MakeMatrix(j, k);

    Multiply(*A, *B, *C, i, j, k, l);

    return 0;
}
}
jonnin (11495)
*A turns a ** to a * … its a dereference. int *a; … *a = 3; //sets the value, not the pointer, and is identical to a[0] = 3;
just call it with A, B and C without *s.
Last edited on
lastchance (6980)
Multiply(A, B, C, i, j, k, l);

You will need to remove the extraneous curly brace at the end, too.
Shishykish (140)
Hello,

Eventually I went back to square 1. Here's my solution

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#include <iostream>
#include <cassert>

using namespace std;

void DisplayMatrix(double**, int, int);
void EnterElements(double**, int, int);
void Multiply(double**, double**, double**, int, int, int, int);

void DisplayMatrix(double** matrix, int nrows, int ncols)
{
    cout << "The elements of your matrix are" << endl;
    cout << "{";
    for (int i = 0; i < nrows; i++)
    {
        for (int j = 0; j < ncols; j++)
        {
            cout << matrix[i][j] << ", ";
        }
    }
    cout << "}" << endl;
}

void EnterElements(double** matrix, int nrows, int ncols)
{
    cout << "Enter elements for your matrix" << endl;
    for (int i = 0; i < nrows; i++)
    {
        for (int j = 0; j < ncols; j++)
        {
            cin >> matrix[i][j];
            cout << endl;
        }
    }
}

void Multiply(double** matrix1, double** matrix2, double** matrix3, int nrows1, int ncols1, int nrows2, int ncols2)
{
    for (int i = 0; i < nrows1; i++)
    {
        for (int j = 0; j < ncols2; j++)
        {
            matrix3[i][j] = 0.0;
        }
    }

    for (int i = 0; i < nrows1; ++i)
    {
        for (int j = 0; j < ncols2; ++j)
        {
            for (int k = 0; k < ncols1; ++k)
            {
                matrix3[i][j] += matrix1[i][k] * matrix2[k][j];
            }
        }
    }
}

int main()
{
    int numRows1,numCols1, numRows2, numCols2;

    cout << "How many rows in your first matrix?" << endl;
    cin >> numRows1;
    cout << "How many columns in your first matrix?" << endl;
    cin >> numCols1;

    cout << "How many rows in your 2nd matrix?" << endl;
    cin >> numRows2;
    cout << "How many columns in your 2nd matrix?" << endl;
    cin >> numCols2;

    cout << "Matrix A has rows = " << numRows1 << " and columns = " << numCols1 << endl;
    cout << "Matrix B has rows = " << numRows2 << " and columns = " << numCols2 << endl;

    double** A = new double* [numRows1];

    for (int i = 0; i < numRows1; i++) {
        A[i] = new double[numCols1];
    }

    double** B = new double* [numRows2];

    for (int i = 0; i < numRows2; i++) {
        B[i] = new double[numCols2];
    }

    assert(numCols1 == numRows2);
    {
        cout << "The number of columns in the 1st matrix don't match rows in the 2nd." << endl;
    }

    double** C = new double* [numRows2];

    for (int i = 0; i < numRows2; i++) {
        C[i] = new double[numCols1];
    }

    EnterElements(A, numRows1, numCols1);
    EnterElements(B, numRows2, numCols2);

    DisplayMatrix(A, numRows1, numCols1);
    DisplayMatrix(B, numRows2, numCols2);

    Multiply(A, B, C, numRows1, numCols1, numRows2, numCols2);

    DisplayMatrix(C, numCols1, numRows2);

    for (int i = 0; i < numRows1; ++i)
    {
        delete[] A[i];
    }
    delete[] A;

    for (int i = 0; i < numRows2; ++i)
    {
        delete[] B[i];
    }
    delete[] B;

    for (int i = 0; i < numRows2; ++i)
    {
        delete[] C[i];
    }
    delete[] C;

    return 0;
}
Shishykish (140)
Hi,

Following your feedback, I went back to the old code and did this. Thanks!

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#include <iostream>
#include <cassert>

using namespace std;

void Multiply(double* [], double* [], double* [], int, int, int, int);
void DisplayMatrix(double**, int, int);
double** MakeMatrix(int, int);
bool MatrixMultiplicationCheck(int, int);
void DataEntryMatrix(double**, int, int);
void FreeMatrixMemory(int numRows, double** matrix);

void DisplayMatrix(double** matrix, int rows, int cols)
{
    for (int i = 0; i < rows; i++)
    {
        for (int j = 0; j < cols; j++)
        {
            cout << matrix[i][j] << ", ";
        }
    }
}

double** MakeMatrix(int numRows, int numCols)
{
    double** matrix;
    matrix = new double* [numRows];
    for (int i = 0; i < numRows; i++)
    {
        matrix[i] = new double[numCols];
    }
    return matrix;
}

void FreeMatrixMemory(int numRows, double** matrix)
{
    for (int i = 0; i < numRows; i++)
    {
        delete[] matrix[i];
    }
    delete[] matrix;
}

void DataEntryMatrix(double** x, int numRows, int numCols)
{
    double temp;

    for (int i = 0; i < numRows; i++)
    {
        for (int j = 0; j < numCols; j++)
        {
            cout << "What is number you would like to enter?" << endl;
            cin >> temp;
            x[i][j] = temp;
        }
    }
    cout << "The elements of your matirix are " << endl;
    cout << "{ ";

    for (int i = 0; i < numRows; i++)
    {
        for (int j = 0; j < numCols; j++)
        {
            cout << x[i][j] << ", ";
        }
    }
    cout << "}" << endl;
}

bool MatrixMultiplicationCheck(int columns1, int rows2)
{
    assert(columns1 != rows2);
    {
        cout << "Matrices cannot be mutiplied" << endl;
        return false;
    }

    return true;
}

void Multiply(double* x[], double* y[], double* z[], int numRows1, int numCols1, int numRows2, int numCols2)
{
    for (int i = 0; i < numCols1; i++)
    {
        for (int j = 0; j < numCols1; j++)
        {
            z[i][j] = 0;
            for (int k = 0; k < numCols1; k++)
            {
                z[i][j] += x[i][k] * y[k][j];
            }

        }
    }
}

int main()
{

    int i, j, k, l = 0;

    cout << "How many rows in your first matrix?" << endl;
    cin >> i;
    cout << "How many columns in your first matrix?" << endl;
    cin >> j;

    cout << "How many rows in your 2nd matrix?" << endl;
    cin >> k;
    cout << "How many columns in your 2nd matrix?" << endl;
    cin >> l;

    cout << "Matrix A has rows = " << i << " and columns = " << j << endl;
    cout << "Matrix B has rows = " << k << " and columns = " << l << endl;

    //MatrixMultiplicationCheck(j, k);

    double** A;
    double** B;
    double** C;

    A = MakeMatrix(i, j);
    B = MakeMatrix(k, l);
    C = MakeMatrix(j, k);

    DataEntryMatrix(A, i, j);
    DataEntryMatrix(B, k, l);

    Multiply(A, B, C, i, j, k, l);
    DisplayMatrix(C, j, k);

    FreeMatrixMemory(i, A);
    FreeMatrixMemory(l, B);
    FreeMatrixMemory(j, C);

    return 0;
}
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