CODEWITHSUNDEEP
c++arraysfunctiondynamicmatrix
d2key
d2key

Reputation: 71

Dynamic arrays C++

i am beginner with c++ and i need to make a program to multiply two matrices. I already understand the array of array concept in order to make a dynamic matrix. The problem that im an facing after a make and fill the matrix is that i cannot access it. it suddenly stops when i run the program and just finished filling the second array with the function: 

void read_matrix(int** matrix, int row, int col)
{

cout << "Enter a matrix\n";

matrix = new int*[row];
for(int i = 0; i < row; i++)
    matrix[i] = new int[col]; 

if (!matrix){
    cerr << "Can't allocate space\n";
}

for(int i = 0; i < row; i++){
    for (int j = 0; j < col; j++){
        cin >> matrix[i][j];
    }
}
}

but according to my compiler, after the program stops there is an arrow pointing after the last loop of this function 

void multiply_matrix(int** matrix1, int rows1, int cols1, int** matrix2, int rows2, int cols2, int** result)
{

for(int i = 0; i < rows1; i++){
    for(int j = 0; j < cols2; j++){
        for (int k = 0; k < rows2; k++){
            result[i][j] += matrix1[i][k] * matrix2[k][j];
        }
    }
}

my main function is 

int main ()
{

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

//TODO: readin matrix dimensions
cout << "Enter matrix dimensions \n";
cin >> rows1 >> cols1 >> rows2 >> cols2;

if(cols1 != rows2){
    cout << "Error!";
    terminate();
}

    //memory for result matrix
    result = new int*[rows1];
       for(int i = 0; i < rows1; i++)
       result[i] = new int[cols2];


// 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;   

}

i can not get why it does not work. what i think is there is something wrong in the way i fil the matrix or o do something wrong in the way i send one matrix from one function to the other.

Thanks,

David

Upvotes: 2

Views: 1265

Answers (3)

molbdnilo
molbdnilo

Reputation: 66371

Your first problem is that you're not allocating the result matrix.

Your second problem is that you're not allocating the other matrices either.

The read_matrix assigns the allocated memory to the parameter you pass it.
Unfortunately, that parameter is a copy of the pointer in main, so the effect is local to the function.

To fix it, you can pass a reference to the variable you wish to assign to:

void read_matrix(int**& matrix, int row, int col);

or, better, just return the correct value from the function:

int** read_matrix(int row, int col);
// ...
matrix1 = read_matrix(rows1, cols1);

On a side note: you only need to input two dimensions - if the first matrix is M x N, the other must be N x M.

Upvotes: 1

Zac Howland
Zac Howland

Reputation: 15872

Instead of (incorrectly) allocating and (not) deallocating your memory manually, you should use either std::array or 'std::vector for your arrays. Since you are pulling in the matrix sizes dynamically, that would lead you to std::vector.

Simple example:

std::vector<std::vector<int> > matrix;
for (int i = 0; i < ROW_COUNT; i++)
{
    std::vector<int> row;
    for (int j = 0; j < COL_COUNT; j++)
    {
        row.push_back(RECORD[i][j]);
    }
    matrix.push_back(row);
}

There are ways to do it in a single line of code as well, but I'll leave it like this to demonstrate the basic idea.

Upvotes: 0

Jonathan Potter
Jonathan Potter

Reputation: 37122

You're not allocating your result matrix anywhere, so your multiply_matrix function is dereferencing a null pointer and writing to random memory.

Upvotes: 1

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