Unhandled exception

[bsr]

I wrote an algorithm for matrix multiplication but I have a compiler error "Unhandled exception at 0x0FE8591C (msvcp140d.dll) in matriscarpim.exe: 0xC0000005: Access violation reading location 0xCDCDCDCD. occurred"
does anyone can help me?
all codes here.

#include <iostream>
using namespace std;
void matrixMult(int matrix1, int matrix2, int a, int b);
int main() {
	int a, b;
	cout<<"please enter number of rows and columns"<<endl;
	cin >> a;
	cin >> b;

	int **matrix1 = new int *[a];
	matrix1[a] = new int[b];

	int **matrix2 = new int *[a];
	matrix2[a] = new int[b];


	cout << "enter value of matrix one" << endl;
	for (int i = 0; i < a; i++) {
		for (int j = 0; j < b; j++) {
		
			cout<<"matrix["<<i<<"][" <<j<< "] degerini giriniz:";
			cin >> matrix1[i][j];
		}
	}

	cout << "enter value of matrix two" << endl;
	for (int i = 0; i < a; i++) {
		for (int j = 0; j < b; j++) {
			cout << "matrix[" << i << "][" << j << "] degerini giriniz:";
			cin >> matrix2[i][j];
		}
	}
	matrixMult(matrix1[a][b],  matrix2[a][b], a,  b);
	int n;
	cin >> n;
	return 0;
}
void matrixMult(int matrix1, int matrix2, int a, int b) {
	int **mult = new int *[a];
	mult[a] = new int[b];

	
	for (int i = 0; i < a; i++) {
		for (int j = 0; j < b; j++) {
			for (int k = 0; k < a; k++) {
				int **matrix1 = new int *[i];
				matrix1[i] = new int[k];

				int **matrix2 = new int *[k];
				matrix2[k] = new int[j];

				mult[i][j] += matrix1[i][k] * matrix2[k][j];
			}
			 
		}
	}
	
	for (int i = 0; i < a; i++) {
		for (int j = 0; j < b; j++) {
			cout << mult[i][j]<<endl;
		}
	}
}

[Peter87]

This is not correct.

int **matrix1 = new int *[a];
matrix1[a] = new int[b];

The first line creates an array of a int pointers, OK.
The second line creates one array of b integers assigns the array pointer to matrix1[a] which is out of bounds because the last valid index is a-1 (indexes start at zero).
If you want each pointer in the array to point to its own array of b integers you will have to use a loop and allocate a arrays that you assign to the a array elements.

[bsr]

thank you. actually I want to make a program which asks user to enter size of the matrix. it means I want to create dynamic matrices but it does not work that way.

[Peter87]

Yes. What I meant was something like this.

int **matrix1 = new int *[a];
for (int i = 0; i < a; ++i)
{
	matrix1[i] = new int[b];
}

[bsr]

thank you so much!

[bsr]

but now I have an other error in this line:

matrixMult(matrix1[a][b], matrix2[a][b], a, b);

[MikeyBoy]

What are aand b? The sizes of your matrix? Because, if so, matrix1[a][b] and matrix2[a][b] are beyond the end of your matrix; you're attempting to access memory that is being used for something else.

[bsr]

yes, but I didn't find any other ways to call function with matrices.

[Peter87]

It's because the parameter types are incorrect. int should be int**, same as matrix1 and matrix2.

void matrixMult(int** matrix1, int** matrix2, int a, int b) {

[bsr]

thank you. It works but incorrect :(
this is programs output

 please enter number of rows and columns
2
2
enter value of matrix one
enter value of matrix[0][0] :4
enter value of matrix[0][1] :5
enter value of matrix[1][0] :8
enter value of matrix[1][1] :7
enter value of matrix two
enter value of matrix[0][0] :6
enter value of matrix[0][1] :4
enter value of matrix[1][0] :1
enter value of matrix[1][1] :5
-168958497
-168958497
-168958497
-168958497 

[Peter87]

Go though the matrixMult function and make sure that the logic is correct.

[lastchance]

@bsr, please post your latest code BELOW THIS POST - full compileable version. There is no way anyone can offer advice on matrix multiplication without seeing it. Your original code was multiplying things that hadn't even been initialised.

[bsr]

this is my whole code. I couldn't see any problems in my matrixMult function.

#include <iostream>
using namespace std;
void matrixMult(int** matrix1, int** matrix2, int a, int b);
int main() {
	int a, b;
	cout<<"please enter number of rows and columns"<<endl;
	cin >> a;
	cin >> b;

	int **matrix1 = new int *[a];
	for (int i = 0; i < a; ++i)
	{
		matrix1[i] = new int[b];
	}

	int **matrix2 = new int *[a];
	for (int i = 0; i < a; ++i)
	{
		matrix2[i] = new int[b];
	}

	cout << "enter value of matrix one" << endl;
	for (int i = 0; i < a; i++) {
		for (int j = 0; j < b; j++) {
		
			cout<<"enter value of matrix["<<i<<"][" <<j<< "] :";
			cin >> matrix1[i][j];
		}
	}

	cout << "enter value of matrix two" << endl;
	for (int i = 0; i < a; i++) {
		for (int j = 0; j < b; j++) {
			cout << "enter value of matrix[" << i << "][" << j << "] :";
			cin >> matrix2[i][j];
		}
	}
	matrixMult(matrix1,  matrix2, a,  b);
	int n;
	cin >> n;
	return 0;
}
void matrixMult(int** matrix1, int** matrix2, int a, int b) {
	int **mult = new int *[a];
	for (int i = 0; i < a; ++i)
	{
		mult[i] = new int[b];
	}
	
	
	for (int i = 0; i < a; i++) {
		for (int j = 0; j < b; j++) {
			for (int k = 0; k < a; k++) {

				mult[i][j] += matrix1[i][k] * matrix2[k][j];
			}
			 
		}
	}
	
	for (int i = 0; i < a; i++) {
		for (int j = 0; j < b; j++) {
			cout << mult[i][j]<<endl;
		}
	}
}

[Peter87]

You never initialize the matrix elements. If you want them to be zero-initialized you can add () or {} after the square brackets when creating the integer arrays.

mult[i] = new int[b]{}; // allocates an array of b ints that are initialized to zero. 

[bsr]

thank you so much! it works without any mistake.