Dynamic Multidimensional array in C++ - c++

Question

I made Matrix Multiplication program in c++ using dynamic Multidimensional arrays.
problem is when i enter test values
matrix A = row1{ 1 } , row2 { 2 } matrix B = row1 {1 ,2 , 3} , it stops working at the loop where user
inputs values of first array , i found it using debugging. but program works fine when i enter
matrix A = row1{ 1 , 2 } , row2 { 3 , 4 } matrix B = row1 {5 , 6 } , row2 {7 , 8}
i want this program to be a general program that can multiply all matrixs
#include <iostream>
using namespace std;
class Lab_02
{
public:
void Product(){
int a1Rows, a1Columns;
int a2Rows, a2Columns;
cout << "Plz Enter the no. of rows for Array 1 :";
cin >> a1Rows;
cout << "Plz Enter the no. of columns for Array 1 :";
cin >> a1Columns;
cout << "Plz Enter the no. of rows for Array 2 :";
cin >> a2Rows;
cout << "Plz Enter the no. of columns for Array 2 :";
cin >> a2Columns;
int **dynamicArray = 0;
int **dynamicArray2 = 0;
int **dynamicArray3 = 0;
cout << endl;
for (int i = 0; i < a1Rows; i++)
{
dynamicArray3 = new int *[a1Rows];
}
for (int i = 0; i < a2Columns; i++)
{
dynamicArray3[i] = new int[a2Columns];
}
// memory allocated for elements of rows.
for (int i = 0; i < a1Rows; i++)
{
dynamicArray = new int *[a1Rows];
}
// memory allocated for elements of each column.
for (int i = 0; i < a1Columns; i++)
{
dynamicArray[i] = new int[a1Columns];
}
// memory allocated for elements of rows.
for (int i = 0; i < a2Rows; i++)
{
dynamicArray2 = new int *[a2Rows];
}
// memory allocated for elements of each column.
for (int i = 0; i < a2Columns; i++)
{
dynamicArray2[i] = new int[a2Columns];
}
cout << "enter the values or array 1 \n";
for (int i = 0; i < a1Rows; i++)
{
for (int j = 0; j < a1Columns; j++)
{
cout << "array[" << i << "][" << j << "]\t";
cin >> dynamicArray[i][j];
}
}
cout << "enter the values or array 2 :\n";
for (int i = 0; i < a2Rows; i++)
{
for (int j = 0; j < a2Columns; j++)
{
cout << "array[" << i << "][" << j << "]\t";
cin >> dynamicArray2[i][j];
}
}
int sum;
for (int i = 0; i < a1Rows; i++)
{
for (int j = 0; j < a1Columns ; j++)
{
sum = 0;
for (int k = 0; k < a2Columns ; k++)
{
sum = sum + (dynamicArray[i][k] * dynamicArray2[k][j]);
}
dynamicArray3[i][j] = sum;
}
}
cout <<"Result" << endl << endl;
for (int i = 0; i < a1Rows; i++)
{
for (int j = 0; j < a2Columns; j++)
{
cout << dynamicArray3[i][j] << "\t";
}
cout << endl;
}
}
};
void main(void)
{
Lab_02 object;
object.Product();
}

Answer 1

Your memory allocations for the matricies are the issue.
Change them to something like the following
// memory allocated for elements of rows.
dynamicArray = new int *[a1Rows];
// memory allocated for elements of each column.
for (int i = 0; i < a1Rows; i++)
{
dynamicArray[i] = new int[a1Columns];
}
You need to allocate one array of array for the rows and then you need to loop over the rows and allocate the columns.

Answer 2

The issue with the code is that you are not supposed to allocate the "rows" in a loop. All you need is one allocation for the rows, and then loop to allocate the data for each row.
So for example, instead of this:
for (int i = 0; i < a1Rows; i++)
{
dynamicArray = new int *[a1Rows];
}
// memory allocated for elements of each column.
for (int i = 0; i < a1Columns; i++)
{
dynamicArray[i] = new int[a1Columns];
}
The correct way would be this:
dynamicArray = new int *[a1Rows];
for (int i = 0; i < a1Columns; i++)
{
dynamicArray[i] = new int[a1Columns];
}
You made the same mistake for each of the loops.
Also, some points:
You failed to deallocate the memory that was allocated.
If you used std::vector, then things become much easier.
You need to check if the matrices are multiplyable before performing the sum loop. By multiplyable meaning that the number of columns and rows of matrix A and B satisfy the requirements for multiplication of A and B.
for (int i = 0; i < a1Rows; i++)
{
for (int j = 0; j < a1Columns; j++)
{
sum = 0;
for (int k = 0; k < a2Columns; k++)
sum = sum + (dynamicArray[i][k] * dynamicArray2[k][j]);
dynamicArray3[i][j] = sum;
}
}
This loop will go haywire if the dynamicArray1 and dynamicArray2 do not have the requisite number of columns and rows before multiplying.
First, the following test should be done before multiplying:
if (a1Columns != a2Rows)
return;
Second, your k loop is wrong. It should be this:
for (int k = 0; k < a2Rows; k++)
sum = sum + (dynamicArray[i][k] * dynamicArray2[k][j]);
dynamicArray3[i][j] = sum;