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Problem with class that stores 2d matrix

sys7em (6)
Hello guys,

I have this code which stores 2d matrix in a class matrix (keeping its elements in a dynamically allocated array of integers (int **A

However when I execute the program it reads the matrix and outputs it correctly but when it's being multiplied by a scalar the program crashes.

Why is this ?

Here's my code:

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#include<iostream>
#include <vector>
#include <ctime>
#include <fstream>
#include <algorithm>
#include <string>
#include <cmath>
#include <cstdlib>
#include <ctime>
#include <numeric>
#include <iterator>
#include <iomanip>

class matrix
{
	int **A;                          // pointer to store 2 dimensional array
	int x, y;

public:
	matrix();

	matrix(int, int);
	
	void read();
	void input();
   	void display();
	matrix operator* (int);
};

//
matrix :: matrix()
{
	x = 0;
	y = 0;
}

matrix :: matrix(int a, int b)
{
	x = a;
	y = b;
}



//read matrix
void matrix :: read()
{
	std::cout << " enter number of rows " << std::endl;
	std::cin >> x;
	std::cout << " enter number of columns " << std::endl;
	std::cin >> y;
}

//matrix input
void matrix :: input()
{ 
    	int num1, num2, k;
     	int i, j;
	A = new int *[x];            
	for (k = 0; k < x; k++)     	
		A[k] = new int [y];
	for (num1=0; num1<x; num1++)
	{

		for (num2=0; num2<y; num2++)
		{
			A[num1][num2] = 0;
		}
	} 
	for ( i = 0; i < x; i++)
	{
		for ( j = 0; j < y; j++)
		{
			std::cin >> A[i][j];
     	}

	}

}


//matrix display
void matrix :: display()

{
    	int i, j;
	for (i=0; i<x; i++)
	{
		for (j=0; j<y; j++)
		{
			std::cout << "  " << A[i][j];
		}
		std::cout << "\n";
	}
} 


//matrix mult
matrix  matrix :: operator* (int a)
{
	matrix temp(x, y);
	int num,num1, num2;
        for (num1=0; num1<temp.x; num1++)
	{
		for (num2=0; num2<temp.y; num2++)
		{
			temp.A[num1][num2] = 0;
		}
	}
	int i, j;
	for ( i = 0; i < x; i++)
	{
		for ( j = 0; j < y; j++)
		{
			temp.A[i][j] = a * A[i][j];
		}
	}
	return (temp);
}


// MAIN
int main ()

{

matrix s;
        matrix t = s;

	matrix M1, M2;
	M1.read ();
    	M1.input ();
 	M1.display ();
    	M2 = M1*2;
	M2.display();

	return 0;

}
jsmith (5804)
Because neither of your constructors initialize A.

Then on 101-118 you just assume that temp.A is initialized and actually points to allocated memory.
helios (17574)
Your constructor is not well-written. Instead of taking care of allocation, you're leaving that up to input.
sys7em (6)
Can you point what should I change in my design, cause I'm new in this. thanks in advance. Please post with examples.

Alex from Germany.
jsmith (5804)
The default constructor should initialize A to NULL.

The other constructor I assume is specifying the dimensions, in which case it should allocate a 2d buffer of the given dimensions and point A to it. (Best to initialize the values to zero in the matrix).

input() should not be a member of matrix, but should be a function elsewhere.

you need to add methods that allow you to set the size of the matrix and/or grow/shrink the matrix.

you need to write a copy constructor and an assignment operator that does the right thing.

you need to write a destructor that frees the memory referenced by A.
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