Question

I am new to C++ and am trying to write code for a multi-dimensional array using double pointers. This is my code:

Class Declaration:

class magicMat{

    private:
         int** ptrnum;

    public:
        void init(int);
        void Display(int);
        void set(int);
        void message();
        void errorhandling(int);    
};

Function definitions:

void magicMat::init(int input)
{       
    ptrnum=new int*[input];

    for (int row=0;row<input;row++)
        ptrnum[row]=new int[input]; 

    for(int x=0;x<input;x++)
    {
        for (int y=0;y<input;y++)
        {
            *(ptrnum[x]+y)=0;
        }
    }
}

void magicMat::set(int input)
{
    int row=1,col=input/2,otherdiag=0;

    for(int value=1;value<=input*input;value++)
    {
        if (*(ptrnum[row]+col)>0)
        {
            row=row+2;
            if(row>input)
                row=row-input;

            col--;
            if(col<1)
                col=input;
        }
        *(ptrnum[row]+col)+=value;
        *(ptrnum[0]+col)+=value;
        *(ptrnum[row]+0)+=value;

        if (row==col)
            *(ptrnum[0]+0)+=value;          

        if (row+col==input+1)
            otherdiag+=value;                 
/*                                                                        */
/*       Determine where new row and col are                              */
/*                                                                     */
         row--;
         if (row < 1)                       /* If row exceeds side then   */
            row = input;                    /*  goto other side.          */
         col++;
         if (col > input)                   /* If col exceeds side then   */
            col = 1; 
    }       
}

Main function:

int main()
{
    int num;
    magicMat newMat;
    newMat.message();
    while(1)
    {
        cin>>num;
        if (cin.good())
        {
            newMat.errorhandling(num);
        }
        else if (!isdigit(num))
        {
            cout<<"Please enter only digits"<<endl;
        }    
        newMat.init(num);
        newMat.set(num);
        newMat.Display(num);
    }
    cout<<"\nBye bye!\n"<<endl;
    return 0;
}

It works in the init function but when in the set function I try to check the value it breaks at the first if statement in the set data function.

Answer 1

You are stepping outside the bounds of your array. Just as an example, if I run your code and enter 5 as my first digit which sets the value of input to 5, look at these lines at the end of your set function:

row--;
if (row < 1)
    row = input;

At the end of the first time through the loop in your set function, right before these lines, row is equal to 1. These three lines execute and as a result, row is set equal to 5.

At the beginning of your next loop in set, you do this:

if (*(ptrnum[row]+col)>0)

The problem is that ptrnum is a 5×5 array, which means valid indexes are from 0-4. At the end of the previous loop, however, you set row equal to 5 and as a result, you’re indexing outside the bounds of ptrnum and thus crashing your program.

I highly recommend either stepping through your code in a debugger or if you’re not sure how to do that, at least put in a bunch of cout statements in your set function so you can check that the variables are getting set to what you think they should get set to.

Good luck!