c++ Dynamic Allocation of 2D Array Class

Question

Very new beginner here and I'm at the end of my rope with this assignment and would really appreciate any help :). Apologies in advance for the length.

I'm having some trouble with retrieving and setting values for my dynamically allocated 2D array. I have a class, defined below, that should construct a 2D array, at which point I need the option to set a value to a given point in the array and also to retrieve a value at a given point.

I ran the debugger, and got as far as to figure out that I have a segmentation fault when the setValue function runs. Can anyone help me understand what I'm doing wrong? As a beginner, the easier terms the better :). Thank you kindly in advance.

#include <iostream>
using namespace std;


class array2D
{
    protected:
        int xRes;
        int yRes;
        float ** xtable;

    public:
        array2D (int xResolution, int yResolution);
        ~array2D() {}
        void getSize(int &xResolution, int &yResolution);
        void setValue(int x,int y,float val);
        float getValue(int x,int y);
};


array2D::array2D(int xResolution, int yResolution)
{
    xRes=xResolution;
    yRes=yResolution;

    float ** xtable = new float*[yResolution];

    for(int i=0;i < yResolution;i++)
    {
        xtable[i] = new float[xResolution];
    }

    for(int i=0;i < yRes;i++)
    {
        for(int j=0;j < xRes;j++)
        {
            xtable[i][j]=45;
        }
    }
}

void array2D::getSize(int &xResolution, int &yResolution)
{
    xResolution=xRes;
    yResolution=yRes;
    cout << "Size of Array: " << xResolution << ", " << yResolution << endl;
}

void array2D::setValue(int x,int y,float val)
{
    xtable[x][y] = val;
}

float array2D::getValue(int x,int y)
{
    return xtable[x][y];
}

int main()
{
    array2D *a = new array2D(320,240);
    int xRes, yRes;
    a->getSize(xRes,yRes);
    for(int i=0;i < yRes;i++)
    {
        for(int j=0;j < xRes;j++)
        {
            a->setValue(i,j,100.0);
        }
    }

    for(int i=0;i < yRes;i++)
    {
        for(int j=0;j < xRes;j++)
        {
            cout << a->getValue(i,j) << " ";
        }

        cout << endl;
    }

    delete[] a;
}

Unfortunately std::vector is not something we've covered the use of yet, so it's somewhat 'forbidden' to use for this project :) — JY One, Jun 10 '17 at 06:30
Why are you not incrementing the indices of `xTable` inside the class? — user366312, Jun 10 '17 at 06:37

R Sahu · Accepted Answer · 2017-06-10T06:06:56.133

6

The line

float ** xtable = new float*[yResolution];

creates a function local variable. The member variable of the class still remains uninitialized. That's not what you want. To allocate memory and assign it to the member variable, remove the type specifier from that line. Just use:

xtable = new float*[yResolution];

Also, you need to switch the use of yResolution and xResolution in those lines. Otherwise, getValue and setValue will be using the indices incorrectly.

Swap the use of xResolution and yResolution in the following lines so that you use:

float ** xtable = new float*[xResolution];
for(int i=0;i < xResolution;i++)
{
    xtable[i] = new float[yResolution];
}

Swap the use of xRes and yRes in the following lines so that you use:

for(int i=0;i < xRes;i++)
{
    for(int j=0;j < yRes;j++)
    {
        xtable[i][j]=45;
    }
}

Since your class acquires resources using dynamic memory allocation, you should read up on The Rule of Three and update your class accordingly.

edited Jun 10 '17 at 06:06

answered Jun 10 '17 at 05:36

R Sahu

204,454
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1

and better yet, use `std::vector` – Ap31 Jun 10 '17 at 06:28
How can I ever thank you enough?! This did the trick. I've been studying possible solutions for over 12 hours now, but I've only come to understand now thanks to your response. Truly appreciate this; you are a lifesaver. – JY One Jun 10 '17 at 06:29
1

@KOne, glad I was able to help. As long as you learn from the experience, it's going to help you with your future programming efforts. Good luck. – R Sahu Jun 10 '17 at 06:32

Satya · Answer 2 · 2021-01-11T20:12:37.637

0

int r,c;
cin>>r>>c;
int** p=new int*[r];
for(int i=0;i<r;i++) {
    p[i]=new int[c];
}
for(int i=0;i<r;i++) {
delete [] p[i]; 
}
delete [] p;

Using this code you can create a 2D Dynamic Array in C++ which would allocate the memory of this array in the Heap Memory.

edited Jan 11 '21 at 20:12

answered Jan 11 '21 at 20:05

Satya

1,569
8
13

c++ Dynamic Allocation of 2D Array Class

2 Answers2