dynamic memory allocation using pointer to pointers

the question is; Write a program that prints out the memory addresses of each element in a two-dimensionalarray. Check to see if the values printed out make sense to you based on the way I explained it
before.

Below is the code I have done. I am having problems printing the "-" sign to keep formatting with the board when the user enter in different dimensions other than [4][4].

Code:

---

#include <iostream>


using namespace std;


void printTable (int x, int y) {
    int **p_p_twoDimension = new int* [y];


    for (int i = 0; i < y; i++) {
        p_p_twoDimension[i] = new int [x];
    }
    for (int i = 0; i < y; i++) {
        for (int j = 0; j < x; j++) {
            cout << &p_p_twoDimension[i][j];
            if (j!= x - 1) {
                cout << " | ";
            }
        }
        cout << endl;
        if (i!= y - 1) {
              for (int k = 0; k < (x*10)+1; ++k) {
                cout << "-";
              }
              cout << endl;
        }
    }
}


void freeTable (int **p_p_twoDimension, int y) {
    for (int i=0; i < y; i++) {
        delete [] p_p_twoDimension;
    }
}


int main()
{
    int x;
    int y;
    cout.flush() << "Please Enter the two dimensions you require" << endl;
    cin >> x >> y;
    int **p_p_twoDimension;
    printTable(x, y);
    freeTable(p_p_twoDimension, y);
    return 0;
}

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I ran it. I'm not seeing a problem: If you make the table big enough, the window will wrap the text, but you can't do anything about that.

Code:

---

freeTable(p_p_twoDimension, y);

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This call doesn't make sense. p_p_twoDimension is an uninitialized pointer
The array you're printing is local printTable() and is lost after return from that function -> leak

Kurt

hi thanks think this rectified now.

Code:

---

#include <iostream>


using namespace std;




void printTable (int x, int y) {
    int **p_p_twoDimension = new int* [y];


    for (int i = 0; i < y; i++) {
        p_p_twoDimension[i] = new int [x];
    }
    for (int i = 0; i < y; i++) {
        for (int j = 0; j < x; j++) {
            cout << &p_p_twoDimension[i][j];
            if (j!= x - 1) {
                cout << " | ";
            }
        }
        cout << endl;
        if (i!= y - 1) {
              for (int k = 0; k < (x*10)+1; ++k) {
                cout << "-";
              }
              cout << endl;
        }
    }
    for (int i=0; i < y; i++) {
        delete [] p_p_twoDimension;


    }
}


int main()
{
    int x;
    int y;
    cout.flush() << "Please Enter the two dimensions you require" << endl;
    cin >> x >> y;
    //int **p_p_twoDimension;
    printTable(x, y);
    //freeTable(p_p_twoDimension, y);
    return 0;
}

---

Still leaking. You have to delete all the pointers to int as well.
Kurt

Sorry just noticed that just after posting it.

Code:

---

#include <iostream>


using namespace std;




void printTable (int x, int y) {
    int **p_p_twoDimension = new int* [y];


    for (int i = 0; i < y; i++) {
        p_p_twoDimension[i] = new int [x];
    }
    for (int i = 0; i < y; i++) {
        for (int j = 0; j < x; j++) {
            cout << &p_p_twoDimension[i][j];
            if (j!= x - 1) {
                cout << " | ";
            }
        }
        cout << endl;
        if (i!= y - 1) {
              for (int k = 0; k < (x*10)+1; ++k) {
                cout << "-";
              }
              cout << endl;
        }
    }
    for (int i=0; i < y; i++) {
        delete [] p_p_twoDimension[i];


    }
}


int main()
{
    int x;
    int y;
    cout.flush() << "Please Enter the two dimensions you require" << endl;
    cin >> x >> y;
    printTable(x, y);
    return 0;
}

---

now only a single call to

Code:

---

delete [] p_p_twoDimension;

---

is missing
Kurt

I thought that I only had to release the memory once? Is that not the case?

Below is what I have adjusted. Thanks for the help.

Code:

---

#include <iostream>


using namespace std;




void printTable (int x, int y) {
    int **p_p_twoDimension = new int* [y];


    for (int i = 0; i < y; i++) {
        p_p_twoDimension[i] = new int [x];
    }
    for (int i = 0; i < y; i++) {
        for (int j = 0; j < x; j++) {
            cout << &p_p_twoDimension[i][j];
            if (j!= x - 1) {
                cout << " | ";
            }
        }
        cout << endl;
        if (i!= y - 1) {
              for (int k = 0; k < (x*10)+1; ++k) {
                cout << "-";
              }
              cout << endl;
        }
    }
    for (int i=0; i < y; i++) {
        delete [] p_p_twoDimension[i];
    }
    delete p_p_twoDimension;
}


int main()
{
    int x;
    int y;
    cout.flush() << "Please Enter the two dimensions you require" << endl;
    cin >> x >> y;
    printTable(x, y);
    return 0;
}

---

It's ok now. You need a call to delete for every call to new.
Kurt

Thanks. Think I need to go back to some old problems and sort them out!:)

Quote:

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Originally Posted by mp252

Write a program that prints out the memory addresses of each element in a two-dimensional array.

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Quote:

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dynamic memory allocation using pointer to pointers

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Did the problem statement mention that you should use pointer to pointers? Technically that's not a 2 dimensional array. Example of two dimensional array (note, NROW could be a variable, but NCOL needs to be a constant).

Code:

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#define NROW 4
#define NCOL 5

    int (*prow)[NCOL] = new int[NROW][NCOL];    // pointer to first row of two dimensional array

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Also just throwing in that the usual way to use a 2D array in C++ is to do:

std::vector<std::vector<T>> vector(N, std::vector<T>(M)); // <--- dynamic array; N and M must be known at runtime

std::array<std::array<T, M>, N> array; // <--- static array; N and M must be known at compile time. Also requires C+11.

Knowing how to allocate 2d arrays with pointers is all fine and dandy, but know that in production code - and if you have a choice - you should prefer using a vector or array construct. It's safer, faster and you don't have to deallocate the memory manually.

Quote:

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Originally Posted by Elysia

Also just throwing in that the usual way to use a 2D array in C++ is to do ... vector of vectors

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I'm wondering if this specific problem statement is asking to create a true 2D array, where the memory for the entire 2D array is contiguous and only contains values (no instances of pointers within the array).