C++ Accessing Values at pointer of 2D Array

Question

I need a dynamic 2D array of ints, it will represent a standard matrix. The size and elements are read in from a file at runtime.

Taking direction from other stack posts I've setup my array as follows;

void buildArray(ifstream &file, int** 2dArray);
void buildQueue(Queue<int> &Q, int** 2dArray);

int main()
{
    int** 2dArray;
    Queue<int> Q;
    //...
    // open file
    //...
    buildMatrix(file, 2dArray)
    buildQueue(Q, 2dArray)
}

void buildArray(ifstream &file, int** 2dArray)
{
    int size, element;
    while (file.good()) {
        file >> size;

        2dArray = new int*[size];
        for (int i = 0; i < size; i++)
            2dArray[i] = new int[size];

        // now I should be able to use 2dArray[r][c]

        for (int i = 0; i < size; i++) {
            for (int j = 0; j < size; j++) {
                file >> element;
                2dArray[i][j] = element;
            }
        } 
}

Then I need to read the ints stored at each position [r][c] and build a queue. I think my problem is dereferencing the pointers... but I'm not sure.

void buildQueue(Queue<int> &Q, int** 2dArray)
{
    int row, column, element;
    // size is passed in as well, size is our rows or columns size here
    for (int i = 0; i < size; i++) {
            for (int j = 0; j < size; j++) {
                row = i;
                column = j;
                element = 2dArray[i][j];   // getting seg fault here!

                Q.push_back(row, column, element);
            }
     }
 }

I should add that I know I could use vectors. I'm challenging myself here to responsibly build, use and deallocate a 2D array using pointers.

Wouldn't it be overall simpler to have actual 2D array, in single memory block int *table = new int [size*size]; and accessed with table[column+size*row] (or make simple wrapper class around this)? — hyde
– hyde, Commented Feb 27, 2013 at 18:07
It might be. I was considering this approach earlier in this project. The pitfall here is that second array normally. — frankV
– frankV, Commented Feb 27, 2013 at 18:25
Your comment seems cut off, but you can still easily generate extra array with row pointers: int *rows[] and avoid overhead of having heap block for every row. Or just int *rowarray = table + row * size. — hyde
– hyde, Commented Feb 27, 2013 at 18:42

user93353 · Accepted Answer · 2013-02-28 00:01:27Z

1

What is table?

table = new int*[size];

Shouldn't this be

2dArray = new int*[size];

Also return the pointer from the function

Change

void buildArray(ifstream &file, int** 2dArray)

to

int ** buildArray(ifstream &file)

because otherwise buildArray gets a copy of the variable. And this copy now holds the pointer to the allocated memory. The pointer in the calling function never gets updated.

In your original code add a printf 2dArray before and after the buildArray call.

printf("%p, 2dArray);
buildMatrix(file, 2dArray);
printf("%p, 2dArray);

You will see that 2dArray doesn't change. So you are essentially passing an uninitialized pointer to buildQueue.

buildArray and call should be changed like this

void buildArray(ifstream &file, int** arr)
{
    int size, element;
    while (file.good()) {
        file >> size;

        arr = new int*[size];
        for (int i = 0; i < size; i++)
            arr[i] = new int[size];


        for (int i = 0; i < size; i++) {
            for (int j = 0; j < size; j++) {
                file >> element;
                arr[i][j] = element;
            }
        }
    }

    return arr;
}

int main()
{
    int** 2dArray;
    Queue<int> Q;

    //...
    // open file
    //...
    2dArray = buildMatrix(file);
    buildQueue(Q, 2dArray);
}

edited Feb 28, 2013 at 0:01

answered Feb 27, 2013 at 17:56

user93353

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4 Comments

frankV Over a year ago

I didn't catch that when I first posted the question. It has since been edited.

frankV Over a year ago

So you don't pass 2dArray into the function? Something about that is confusing me. How would you access it? (sorry if that sounds like a stupid question)

frankV Over a year ago

Or is the solution to make 2dArray global?

user93353 Over a year ago

@frankV - create local variable - int ** 2dArray - inside the function and return it. It doesn't even have to be the same name. i.e. (int ** x; x= new int*[size]; etc; x[i][j] = element; etc etc; return x` – user93353 - Updated the answer with code.

Jonathan Henson · Accepted Answer · 2013-02-27 18:05:29Z

1

If you are allocating the initial array inside a function, you either need to pass it as a triple pointer and change to:

*2dArray = new int*[size],

or return an int**

Otherwise you will lose the pointer to your new memory.

Don't forget, pointers are passed by value. As is, after your buildArray() call, you still will not have a pointer to valid memory.

edited Feb 27, 2013 at 18:05

answered Feb 27, 2013 at 18:00

Jonathan Henson

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Comments

Hulor · Accepted Answer · 2013-02-27 22:36:05Z

0

When you allocate in a function a pointer passed in argument you only change the copy value. In C/C++ when you send a parameter to a function a copy of this is create. So, like the other answers hasaid before, it's possible to return the new value of the pointer changing your void buildArray(...) to int** buildArray(...) or sending a pointer on your pointer (it's look difficult), or finally, you're in C++ so you can send a reference, void buildArray(ifstream &file, int**& 2dArray) also 2dArray will be the real pointer from the main and you should change his value without changing your code.

answered Feb 27, 2013 at 22:36

Hulor

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