2D dynamic memory allocation array in C++

Question

A few days ago I learned about creating 2D allocated memory arrays from the internet, it works perfect. To access the array we just simply use matrix[i][j], however is there any way that I can dereference this 2D array by using * notation instead of [] for input as well as other methods?

First questions is solved I can use *(*(matrix + i) + j)

Now I got another question, last code segment is to free the allocated memory (I got it from internet as well), but I don't understand it, why cant I just use delete [] matrix ?

int **matrix;

// dynamically allocate an array
matrix = new int *[row]; 
for (int count = 0; count < row; count++)
{
    matrix[count] = new int[col];
}

// input element for matrix
cout << endl << "Now enter the element for the matrix..."; 
for (int i=0; i < row; i++) 
{
    for (int j=0; j < col; j++)
    {
        cout << endl << "Row " << (i+1) << " Col " << (j+1) << " :";
        cin >> matrix[i][j]; // is there any equivalent declaration here?
    }
}

// free dynamically allocated memory
for( int i = 0 ; i < *row ; i++ )
{
    delete [] matrix[i] ;   
}
delete [] matrix ;

If I understand the question, *(*(matrix+i)+j)

WhozCraig
– WhozCraig

2013-02-12 09:21:14 +00:00
Commented Feb 12, 2013 at 9:21 — WhozCraig
– WhozCraig, Commented Feb 12, 2013 at 9:21

JBL · Accepted Answer · 2015-04-29 07:45:42Z

6

Answering your second question: when you allocate a 2D array with the following code

// dynamically allocate an array
    matrix = new int *[row]; 
    for (int count = 0; count < row; count++)
        matrix[count] = new int[col];

you are in fact allocating one array of pointers (your matrix variable, which is a double pointer) and "row" arrays of integers (each one representing one row in your matrix, of size "col"), which are matrix[0], matrix[1], etc. up to matrix[row-1].

Thus, when you want to free your matrix, you'll first need to free every single row (the arrays allocated within the loop), and then the array which held the rows. In your case, the code you use to free your matrix is partly wrong, and should be more like the following :

// free dynamically allocated memory
for( int i = 0 ; i < row ; i++ )
{
    //first we delete each row
    delete [] matrix[i] ;
}
//finally, we delete the array of pointers
delete [] matrix ;

The delete within the loop will free each row of your matrix, and the final delete will free the array of rows. In your code, you use delete row times on your double pointer (matrix), which makes no sense.

Finally, using a single delete on the double pointer is wrong, because it would end up in a memory leak as you aren't freeing the memory allocated for each row, only the pointers referring to it.

edited Apr 29, 2015 at 7:45

answered Feb 12, 2013 at 14:04

JBL

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1 Comment

Timʘtei Over a year ago

It's working now. I was making another mistake. I will delete these comments.

Pubby · Accepted Answer · 2013-02-12 09:20:29Z

3

Since a[b] is just *(a + b) you can of course do this:

*(*(matrix + i) + j)

Anyway, those new allocations are error prone. If one of the nested news throws then you'll have a leak. Try using std::vector instead.

answered Feb 12, 2013 at 9:20

Pubby

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Comments

Barmar · Accepted Answer · 2013-02-12 09:39:32Z

2

Something like this would work:

int **matrix;

// dynamically allocate an array
matrix = new (std::nothrow) int *[row];
if (matrix == NULL)
{
      // handle the error
}
for (int count = 0; count < row; count++)
{
    *(matrix + count) = new (std::nothrow) int[col];
    if (matrix[count] == NULL)
    {
          // handle the error
    }
 }

cout << "\nNow enter the element for the matrix..."; 
for (int i=0; i < row; i++)
{
    for (int j=0; j < col; j++)
    {
        cout << "\nRow " << (i+1) << " Col " << (j+1) << " :";
        cin >> *(*(matrix + i) + j);
    }
}

edited Feb 12, 2013 at 9:39

Barmar

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answered Feb 12, 2013 at 9:25

SeedmanJ

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4 Comments

Casper Over a year ago

the notation works well, is this the better way to make 2D allocated memory array? whats wrong with mine?

SeedmanJ Over a year ago

It's an equivalent of malloc in C, as exceptions are a new feature in C++, you'd have to put new int[col] into a try bracket, and eventually catch an exception. If you precise the std::nothrow, in error case, your ptr will point to NULL.

mlstudent Over a year ago

Your way is the same, but easier to read, so that if you're writing it, you're more likely to write it correctly.

SeedmanJ Over a year ago

Yep it does exactly the same, but doesn't throw an exception. It returns NULL on error.

mlstudent · Accepted Answer · 2013-02-12 09:22:34Z

1

Yes, you use pointer addition, but you need to understand how the memory is laid out. Say x is a pointer to the first element of an array of ints, if you want to access x[2], you can use *(x+2). However, with matrices it can get quite confusing and you're a lot more likely to access wrong indices in your matrix if you do this, so I wouldn't advise it.

answered Feb 12, 2013 at 9:22

mlstudent

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1 Comment

Casper Over a year ago

thank you very informative, since my textbook doesnt provide method to make 2D array so I got to search the internet

Victor Zamanian · Accepted Answer · 2013-02-12 09:21:53Z

0

You could do *(*(matrix+i)+j). It should be equivalent to bracket notation. What is happening with both notations is simply pointer arithmetic.

answered Feb 12, 2013 at 9:21

Victor Zamanian

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