Looping a dynamically allocated 2d array with pointer arithmetic

Question

I have a dynamically allocated 2d array and would like to loop through it with pointer arithmetic because I won't know the number of rows and number of cols before runtime.

I know how to do this with a 1d array:

int *arr = new int[size];

and to loop through it:

for (int *i = arr; i < arr + arr.size(); i++){
    *i = 20; //sets all elements to 20
}

However, it's at the 2d level that I get stuck. Here's what I have so far:

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

To loop through all values:

for(int **i=arr; i < arr + row; i++){
    for(int *j=*i; j < j + col; j++){
        *j = 20; // set all values to 20
    }
}

The second loop is obviously incorrect, I just don't know what else to try.

Answer 1

You should do the same thing to j as you did to i.

for(int **i=arr; i < arr + row; i++){
    for(int *j=*i; j < *i + col; j++){
        *j = 20; // set all values to 20
    }
}