Question

I have an example involving a pointer to a 2D array. Can someone help me understand what is going on in this example?

int main()
{

    int i = 0, j=0, sum0=0, sum1=0;
    int data[4][3] = { {23,55,50},{45,38,55},{70,43,45},{34,46,60}};
    int *Ptr;
    Ptr = *data;    //Why is the indirection operator used here? 
                    // Does Ptr = 23 by this assignment?

    for (i=0; i<4; i++) {
        sum1 = 0;
        for (j = 0; j < 3; j++) {
            sum1 += data[i][j];
        }
        if (sum1 > sum0) {
                 sum0 = sum1;
                 Ptr = *(data + i);     // Seems like this statement makes Ptr
        }                               // point one row below ... what syntax
    }                                   // can you use to access columns then?
                                       // Is it possible to use pointer arithmetic
    for (i=0; i<3; i++)                 // to access elements of data[i][j] that
        printf("%d\n", Ptr[i]);          // are not at j = 0?

  return 0;
}

Answer 1

data is a 2 dimentional array, which has 4 rows and each row has 3 elements (ie 4 X 3).

Now, Ptr = *data; means you are storing the starting address of 1st row to the pointer variable Ptr. This statement is equivalent to Ptr = *(data + 0). Ptr = *(data + 1) – this means we are assigning 2nd row’s starting address.

Then *Ptr or *(Ptr + 0) will give you the value of the first element of the row to which is pointing. Similarly, *(Ptr + 1) will give you the value of the second element of the row.

The for loop in your program is used to identify which row has the maximum value of the sum of its elements (3 elements). Once the control comes out of that for loop, Ptr will be pointing to the row which has the maximum sum of its elements and sum0 will have the value of the sum.

Consider an array int a[5];, I hope you know that a[0] and 0[a] is the same. This is because a[0] means *(a+0) and 0[a] means *(0 + a). This same logic can be used in 2 dimensional array.

data[i][j] is similar to *(*(data + i) + j). We can write it as i[data][j] also.

For more details please refer to the book "Understanding Pointers in C" by Yashavant Kanetkar.