3d array using pointers

Question

I am new to C programming. Currently am trying to learn 3d array using pointers. Below is a program I am trying to debug. Can any one explain the difference between the two programs given below?

code1:

#include <stdio.h>
int main()
{
    int a;
    int d[2][2][2] = {1, -2, -3, 0, -9, -1, 3, -1};
    printf("%d\n",*(*(*(d +1)+1)+1));
    if(*(*(*(d +1)+1)+1) <(a= sizeof( int )))
        puts(" u got it ");
    else
        puts (" but wrong");
    return 0;
}

code2:

#include <stdio.h>
int main()
{
    int a;
    int d[2][2][2] = {1, -2, -3, 0, -9, -1, 3, -1};
    if(*(*(*(d +1)+1)+1) <(sizeof( int )))
        puts(" u got it ");
    else
        puts (" but wrong");
    return 0;
}

In the first code I am getting the […incomplete…]

Answer 1

 int d[2][2][2] = {1, -2, -3, 0, -9, -1, 3, -1}; 

The initializer is not fully braced but in this situation the initializers apply to the next element of the array in memory, so this assigns d[0][0][0] = 1 , d[0][0][1] = -2 , d[0][1][0] = -3 , etc.

 printf("%d\\n",*(*(*(d +1)+1)+1)); 

The thing full of stars is an obfuscated way of writing d[1][1][1] . The definition of X[Y] is *(X+Y) .

 (a= sizeof( int ))) 

The type of an assignment expression is the type of the left-hand operand. So the first program does (int)-1 < (int)4 . The second program does (int)-1 < (size_t)4 . (assuming your ints are 4 bytes big).

In the first case this is true . In the second case it is a type mismatch. The type mismatch has to be fixed before the comparison can occur. The rules of C say that in this case the signed type is converted to the unsigned type, giving (size_t)-1 < (size_t)4 . Since (size_t)-1 is actually the largest possible size_t value, this comparison is false .

Answer 2

It is basically an integer type comparison problem. First program compares a signed int (-1) with a signed int (a) and the second, a signed int (-1) with an unsigned int (sizeof()). Integer promotion happens in the second case, where signed int (-1) gets converted to unsigned int (-1) -> SIZE_MAX. For more details on type comparison check the thread, What are the general rules for comparing different data types in C?