Question

Here is a code which print the address of the first element of an 2D array followed by an addition of 1. Though all 4 base addresses are same but their addition with 1 is obviously not giving the same result due the their different “types”. I can figure out the type for some(in case they are correct) but not for all.

int main()
{
    int array[4][3];
    printf("array %u\n",array);   //of type int(*)[3]
    printf("array+1 %u\n",array+1);
    printf("&array %u\n",&array);   //....???
    printf("&array+1 %u\n",&array+1);
    printf("array[0] %u\n",array[0]);    //of type int*
    printf("array[0]+1 %u\n",array[0]+1);
    printf("&array[0] %u\n",&array[0]); //....???
    printf("&array[0]+1 %u\n",&array[0]+1);
} 

Can you explain the “type” of each base address in detail in order to understand the pointer arithmetic involved after adding 1. A sample output for gcc machine is given below for quick reference.

array       3214383040 
array+1     3214383052 
&array      3214383040 
&array+1    3214383088 
array[0]    3214383040 
array[0][phone] 
&array[0]   3214383040 
&array[0][phone] 

Answer 1

Having int array[4][3]; the following applies

array is an array with 4 elements. Each element is an array with 3 ints. In most circumstances using the name by itself makes the array decay to a pointer to its first element; then array becomes a pointer to arrays of 3 ints

array + 1 is a pointer to an array of 3 ints. Here array decays to pointer and the 1 refers to array of 3 int

&array is the address of the whole array. It points to objects of type array of 4 arrays of 3 ints

&array + 1 is the 2nd (which really does not exist) element of a pseudo-array of arrays of 4 arrays of 3 ints

array[0] is an array of 3 ints. It usually decays to a pointer to the first element

array[0] + 1 points to the 2nd int in array[0]

&array[0] address of an object of type array of 3 ints

&array[0]+1 2nd element of an array of arrays of 3 ints

PS. I’ll try to make a drawing (ASCII) after lunch.

---

Hmmm … drawing is tough 🙂

Before trying, I thought I could make a better drawing.
This is the best I could come up with …

    int array[4][3]        ........[aaabbbcccddd]........
                              where aaa, bbb, ccc, ddd are arrays of 3 ints'

    the [] represent the object itself; the {} represent pointers.

    array (object)         ........[AAABBBCCCDDD]........       int[4][3]
    array (decayed)    ==> ........{aaa}bbbcccddd........       int(*)[3]

    array + 1          ==> ........aaa{bbb}cccddd........       int(*)[3]

    &array             ==> ........{aaabbbcccddd}........       int(*)[4][3]
    &array + 1         ==> ........aaabbbcccddd{xxxxxxxxxxxx}........       int(*)[4][3]

    array[0] (object)      ........[AAA]bbbcccddd........       int[3]
    array[0] (decayed) ==> ........{a}aabbbcccddd........       int*

    array[0] + 1       ==> ........a{a}abbbcccddd........       int*

    &array[0]          ==> ........{aaa}bbbcccddd........       int(*)[3]
    &array[0] + 1      ==> ........aaa{bbb}cccddd........       int(*)[3]