Question

I have defined an array:

float array[3][4][5];

then, what is the difference when

array, array[0], array[0][0], &array[0][0][0]

used as function argument?

Answer 1

The important thing to learn is that, in C, arrays aren’t passed as parameters in their entirety. Instead, the pointer to the first element of the array is passed.

So, given the definition float array[3][4][5];…

array (as a parameter) will be of type float (*)[4][5], a pointer to a two-dimensional array of floats (explanation: we can’t pass the array, we pass the pointer to its first element, which is a 4×5 array, hence float (*)[4][5]).

array[0] (as a parameter) will be of type float (*)[5], a pointer to a one-dimensional array of floats (explanation: array[0] is a 4×5 array, we can’t pass the array, we pass the pointer to the first element of it, the first element being an array of 5 elements, hence float (*)[5]).

array[0][0] (as a parameter) will be of type float *, a pointer to a float (explanation: array[0][0] is an array of 5 elements, we can’t pass the array, we pass the pointer to the first element of it, the first element being a float, hence float *).

&array[0][0][0] (as a parameter) will be of type float *, a pointer to a float (explanation: array[0][0][0] is a float, we pass a pointer to it, hence float *).

Perhaps, a more elaborate example:

#include <stdio.h>

int x[3][5] =
{
  {  1,  2,  3,  4,  5 },
  {  6,  7,  8,  9, 10 },
  { 11, 12, 13, 14, 15 }
};

int (*pArr35)[3][5] = &x;
// &x is a pointer to an array of 3 arrays of 5 ints.

int (*pArr5a)[5] = x;
// x decays from an array of arrays of 5 ints to
// a pointer to an array of 5 ints,
// x is a pointer to an array of 5 ints.

int (*pArr5b)[5] = &x[0];
// &x[0] is a pointer to 0th element of x,
// x[0] is an array of 5 ints,
// &x[0] is a pointer to an array of 5 ints.

int *pInta = x[0];
// x[0] is 0th element of x,
// x[0] is an array of 5 ints,
// x[0] decays from an array of 5 ints to
// a pointer to an int.

int *pIntb = *x;
// x decays from an array of arrays of 5 ints to
// a pointer to an array of 5 ints,
// x is a pointer to an array of 5 ints,
// *x is an array of 5 ints,
// *x decays from an array of 5 ints to
// a pointer to an int.

int *pIntc = &x[0][0];
// x[0][0] is 0th element of x[0],
// where x[0] is an array of 5 ints,
// x[0][0] is an int,
// &x[0][0] is a pointer to an int.

int main(void)
{
  printf("&x=%p x=%p &x[0]=%p x[0]=%p *x=%p &x[0][0]=%p\n",
         pArr35, pArr5a, pArr5b, pInta, pIntb, pIntc);

  return 0;
}

Output (ideone):

&x=0x804a040 x=0x804a040 &x[0]=0x804a040 x[0]=0x804a040 *x=0x804a040 &x[0][0]=0x804a040