C: How much space allocated if I declare array as int [][5] vs int [2][5]

Question

See the following two program

#include<stdio.h>
int main()
{
  int a[][5] = { 1}; 
  printf("%d", sizeof(a)); 
}

Output: 20

#include<stdio.h>
int main()
{
  int a[2][5] = { 1}; 
  printf("%d", sizeof(a)); 
}

Output: 40

Can someone explain the logic why there is such difference of the size?

Answer 1

It is very simple to understand above two code programs.
In the first program, only a single value is assigned to an array where no. of rows are not defined but the number of columns are defined. If you assigned more than 5 elements in the array, it will consider into second row . Similarly, if number of elements assigned to array are more than ten then elements considered into third row automatically.

While in the second program, array size has already been defined. I meant to say that it is 10 elements in advance. And each integer is 4 bytes long that's why result is 40.