<SOLVED> weird (int)[array] or (int)[ T* ] operation

Question

Recently i did a challenge where i one of the rounds I had to find the output on this code:
int x[5]={1,2,3,4,5};
std::cout<<3[x]<<std::endl;
the answer was: 4
is the (int)[int*] or (int)[int[]] a valid operation? Is it a UB? And why is that the output?

Answer 1

<arrray>[<index>]
<index>[<array>]
*(<array> + <index>)
*(<index> + <array>)
Are alike, so for your case here, 3[x] is similar to x[3]. And the element at 3rd index is 4. Odd isn't it? I was also so confused about this once XD
(Edited)

Answer 2

the compiler converts the array operation in pointers before accessing the array elements, which means:
array[index] == *(array + index);
therefore x[3] will evaluate to → *(x+3), and 3[x] will evaluate to → *(3 + x) which is the same as *(x + 3) "addition is commutative", so output will be 4.
NOTE: don’t use this syntax in real code. this only works for the built-in subscript operator.

Answer 3

It looks like an alternate way to access the elements in the array. Its the same as x[3]

Answer 4

xaralampis_
It's also the same when we do
3[x]
We can also see it like this,
*(3 + x)
Or like this,
*(x + 3)
The literal 3 can be put before or after <x>.

Answer 5

Ipang Yeah, it is... the plus and dereference wayo was familiar to me sure. But I had never seen the second one before

Answer 6

Mohamed ELomari that makes a lot of sence