Shifting 1001 certain value gives a different result than expected

Question

I am doing the following. I would like to shift 1001 to the left

int a = 0;
a = (1001 << 2);
std::cout << a; //Gives 4004

I was expecting it to be in binary 100100 which is 36. Why am I getting 0000111110100100 ?

Answer 1

1001 is a decimal constant. All integers literals are decimal unless prefixed accordingly. If you wish to obtain a binary constant, then prefix it with 0b (only available since C++14 though).

a = (0b1001 << 2);

Otherwise, hexadecimal is a decent alternative:

a = (0x9 << 2);