Explanation of shift operationals use in condition

Question

I have a c code given to me to fill for my thesis. Can you please explain what the following segment does, because i'm very buffled with it.

int i;
_int8 codeword[64800];

//loop running through codeword
if (codeword[i << 1] << 1 | codeword[i << 1 | 1])
    {
      //more code here
    }

where i is a loop counter and codeword[] is an 1d matrix of ones and zeros

I mostly seek explanation of the operations taking place if, for example, codeword[i] is 1.

@user2079303 Given the access pattern here, I'd say it's a heap. — EOF, Jul 20 '17 at 19:20
If you are confused, split it up into several lines and use temp booleans, preferably ones with meaningful names, to avoid 'one-liner' compound expressions. — Martin James, Jul 20 '17 at 19:23
The second shift in first expression is unnecessary as any Nonzero value will make the condition true — 0___________, Jul 20 '17 at 19:33
@PeterJ If `codeword[i << 1]` is `unsigned` with only its MS bit set, `codeword[i << 1] << 1` is not true, so the `if()` depends on the next element. — chux - Reinstate Monica, Jul 20 '17 at 19:40
@Christos Mordons Post lacks context and what types are `codeword, i`? — chux - Reinstate Monica, Jul 20 '17 at 19:42
without the declaration of `i` and `codeword` any answer would have to do some guessing. You just need to add few lines to make it an [mcve] — 463035818_is_not_an_ai, Jul 20 '17 at 19:47
Which language, C or C++? In C, `<<` is the bit shift left operator. In C++, you can overload the operator (for example, for I/O it is overloaded). — Thomas Matthews, Jul 20 '17 at 20:23
Original post edited. Sorry for the inconvenience as this is my first post ever. The code is C. — Christos Mordons, Jul 22 '17 at 17:47

score 1 · Answer 1 · answered Jul 23 '17 at 16:16

The test combines the 2 bits in codeword at offset 2 * i and 2 * i + 1 and evaluates the body if they are not both 0. The expression is parsed as:

int i;
_int8 codeword[64800];

//loop running through codeword
if ((codeword[i << 1] << 1) | codeword[(i << 1) | 1]) {
    // more code here
}

Note that the expression would be equivalent but more readable as:

int i;
_int8 codeword[64800];

//loop running through codeword
if (codeword[2 * i] || codeword[2 * i + 1]) {
    // more code here
}

Explanation of shift operationals use in condition

1 Answers1