bit operation problem

This is a discussion on bit operation problem within the C Programming forums, part of the General Programming Boards category; I got a interview question previous like this: find what might be wrong if (mask & ASYNC_BIT == 0x0100) {.... ...

  1. #1
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    bit operation problem

    I got a interview question previous like this:
    find what might be wrong

    if (mask & ASYNC_BIT == 0x0100) {....

    No values were defined for both "mask" and ASYNC_BIT in the question, just wonder in what condition the result of "AND" operation could not compare with hex value.


  2. #2
    and the Hat of Guessing tabstop's Avatar
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    Syntactically: == comes first, & comes second.

  3. #3
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    The order of precedence between those operations are "bad": you first do the ==, then do the and for some reason. You need parenthesis to make it right, e.g.
    Code:
    if ((mask & ASYNC_BIT) == 0x0100)
    Without parenthesis, it becomes:
    Code:
    if (mask & (ASYNC_BIT == 0x0100))
    which turns into an and with either 1 or 0.

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    Oh that's really basic thing and I understand why i made a mistake then.
    thx

  5. #5
    and the hat of sweating
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    I wouldn't have known that without looking at a precedence table. I'd tell the interviewers I can answer the question if they either show me a precedence chart or put some parentheses in.

  6. #6
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    Quote Originally Posted by pinklax View Post
    Oh that's really basic thing and I understand why i made a mistake then.
    thx
    Aside from cpjust's answer, I have made the mistake of writing code like that so many times that I'm now ACTIVELY thinking about it when I'm writing bit-checking/bit-twiddling code - a bit like someone who's flooded the bathroom 6 times will double and tripple-checkl that they haven't left the taps on.

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    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

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