sqrt or negative zero?
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Thread: sqrt or negative zero?

  1. #1
    Join Date
    Jan 2011

    Question sqrt or negative zero?


    I'm having a bit trouble with this equation:

    sum = pow(cos(x),2) + pow(cos(y),2);
    z = acos ( sqrt ( 1 - sum ) );

    so when sum == 1, it should be sqrt(1-1) i.e. sqrt(0) which should be 0..

    however I get an error as .. Nan..

    a little help please.. how do i tell c++ that it is 0 not -0

  2. #2
    Lindley is offline Elite Member Power Poster
    Join Date
    Oct 2007
    Seattle, WA

    Re: sqrt or negative zero?

    Unfortunately floating point math is not exact. Therefore, it's possible that "sum" could be ever so slightly greater than 1, and therefore you would be trying to take the square root of an actual negative (not just -0).

    One solution in this case would be to use fabs(1 - sum). Assuming that sum is never much more than 1, which it shouldn't be, this wouldn't change your answer too much but would ensure you never put a negative into the sqrt().

  3. #3
    Join Date
    Jan 2009

    Re: sqrt or negative zero?

    Yeah, the rounding errors must be pushing it to negative numbers. sqrt(-0) will return -0; Only negative numbers will go into imaginaries.

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