HELP:Matematical Expression

This is a discussion on HELP:Matematical Expression within the C Programming forums, part of the General Programming Boards category; Hi, Could you explain how th eexpression below works (c-x)/(c-b)*(b<=x)*(x<=c) It is difficult for me to understand there is a ...

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    HELP:Matematical Expression

    Hi,

    Could you explain how th eexpression below works

    (c-x)/(c-b)*(b<=x)*(x<=c)

    It is difficult for me to understand there is a multiplication with (b<=x) or (x<=c)

    Thank you in advance

    Nuray

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    C++ Enthusiast jmd15's Avatar
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    In programming x<=c means x is less than or equal to c. There is a symbol the < with a _ under it for math(which means the same thing) but <= is its "computer equivalent". Please correct me if that's not what you mean. What's the equation for, btw?
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    Just Lurking Dave_Sinkula's Avatar
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    Quote Originally Posted by nuray
    Could you explain how th eexpression below works

    (c-x)/(c-b)*(b<=x)*(x<=c)

    It is difficult for me to understand there is a multiplication with (b<=x) or (x<=c)
    If b <= x and x <= c, the result will be (c - x) / (c - b). Otherwise, the result will be 0.

    [edit]Unless c - b is zero.
    Last edited by Dave_Sinkula; 10-14-2005 at 08:45 PM.
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    Thank you.

    Yours
    Nuray

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    and the hat of wrongness Salem's Avatar
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    > (c-x)/(c-b)*(b<=x)*(x<=c)
    What (or who) thought this was a good idea?
    What's wrong with the use of an if() statement?
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    Just Lurking Dave_Sinkula's Avatar
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    Quote Originally Posted by Salem
    > (c-x)/(c-b)*(b<=x)*(x<=c)
    What (or who) thought this was a good idea?
    What's wrong with the use of an if() statement?
    Definitely not my arena, but I've heard of avoiding branches for valid reasons.
    The downside of a long pipeline is when a program branches, the entire pipeline must be flushed, a problem that branch predicting helps to alleviate. Branch predicting itself can end up exacerbating the problem if branches are predicted poorly. In certain applications, such as supercomputing, programs are specially written to rarely branch and so [...]
    Last edited by Dave_Sinkula; 10-15-2005 at 02:09 PM. Reason: Changed 'avoiding comparisons' to 'avoiding branches'.
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    40. There are two ways to write error-free programs; only the third one works.*

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    and the hat of wrongness Salem's Avatar
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    I see a lot of potential for division by zero going on here.

    I don't think there's anything in C which would guarantee that the sub-expression (b<=x)*(x<=c) would always compile down to something without any branches at all. It's certainly assuming a lot about the architecture of the underlying machine.

    Code:
    if ( (b<=x) && (x<=c) ) {
      result = (c-x)/(c-b);
    } else {
      result = 0;
    }
    Potentially saves a comparison, and a number of arithmetic operations, through short-circuit evaluation.
    A comparison and branch is usually a lot less expensive than a pair of multiplies, and could well save the cost of pointless division as well (if the test fails).

    With the added bonus that it's much easier to figure out what is going on
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
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