Problem with rounding floating points

This is a discussion on Problem with rounding floating points within the C++ Programming forums, part of the General Programming Boards category; Code: int convMinutes(double x) { int mins = static_cast<int>(x) * 60; cout << x - static_cast<int>(x); mins = (x - ...

  1. #1
    Registered User MacNilly's Avatar
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    Problem with rounding floating points

    Code:
    int convMinutes(double x)
    {
        int mins = static_cast<int>(x) * 60;
        cout << x - static_cast<int>(x);
        mins = (x - static_cast<int>(x)) * 100;
        cout << mins;
        
        return mins;
    }
    The problem is to take a floating point number (x) and look at it as if it's a time input in format HH.MM. If I enter, say 12.59 as x, the first cout outputs .59. The second one outputs 58. .59 * 100 is not 58. I can only assume some internal rounding is messing up my conversion.

    Any insights, ideas, would be much appreciated.
    Last edited by MacNilly; 02-17-2006 at 01:13 AM.

  2. #2
    Algorithm Dissector iMalc's Avatar
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    You could add half a percent to round it:
    mins = (x - static_cast<int>(floor(x)) + 0.005) * 100;

  3. #3
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    .59 * 100 is not 58.
    No, but .589999999999999999 cast to an int and THEN multiplied by 100 is 58. Everything stored on a computer is stored in binary--that's the only thing computers know how to do--and there is no exact conversion from decimal to binary for some numbers.

  4. #4
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    >No, but .589999999999999999 cast to an int and THEN multiplied by 100 is 58.
    Actually it's more like 0.

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