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Thread: Silly error

  1. #1
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    Default Silly error

    System.out.println(2.6%1.1)// gives 0.39999999999
    System.out.printf("%f",2.6%1.1)//gives 0.40000000

    Anyone know why ?


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    Super Moderator helloworld922's Avatar
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    Default Re: Silly error

    Because floating point numbers have limited precision. So, when you calculate the modulus, the slight error due to the hardware limitations (known as the "ulp" or "unit in the last place") makes the number slightly off from exactly 0.4.

    Printf is likely rounding off items within a few ulps so it prints out 0.4, but println doesn't so it prints out the number that's slightly off. This is also why using the == operator with floating points is very dangerous (and rather silly).

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    Administrator copeg's Avatar
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    Default Re: Silly error

    In the first case, the parameter numbers are treated as doubles (the default for primitive floating point numbers in java). The second instance it is being interpreted as a float. As an example:
    System.out.println(2.6%1.1)// gives 0.39999999999
    System.out.println((float)(2.6%1.1))// gives 0.4
    System.out.printf("%f",2.6%1.1)//gives 0.40000000
    Last edited by copeg; August 22nd, 2010 at 08:35 PM.

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    Default Re: Silly error

    oh, well it's rounding off more than a few ulps

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    American (August 23rd, 2010)

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    Default Re: Silly error

    Thanks to you all. Interesting....
    Last edited by American; August 23rd, 2010 at 03:55 PM.

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    Default Re: Silly error

    I actually didn't know this either. Thank you Copeg and HelloWorld922!

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