how to compare decimals in c programming

This is a discussion on how to compare decimals in c programming within the C Programming forums, part of the General Programming Boards category; hi i am using unix cygwin compiler, courtesy of NUS. i know we can use if statements to compare integers ...

  1. #1
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    how to compare decimals in c programming

    hi i am using unix cygwin compiler, courtesy of NUS.
    i know we can use if statements to compare integers but how about decimals(just as they are) ??? Thank you and sorry if this is a petty question.

  2. #2
    spurious conceit MK27's Avatar
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    You want to use a float type, otherwise it is the same.

    HOWEVER, there is a caveat with comparing floats. Because of the nature of floating point numbers in a computer, they are only exactly equal to decimal values that are an inverse power of two -- 0.5, 0.25, 0.125, 0.0625, and so on. Notice 0.1 is not on the list.

    Try this:

    Code:
    #include <stdio.h>
    
    int main() {
            float i;
            for (i=0.0f; i<20; i+=0.1f) {
                    printf("%f\n",i);
            }
            return 0;
    }
    It does not do what you would think:

    [...]
    2.500000
    2.600000
    2.700000
    2.799999
    2.899999
    2.999999
    [...]

    So much for 0.1! This happens at different points for 32-bit and 64-bit systems.

    Because of that, use a range instead of ==:

    Code:
    float x;
    [...]
    if (x >= 10.1999 &&  x <= 10.2001)
    Instead of

    Code:
    if (x == 10.2)
    Last edited by MK27; 05-15-2011 at 09:49 AM.
    C programming resources:
    GNU C Function and Macro Index -- glibc reference manual
    The C Book -- nice online learner guide
    Current ISO draft standard
    CCAN -- new CPAN like open source library repository
    3 (different) GNU debugger tutorials: #1 -- #2 -- #3
    cpwiki -- our wiki on sourceforge

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