(un)natural logarithm function using the wrong base

This is a discussion on (un)natural logarithm function using the wrong base within the C Programming forums, part of the General Programming Boards category; The following code produces logarithms apparently using base 2.71826 instead of base 10. What could I be doing wrong in ...

  1. #1
    Registered User Gerling's Avatar
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    (un)natural logarithm function using the wrong base

    The following code produces logarithms apparently using base 2.71826 instead of base 10. What could I be doing wrong in a function so simple?
    Code:
    #include <stdio.h>
    #include <math.h>
    
    int main()
    { double d = 100.0;
      double num;
        printf("Ready\n");
        scanf("%g", &num);
        printf("log (%g) = %g\n", num, log(num));
        printf("log (%g) = %g\n", d, log(d));
        return (0);
    }
    Input to scanf:
    4
    Output
    Code:
    Ready
    4  
    log (1.21742e-306) = -704.394
    log (100) = 4.60517
    Last edited by Gerling; 08-12-2010 at 09:25 AM. Reason: Forgot input and ouput

  2. #2
    and the hat of wrongness Salem's Avatar
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    Which compiler are you using?

    What results do you get?
    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.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

  3. #3
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    I think you're looking for log10.
    If I did your homework for you, then you might pass your class without learning how to write a program like this. Then you might graduate and get your degree without learning how to write a program like this. You might become a professional programmer without knowing how to write a program like this. Someday you might work on a project with me without knowing how to write a program like this. Then I would have to do you serious bodily harm. - Jack Klein

  4. #4
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    log - C++ Reference


    ^ is the natural logarithm

  5. #5
    Guest Sebastiani's Avatar
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    Just FYI, you can calculate the logarithm of a number to any base using log(number)/log(base).
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

  6. #6
    Registered User Gerling's Avatar
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    Wow. Thanks for telling me about log10. I got confused and thought log base 10 was natural. Now I remember what e is.

  7. #7
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    Common logarithm is to base 10. Natural logarithm is to base e (approximately 2.71828...).

    Mathematically the natural log of x is often described using the notation ln(x) - goes to show that the people who specified the C standard library were not actually mathematicians.

    For those familiar with calculus: the term "natural" comes the fact that the derivative of ln(x) with respect to x is 1/x.
    Right 98% of the time, and don't care about the other 3%.

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