calculate the ln of a number?

This is a discussion on calculate the ln of a number? within the C Programming forums, part of the General Programming Boards category; hey, i was wondering if there was a function you could use to find the ln of a number ( ...

  1. #1
    Shibby willc0de4food's Avatar
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    calculate the ln of a number?

    hey, i was wondering if there was a function you could use to find the ln of a number ( such as: ln(2) ). i've done some searching on google about it, but either i've been using the wrong keywords or i didn't look deep enough, lol. i also tried declaring a double and having this:
    Code:
    lntwo = ln(2);
    which..doesn't work. lol thanks
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  2. #2
    & the hat of GPL slaying Thantos's Avatar
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    Code:
    lntwo = log(2);

  3. #3
    Shibby willc0de4food's Avatar
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    Code:
    log(2) != ln(2)
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    Then you need to base convert it.
    Code:
    log(2) / log(2.7182)
    http://mathforum.org/library/drmath/view/55568.html
    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

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    Just Lurking Dave_Sinkula's Avatar
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    http://www.rt.com/man/log.3.html
    The log() function returns the natural logarithm of x.
    Code:
    #include <stdio.h>
    #include <math.h>
    
    int main(void)
    {
       double lntwo = log(2);
       printf("lntwo = %g\n", lntwo);
       return 0;
    }
    
    /* my output
    lntwo = 0.693147
    */
    Last edited by Dave_Sinkula; 04-04-2005 at 03:43 PM. Reason: Added code.
    7. It is easier to write an incorrect program than understand a correct one.
    40. There are two ways to write error-free programs; only the third one works.*

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    Ha...maybe you don't. My bad.
    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

  7. #7
    Shibby willc0de4food's Avatar
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    awesome thank you
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    & the hat of GPL slaying Thantos's Avatar
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    Hehe I was wondering if you'd fail for that

    Remember in higher mathematics when you say log you are really saying ln not log10

  9. #9
    Registered User Micko's Avatar
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    Quote Originally Posted by Thantos
    Hehe I was wondering if you'd fail for that

    Remember in higher mathematics when you say log you are really saying ln not log10
    Well it depends, many universitybooks log for base e (2.71...) denote like ln while log is actuallly log10. That is on also used on windows calculator. But in programming world and because of programming world log denots ln not log10. For example in MATLAB, C and C++ (and many others prog languages) log means ln.

    And just in case you wondering about pianorain's formula consider this problem.
    How to calculate logarithm log4(15) (base is 4)?
    Simple
    Let log4(15) is equal x so x = log4(15). Now that means 4^x = 15.
    Now if we take logarithm (base e ln ) we have ln(4^x) = ln(15)=>
    x*ln(4) = ln(15) (logarithms property)=> x=ln(15)/ln(4). And remeber now what x is?
    x is log4(15). That means we got a formula for logarithms converting: log4(15) = ln(15)/ln(4).
    On you0re calaculator you usually don't have opportunity to calculate logarithms for different bases (usually e and 10) so knowing this makes calculation of logarithm of any base simple.
    I don't know why I wrote this.
    Maybe someone will find it useful.

    Micko
    Last edited by Micko; 04-04-2005 at 04:59 PM.
    Gotta love the "please fix this for me, but I'm not going to tell you which functions we're allowed to use" posts.
    It's like teaching people to walk by first breaking their legs - muppet teachers! - Salem

  10. #10
    & the hat of GPL slaying Thantos's Avatar
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    Well it depends, many universitybooks log for base e (2.71...) denote like ln while log is actuallly log10. That is on also used on windows calculator. But in programming world and because of programming world log denots ln not log10. For example in MATLAB, C and C++ (and many others prog languages) log means ln.
    I haven't see log in my text books since trig or so. You know why log is ln in programming langauges? Because they were created by mathematicans

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