log function causing an error

This is a discussion on log function causing an error within the C Programming forums, part of the General Programming Boards category; I am currently working on a program that includes a special exponential function that calls the log function from with ...

  1. #1
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    log function causing an error

    I am currently working on a program that includes a special exponential function that calls the log function from with in it. (*the code is below). I am getting the following error:

    gcc -c gas.c
    gcc gas.o list.o queue.o -o gas.exe
    Undefined first referenced
    symbol in file
    log gas.o

    which seems to mean that it is not understanding the log function, however I am certain that the math.h library is inclued. Can there be another reason for this error?

    #include <stdio.h>
    #include <math.h>
    #include <stdlib.h>
    #include "bool.h"
    #include "gas.h"

    double exponential(double mean)
    {
    double rnx;
    rnx = drand48();
    return (mean * -1.0 * log(rnx));
    }

  2. #2
    Code Goddess Prelude's Avatar
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    9,796
    You included math.h, but forgot to link with the math library.
    Code:
    % gcc gas.o list.o queue.o -o gas.exe -lm
    My best code is written with the delete key.

  3. #3
    ZAK
    ZAK is offline
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    Also, you have a bug in your code if your variable rnx = 0 then the log function will tend towards infinity, theres an asymptote at log(0), you can think of it being undefined you may want to make sure you don't allow rnx to be 0. This is ofcourse assuming that your random number generator is Gaussian (ie. generating a normal distribution of values between 0.0 and 1.0)
    "Computer Science is no more about computers than astronomy is about telescopes"
    - E.W. Dijkstra

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