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NAN handling in C

  1. Dec 11, 2003 #1
    I'm trying to write a program but I face a little problem (and probably very simple): there is a function that sometimes is calculated outside of its domain and then gives the result "nan".

    I need to put an "if" that says that everytime that function returns "nan" the program will execute some procedure.

    Can anyone help me with this?
  2. jcsd
  3. Dec 11, 2003 #2


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    Staff Emeritus

    If you don't known what nan is, it means "not a number." This occurs when you do things like take the square root of negative one.

    To check for this, math.h includes this function:
    int isnan( float value )

    math.h also includes isinf() and finite() if your interested.

    If you just want to include an if statement without using math.h simply use this:

    if( x != x )

    It might look strange, but it works (most of the time). It sometimes doesn't work because of your compiler optimization options. It will just think the statement is always false.

    In the rare ocassions that I get a nan I just rewrite my code to handle exceptional cases better so i don't have to use isnan().
  4. Dec 11, 2003 #3
    Thanks for the help.

    I cannot rewrite the code, because I need to identify the "nan" in the program: there is a function that calculates a logarithm in my program and everytime the log gives a nan, I need to change the flow of the program to a special subroutine that will use another function only in this case.

    I will try to use the other solutions you gave.

    Thanks again.
  5. Dec 11, 2003 #4
    I've just tested the suggestions and they worked well.

    Thanks for saving me a lot of time!
  6. Dec 11, 2003 #5


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    Staff Emeritus
    Science Advisor
    Gold Member

    What dduardo was suggesting is that you rewrite the function that calculates the logarithm to signal an error in some way if the argument to the logarithm is nonpositive. e.g.:

    Code (Text):

    int foo(double param, double *result)
      if(param <= 0) return 1;
      *result = log(param);
      return 0;

    void bar(double param)
      double value;

      if(foo(param, &value)) {
        printf("Invaild argument.\n");

      printf("The logarithm is %f.\n", value);
  7. Dec 12, 2003 #6
    I'm grateful for the suggestion, but the problem is that my function is a summation of logarithms. I will explain: I have a minimization problem with a barrier function. The minimization involves a d-dimensional variable with components that cannot be negative because they represent probabilities. But when I use backtracking line search with this function, sometimes the function calculated at the new point (without any scale factor multiplying the step) falls outside the barrier and gives the nan value. So, what I need is just test if the function is nan to rescale the step size such that the new point will fall inside the domain of the function plus the barrier.

    But thanks anyway.
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