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is there any way to calculate the values of the partial derivates of a function in c, which is unknown so far? I read here: Compute a derivative using discrete methods about how to implement the derivate, which is only possible when I know exactly the function f(x) with x=(x0,x1,...,xn). But what, if I want to pass the function f(x) as parameter, is this somehow possible? I imagine that not because I would need to somehow parse the string (must be passed as string obviously, like 'x0*exp(x1)*ln(x2*x1)')!? thanks so far!

edit: My question isn't about how to pass a function as a pointer in c, but about how to evaluate a mathematical function which is e.g. entered by the user as string.

Community
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tim
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3 Answers3

4

You could pass in some sort of parse-tree representing the expression instead of a string. Symbolic differentiation is an easy and completely mechanical task for non-implicit functions, so differentiating an expression once you've decided how to represent it would be relatively easy.

James
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  • Okay thanks so far, so there's no easier way to do it, right? No built-in librarys or something like that? – tim Apr 28 '11 at 09:10
2

Perform a search on function pointers in C, that should help you.

edit: adding an example so it is more obvious yet...

#include <stdio.h>

typedef float (*real_univariate_function)(float x);

float calculate_derivative(real_univariate_function f, float x, float h) {
    return (f(x+h)-f(x))/h;
}

float square(float x) {
    return x*x;
}

int main() {
    printf("derivative for x^2 in 2 with h 0.001 is: %f\n",
            calculate_derivative(&square, 2, 0.001));
}

there might be some syntax errors, my C is getting rusty...

fortran
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  • Function pointers will only give a pointer to the code in memory. Yes, it would be theoretically possible to look at the machine code in memory at run-time, but I don't think that's what the poster is asking. – Joe Apr 28 '11 at 09:00
  • Also can't see how this could help me really. – tim Apr 28 '11 at 09:09
  • I think the op is asking how to pass around a function as a parameter so he can evaluate it at some points, and that is what function pointers are used for. – fortran Apr 28 '11 at 09:11
  • Oh sorry, I meant a "mathematical functions" which could come from a user input, something like the user enters "x^2*y^2*z^2" as function to be evaluated at a certain point (x,y,z). I didn't mean coded c-functions. – tim Apr 28 '11 at 09:18
  • :-/ the question should be clearer on that... then your question is really on parsing, not on function passing. – fortran Apr 28 '11 at 09:21
  • OKay thanks though, this is also a cool example for this sort of question! I will edit my question above! – tim Apr 28 '11 at 09:26
1

I know I'm late, but maybe this can help someone. I've written a library that does exactly that: https://github.com/B3rn475/MathParseKit

You can parse a string into a function tree and compute the derivatives.

B3rn475
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