Solving a Variable Equation defined by the User

Question

Answers in C, Python, C++ or Javascript would be very much appreciated. I've read a few books, done all the examples. Now I'd like to write a simple program. But, I already ran into the following roadblock:

My intention is to take an equation from the user and save it in a variable, For example:

-3*X+4 or pow(2,(sin(cos(x))/5))       >  [In valid C Math syntax]

And then calculate the given expression for a certain X-Value. Something like this:

printf("%g", UserFunction(3.2))   // Input 3.2 for X in User's Function and Print Result

Any ideas? For the life of me, I can't figure this out. Adding to my frustration, the solution is likely a very simply one. Thank you in advance.

Wouldn't you use `-3 * x` instead of `-3x`? You could use `eval`, but that isn't safe if you're expecting arbitrary user input. — Blender, May 16 '13 at 01:52
You're right, -3 * X would be the appropriate syntax. Regarding eval, what do you mean by "not safe"? — user2388026, May 16 '13 at 01:57
`eval("__import__('os').system('rm -Rf /')")` does just what you think it does. — Blender, May 16 '13 at 01:58
Thank you, but I don't imagine a User will input "rm -Rf /" when asked for an equation they want to solve. Any chance you could give me an example of its usage? It wouldn't be as simple as eval(scanf("%s"), x=5) right? — user2388026, May 16 '13 at 02:05

Shafik Yaghmour · Accepted Answer · 2013-10-09T15:19:21.270

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There isn't a simple way to do this in C but I think muParser may be useful to you, it is written in C++ but has C binding. ExprTk is also an option but looks like it is C++ only, on the plus side it looks much easier to get interesting results with.

Another option may be the Expression Evaluation which is part of Libav. It is in C and the eval.h header has some good descriptions of the interface.

edited Oct 09 '13 at 15:19

answered May 16 '13 at 01:52

Shafik Yaghmour

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Sadly, as a beginner I have no idea how to use that. Thank you anyways for your answer, Mr. Yaghmour. – user2388026 May 16 '13 at 02:19

score 0 · Answer 2 · answered May 16 '13 at 02:00

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In compiled languages like C, C++, or Java there is no easy way to do this--you basically have to rewrite a whole compiler (or use an external library with an interpreter). This is only trivial in "scripting" languages like Python and Javascript, which have a function (often called "eval()") that evaluates expressions at runtime. This function is often dangerous, because it can also do things like call functions with side effects.

answered May 16 '13 at 02:00

Lee Daniel Crocker

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Thank you for your answer, Mr. Crocker. It looks like I'm in way over my head. My idea was to write a simple program to approximate an integral from a to b using Simpson's and Trapezoid rule. I would then solve the function for the appropriate x-values determined by the length and n, then multiply the answers by the appropriate coefficients, .... But it all depends on being able to calculate the user's function for a given X. So it seems I will have to use a simpler language. Thanks again. – user2388026 May 16 '13 at 02:16
Couldn't this be done by implementing a sudo try-catch in C, like described here http://stackoverflow.com/questions/10586003/try-catch-statements-in-c? – user3282276 Sep 29 '14 at 05:16

score 0 · Answer 3 · answered May 16 '13 at 02:29

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Ffmpeg/libav has a nice simple function evaluator you could use.

answered May 16 '13 at 02:29

R.. GitHub STOP HELPING ICE

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Solving a Variable Equation defined by the User

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