How to give command line input to this C++ program - c++

Question

I am running a simple program:
#include<iostream>
#include<math.h>
using namespace std;
double fu (double x) {
double func = pow(x,0.5);
return func;
}
int main (int argc, char* argv[]) {
double x = 2;
double func = fu(x);
cout<<"f(x) = "<<func<<endl;
return 0;
}
Here func is a function of which value is calculated at x.
Suppose, I need to use this program from another program or if I want to give a function, such as pow(x,0.5) or sqrt(1+sin(x)) during command line running of this program.
How I can do that? If I do it using argv, then can you suggest how can I convert a string into an expression func (that can be evaluated by C++ compiler)?
Any suggestions?

Answer 1

There's no easy way to automatically evaluate that kind of math expressions from a string in c++ (like you may have experienced with kind of eval() expressions in scripting languages).
C++ is a compiled language, and the compiler can't actually resolve anything to concrete function calls and parameter values at compile time.
You have to parse these strings at runtime, break down the input tokens and parameters, and map the parsing results to concrete function calls of the appropriate <cmath> functions applying the parsed parameter values, from within your program.
That's not trivial, and there are a number of solution approaches available. But these completely depend on the kind of math language you want to parse from the command line arguments.

Answer 2

There is not such thing as 'execute an expression in a string' in standard C++. The reason is because C++ is a compiled language (i.e. the output is binary code that the processor can directly execute without any intermediate interpreter). So it cannot just interpret an expression (or code in general) from a string.
I have an implementation in Snap Websites, there is C++ code:
https://sourceforge.net/p/snapcpp/code/ci/master/tree/snapwebsites/lib/snap_expr.cpp
Go here to find the other files:
https://sourceforge.net/p/snapcpp/code/ci/master/tree/snapwebsites/lib
However, instead of implementing an expression parser and execution environment, you may want to make use of an existing library (mine is an example, obviously, actually I have a tool named snapexpr under snapwebsites/src which will do exactly what you are talking about!) In that case you could also choose a different language. For example, Qt offers QScript:
http://qt-project.org/doc/qt-5/qtscript-index.html
In that case, you have to write JavaScript code, but since JavaScript is relatively close to C++, it may work in your environment.