Pass lambda expression on command line

Question

I created a program to calculate the defined integral of a function, but I want to be able to run it through the terminal.

#include <iostream>
#include <functional>
#include <cmath>

#define PI acos(-1)

using namespace std;

// Função que calcula a integral de uma função matemática (func) no intervalo
// [inf, sup] com Δx = (inf - sup) / n e inf <= sup
inline long double integral (const function<long double (const long double&)> &func, const long double &inf, const long double &sup, const long double &n) {
  if (inf > sup) return 0;
  if (n <= 0) return 0;

  register long double &&area = 0;
  register long double &&delta = (sup - inf) / n;

  for (register long double i = inf; i <= sup; i += delta) {
    area += delta * func(i);
  }
  return area;
}

int main () {
  const function<long double (const long double&)> &&f1 = [](const long double &x)-> long double {return pow(sin(x), 3) * cos(x);};
  const long double &vf1 = integral(f1, 0, PI / 2, 10000000.0);

  const function<long double (const long double&)> &&f2 = [](const long double &x)-> long double {return sin(x);};
  const long double &vf2 = integral(f2, 0, 2 * PI, 10000000.0);

  const function<long double (const long double&)> &&f3 = [](const long double &x)-> long double {return x * x;};
  const long double &vf3 = integral(f3, 0, 1, 10000000.0);

  const function<long double (const long double&)> &&f4 = [](const long double &x)-> long double {return sqrt(1 - x * x);};
  const long double &vf4 = integral(f4, 0, 1, 10000000.0);

  cout << "Area: " << vf1 << " u.a" << endl;
  cout << "Area: " << vf2 << " u.a" << endl;
  cout << "Area: " << vf3 << " u.a" << endl;
  cout << "Area: " << vf4 << " u.a" << endl;
  return 0;
}

I wish I could do something like that on the command line:

integral "x*x" 0 1 100000

Any suggestions/hint?

Answer 1

Lambda is something internal to the code (essentially a pointer to a function), it does not pass through the command line (it is not a magic thing that turns into executable code).

The only way to achieve something close to what you want is to create a parser (even if simplified to fit only some types of code) of the content passed to the parameter of the main() and select the lambda you want according to what you find. It’s a very complex thing to do for something that seems to be just a simple exercise.

What you could do more simply is to have a parameter that would receive a number that would serve as an index for a vector of Amble predefined with some operations.

register is innocuous in C++. You probably shouldn’t use inline. I think there are other unnecessary complications in this code.