calculating numerical integral in c++ [closed] - c++

Question

Closed. This question needs details or clarity. It is not currently accepting answers.
Want to improve this question? Add details and clarify the problem by editing this post.
Closed 6 years ago.
Improve this question
I want to write a function that can calculate me inetegral e^(cos x) on range (a,b)
double integral(double(*f)(double x), double a, double b, int n) {
double step = (b - a) / n; // width of each small rectangle
double area = 0.0; // signed area
for (int i = 0; i < n; i ++) {
area += f(a + (i + 0.5) * step) * step; // sum up each small rectangle
}
return area;
}
This is what I have found but I`m new in c++ and I can't work with pointers
if there is another way please help me.

Answer 1

That function you found allows you to integrate any function you want, and that funcion is the first parameter of the integral method. You can just remove the first argument ('double(*f)(double x)' ), that is because the function you want to integrate is known ( e^cos(x)), so you don't need to give it as an argument. Then, in the for loop, you just replace de f function for e^cos(x). The method will look like this:
double integral(double a, double b, int n){
double step = (b - a) / n; // width of each small rectangle
double area = 0.0; // signed area
for (int i = 0; i < n; i ++) {
area += exp(cos(a + (i + 0.5) * step)) * step; // sum up each small rectangle
}
return area;
}

Answer 2

#include <functional>
template<typename T>
T integral(const std::function<T(T)>& f, T a, T b, int n) {
auto step = (b - a) / n; // width of each small rectangle
auto area = static_cast<T>(0); // signed area
for (auto i = 0; i < n; i++)
{
// sum up each small rectangle
area += f(a + (i + static_cast<T>( 0.5)) * step) * step;
}
return area;
}
int main()
{
std::function<float(float)> f_sine = [](float in) { return sin(in); };
auto two = integral(f_sine, 0.0f, 3.14f, 20);
return 0;
}
That will be $3.50