function does not return the value

[john98]

i just started coding in c++. i wrote a function to approximate pi value in monte carlo method but it does not return the value.

#define _USE_MATH_DEFINES
#include <iostream>

#include <cmath>
using namespace std;
#include <cstdlib>  // pour srand et rand
#include <ctime>  // pour time
#include <iomanip>



double getpi()
{
	double compteur = 0;
	double nb_iteration;
	
	cout << "entrer le nombre d'iteration ";
	cin >> nb_iteration;
	srand(unsigned(time(nullptr)));
	for (int i = 1; i <= nb_iteration; i++)
	{
		double x = -1.0 + static_cast <double> (rand()) / (static_cast <double> (RAND_MAX / (1.0 - (-1.0))));
		double y = -1.0 + static_cast <double> (rand()) / (static_cast <double> (RAND_MAX / (1.0 - (-1.0))));
		if (x * x + y * y < 1)
		{
			compteur += 1;
		}
		
	}
	cout << fixed;
	cout << setprecision(6);
	double pi = 4 * (compteur / nb_iteration);
	double ecart = abs((pi - M_PI)) * 100 / M_PI;
	return pi;
}
		



int main() {
	double answer= getpi();
}
  

[keskiverto]

How do you know that it does not return a value?

[john98]

i tried to run in visual studio but i did not work. it asks me to enter my cin value and after that it shows nothing

[keskiverto]

Which line of code would show something?
You do get 'answer' on line 41 and do exactly nothing with it.

[john98]

the line 18 run correctly because it asks me to entre the nb_iteration and after that nothing happens

[john98]

i tried it with a void function . and i put cout << pi; instead of return it works perfectly

[keskiverto]

Wrong. Everything happens there -- just as you wrote it.

Add one line:

int main() {
	double answer= getpi();
	cout << answer << '\n';
}

[john98]

ohhhh thank you so much. i thought that by calling the function the return will show up automatically.

[keskiverto]

The Visual Studio has a debugger. You could with it halt the execution of your program and read what values it has in memory at that point.

Remember that a program could be run from command line and the main() could call many functions and have many variables. Nothing is shown implictly.


The rand() and srand() are functions from C library. C++ library has <random> with much better options.

For example:

#include <iostream>
#include <random>
#include <iomanip>

using std::cout;

double getpi( size_t nb_iteration )
{
	std::default_random_engine generator;
	std::uniform_real_distribution<double> distribution( -1.0, 1.0 );

	size_t compteur = 0;
	for ( size_t i = 0; i < nb_iteration; ++i )
	{
		double x = distribution(generator);
		double y = distribution(generator);
		if (x * x + y * y < 1.0)
		{
			++compteur;
		}
	}
	return (4.0 * compteur) / nb_iteration;
}

int main() {
	size_t nb_iteration {};
	cout << "entrer le nombre d'iteration ";
	std::cin >> nb_iteration;

	double answer= getpi( nb_iteration );
	cout << std::fixed;
	cout << std::setprecision(6);
	cout << answer << '\n';
}

[George P]

I understand creating a custom function to approximate the value of PI is a good learning experience, FYI C++20 has the <numbers> library that includes a PI constant.
https://en.cppreference.com/w/cpp/header/numbers

[john98]

thank you so much guys for the help. appreciate it.