Integer value not having precision with setprecision()

Question

I have a program here to approximate the value of pi using a series, and need it to output pi to a certain degree of accuracy, I have chosen 16 digits. However, when I input 0 for the summation, it returns 4 (which is the correct answer) but with no decimals. The rest of my inputs result in a precision of 16.

#include <iostream>
#include <iomanip>
#include <string>

using std::cin;
using std::cout;
using std::endl;

int myPrompt(void);
double mySum(int);
void myResults(void);

int main() {
    myResults();
    return 0;
}


int myPrompt(void) {
    cout << "Approximating pi using the Madhava-Leibniz series" << endl
         << std::setfill('=') << std::setw(std::size("Approximating pi using the Madhava-Leibniz series")) << "" << endl;
    int input;
    do {
        cout << "Please input an integer greater than or equal to 0: ";
        cin >> input;
    } while (input < 0);
    return input;
}

double mySum(int input) {
    double numerator = -1;
    double output = 0;
    int base = -1;
    for (int k = 0; k <= input; k++) {
        numerator *= base;
        output += numerator / ((2 * k) + 1);
    }

    return (output * 4);
}

void myResults(void) {
    int userInput = myPrompt();
    double output = mySum(userInput);
    cout << std::setprecision(16) << "The approximated value of pi is: " << output << endl;
}

I tried setting multiple variable types to double assuming it was just returning an integer, but I can't seem to find why. I just need it to output 4 with 16 digits of accuracy. Any help is appreciated, thank you!

Answer 1

I think the problem is how std::cout displays floating-point numbers. When you set the precision using std::setprecision(16), it affects the number of decimal places displayed, but it doesn't guarantee that all 16 decimal places will be shown if they are zeros.

You can use std::fixed and std::showpoint in addition to setting the precision:

void myResults(void) {
    int userInput = myPrompt();
    double output = mySum(userInput);
    cout << std::fixed << std::showpoint << std::setprecision(16) << "The approximated value of pi is: " << output << endl;
}