PI number calculating with c++ program

Question

// PI = 4 - (4/3) + (4/5) - (4/7) ... for 100 first statements
#include <iostream>
#include <conio.h>
using namespace std;
int main() {
    double PI = 0.0;
    int a = 1;
    for (int i = 1; i <= 100; i++) {
        if (i % 2 == 1) {
            PI += double(4 / a);
        }
        else {
            PI -= double(4 / a);
        }
        a += 2;
    }
    cout << "PI Number is : " << PI;
    cout << endl;
    return 0;
}

I tried this code in visual studio 2015 to give me the answer of PI number value but it returns "PI Number is : 3" and I want it to return a float or a double number.

What should I do?

Try replacing `double(4 / a)` with `4.0 / a` – UnholySheep Mar 16 '20 at 11:47 — UnholySheep, Mar 16 '20 at 11:47

score 3 · Answer 1 · answered Mar 16 '20 at 11:49

3

In double(4 / a), the 4 / a part evaluates to an integer and it is already truncated by the time you cast it to double. What you want to do is 4.0 / a instead, and no need for an explicit cast.

answered Mar 16 '20 at 11:49

Aykhan Hagverdili

28,141
6
41
93

score 2 · Answer 2 · answered Mar 16 '20 at 11:49

2

4 / a is an integer division and your conversion double(…) happens after that division, so the result will never have something after the decimal point. e.g. 4/5 results in 0.

You need to change 4 from an integer to a double 4.

answered Mar 16 '20 at 11:49

t.niese

39,256
9
74
101

score 1 · Accepted Answer · answered Mar 16 '20 at 11:53

The issue in your code is when it's computing the value:

double PI = 0.0;
int a = 1;
for (int i = 1; i <= 100; i++) {
    if (i % 2 == 1) {
        PI += double(4 / a);
    }
    else {
        PI -= double(4 / a);
    }
    a += 2;
}

You shouldn't do double(4 / a) but rather (double)4 / a or 4.0 / a

double PI = 0.0;
int a = 1;
for (int i = 1; i <= 100; i++) {
    if (i % 2 == 1) {
        PI += (double)4 / a;
    }
    else {
        PI -= (double)4 / a;
    }
    a += 2;
}

PI number calculating with c++ program

3 Answers3