CODEWITHSUNDEEP
c++
Amos Chew
Amos Chew

Reputation: 137

"double" does not print decimals

i was wondering why in this program, "pi_estimated" wouldn't print out as a number with decimal places although the variable was declared as a "double". However, it prints out an integer.

double get_pi(double required_accuracy)
{
    double pi_estimation=0.0;
    int x,y;
    double p=0.0,q=0.0,r=0.0;
    int D=0;
    for(int N=1;N<=1e2;N++)
    {
        x = rand()%100;
        p = (x/50.0 - 1.0)/100.0;
        y = rand()%100;
        q = (y/50.0 - 1.0)/100.0;
        r = p*p + q*q;
        if((sqrt(r))<1.0)
        {
            D++;
           pi_estimation = 4.0*(double (D/N));
        }
        if(double (4/(N+1)) < (required_accuracy*pi_estimation/100.0))
        {
            cout<<pi_estimation<<endl;
            return (pi_estimation);
        }
    }
}

int main()
{
    double pi_approx=0.0, a, actual_accuracy=0.0;
    for(a=0.1;a>=1e-14;a/=10)
    {
        pi_approx = get_pi(a);
        actual_accuracy = (fabs((pi_approx - M_PI)/(M_PI)))*100.0;
        cout<<actual_accuracy<<endl;
    }
}

Upvotes: 2

Views: 3528

Answers (3)

Bathsheba
Bathsheba

Reputation: 234635

Since D and N are both integral types, D/N is performed in integer arithmetic; the cast to double happens too late as precision is lost prior to the cast.

One fix is to write 4.0 * D / N. This will ensure that everything is calculated in floating point. (Since * and / have the same precedence, you don't need to write (double).)

Upvotes: 1

Zac Howland
Zac Howland

Reputation: 15872

The problem is here:

pi_estimation = 4.0*(double (D/N));

D and N are both integers, so D/N is an integer that you are casting to a double and then multiplying by 4.0.

You want to do this:

pi_estimation = 4.0 * (static_cast<double>(D) / N));

Upvotes: 1

tckmn
tckmn

Reputation: 59273

This line is the culprit:

pi_estimation = 4.0*(double (D/N));

Since D and N are both ints, D/N is an int. Casting the int to a double cannot magically make decimals appear out of nowhere.

Here's the line, fixed:

pi_estimation = 4.0 * (((double) D) / N));

You could also multiply first, so you don't need so many parens:

pi_estimation = 4.0 * D / N;

D is being multiplied by 4.0, so it becomes a double because double * int = double. Then it's divided by N. Since (x * y) / z === x * (y / z) (associative property), the expressions are equivalent.

Upvotes: 6

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