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cpi

C rounding double to 6dp

I am using the following code to calculate pi in C but the answer is only being printed to 6dp.

code:

#include<stdio.h>
#include<stdlib.h>

long double calc_pi(int terms) {
    long double result = 0.0F;
    long double sign = 1.0F;

    for (int n = 0; n < terms; n++) {
                result += sign/(2.0F*n+1.0F);
            sign = -sign;
        }

    return 4*result;
}


int main(int argc, char* args[]) {
        long double pi = calc_pi(atoi(args[1]));
    printf("%LF", pi);
}

output:

$ ./pi 10000
3.141493

the 10000 is the number of terms that are used as the code uses an infinite series.

Solution

  • You can add a precision specifier to your printf() format specifier to have it print more digits.

    int main(int argc, char* args[]) {
        long double pi = calc_pi(atoi(args[1]));
    printf("%.30LF", pi); /* have it print 30 digits after the decimal point */
}