C - Subtraction of numbers with many decimals

Question

Why does the C code below output "Difference: 0.000000" ? I need to make calculations with many decimals in one of my university tasks and I don't understand this because I'm new to programming in C. Am I using the correct type? Thanks in advance.

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


int main() {

long double a = 1.00000001;
long double b = 1.00000000;
long double difference = a-b;

printf("Difference: %Lf", difference);

}

I have tried that code and I'm expecting to get the result: "Difference: 0.00000001"

Try `printf("Difference: %.20Lf", difference);` instead. Then read [Is floating point math broken?](https://stackoverflow.com/questions/588004/is-floating-point-math-broken) — Retired Ninja, Nov 26 '22 at 14:44

Jonathan Leffler · Accepted Answer · 2022-11-26T15:03:24.433

You see 0.000000 because %Lf prints a fixed number of decimal places, and the default number is 6. In your case, the difference is 1 in the 8^th decimal place, which shows as 0.000000 when printed to 6 d.p. Either use %Le or %Lg or specify more precision: %.8Lf.

#include <stdio.h>

int main(void)
{
    long double a = 1.00000001;
    long double b = 1.00000000;
    long double difference = a - b;

    printf("Difference: %Lf\n", difference);
    printf("Difference: %.8Lf\n", difference);
    printf("Difference: %Le\n", difference);
    printf("Difference: %Lg\n", difference);
    return 0;
}

Note the minimal set of headers.

Output:

Difference: 0.000000
Difference: 0.00000001
Difference: 1.000000e-08
Difference: 1e-08

score 1 · Answer 2 · edited Nov 26 '22 at 15:05

#include <stdio.h>

int main() {
    
    long double a = 1.000000001;
    long double b = 1.000000000;
    long double difference = a-b;
    
    printf("Difference: %.9Lf\n", difference);

}

Try this code. Actually, you need to specify to the compiler how much precision you need after the decimal point. Here the .9 will print 9 digits after the decimal point. You can adjust this value according to your needs; just don't exceed the range of the variable.

C - Subtraction of numbers with many decimals

2 Answers2