printing doubles from a function outside of main

Question

I am new to c and trying to use a function outside of the main function, I wanted to test what I was getting but found an error when I print the value from the function.

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

/*function prototype f*/
int f(double x);

/*main*/
int main(void){

   double x = 0.1;

   printf("the value is %.3f\n", f(x)); 
   printf("the value is %.3f", sqrt(1.0-(x*x))); 

   return 0;
}

int f(double x){
    double value;
    value = sqrt(1.0-(x*x));}
    return value;
}

This basic code finds a value from a given x that I want to later use in a loop. when I run this however I get the following output

the value is -1.....
the value is 0.995

(… is a random sequence of characters)

I am not sure what the difference is in the function f and the actual value calculated inside the main function and why I am getting this issue

Answer 1

You were using int instead of double for your prototype. This should give you the right answer.

#include <stdio.h>
#include <math.h>
#include <conio.h>

/*function prototype f*/
double f(double x);

/*main*/
int main() {

    double x = 0.1;

    printf("the value is %.3f\n", f(x));
    printf("the value is %.3f", sqrt(1.0 - (x*x)));

    _getch();
    return 0;
}

double f(double x) {
    double value;
    value = sqrt(1.0 - (x*x));
    return value;
}