函数未在C中返回正确的值

Question

I'm trying to make a program that finds the root of an equation. Everything about my program works just fine, except for the equation itself: it keeps returning wrong values (1, infinity, 0, ...).

This is the function I'm evaluating:

double f(x) {
    return exp(-x)-sin(M_PI*x/2.);
}

For example, f(.3) should be .287, but it returns 1.000. Weird thing is, I tried the exact same code on another computer a while ago and it worked just fine.

Answer 1

double f(x) {
    return exp(-x)-sin(M_PI*x/2.);
}

The type for x is not declared, so it defaults to int .

You pass .3 , so it coerced to int and become 0.

exp(-0)-sin(M_PI*0/2.) == exp(0)-sin(0) == 1.0-0 == 1.0

Please find yourself a better compiler, and turn on warnings.

Answer 2

Probably you should cast your variable. That usually happens to me with divisions.

Answer 3

double f(double x) {
 ......
}

Answer 4

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

double f(double x) {
    return exp(-x)-sin(M_PI*x/2.);
}

int main(void){
    printf(".3 = %f",f(.3));
    getchar();
    return 0;
}

results: '.3 = 0.286828'