I wrote a program to calculate the value of cosx by Maclaurin's series and by library function.See the following

#include<stdio.h>
#include<math.h>
#define N 20
int main()
{
    float sum,y;
    int x,i=0;
    while(scanf("%d",&x)!=EOF)
    {   sum=0;
        for(i=0;i<=N;i  )
        {
            y=pow(-1,i)*pow(x,2*i);
        sum=sum y/((float)fact(2*i));
        }
    printf("using Maclaurin's series %.4f and original cosx=%.4f",sum,cos(x));

    }

}

int fact(int x)
{
    int prod=1;
    int i=1;
    for(i=1;i<=x;i  )
        prod=prod*i;
    return prod;
}

when I input 5(or any valid number) the results are quite weird like this

5
using Maclaurin's series -1.#IND and original cosx=0.2837

what is this -1.#IND?

CodePudding user response：

1. You need function prototype before calling it.
2. Add some ifs to check if you do not divide by zero.
3. scanf returns number of successfully scanned elements. Check for 1 in this case.

            int fct = fact(2*i);
            if(fct) 
                sum=sum y/(double)fct;
            else
                {printf("DIVISION BY ZERO!!!!\n"); return 1;}

You will discover that int is to small for factorial function. Change it to double. Also, use double in all floating point operations.

double fact(int x);

int main()
{
    double sum,y;
    int x,i=0;
    double fct;
    while(scanf("%d",&x) == 1)
    {   sum=0;
        for(i=0;i<=N;i  )
        {
            y=pow(-1,i)*pow(x,2*i);
            fct = fact(2*i);
            if(fct != 0.0) 
                sum=sum y/fct;
            else
                {printf("DIVISION BY ZERO!!!!\n"); return 1;}
        }
        printf("using Maclaurin's series %.10f and original cosx=%.10f",sum,cos(x));
    }
}

double fact(int x)
{
    double prod=1.0;
    int i=1;
    for(i=1;i<=x;i  )
        prod=prod*i;
    return prod;
}

https://godbolt.org/z/qsh4qh3d1