pow function in C fails to recognize variable

Question

I have a problem with pow function in c. The variable Qb will give wrong output - it gives 10000 instead of 177827.941004 resulting in final output of 2007 instead of 2009

Compile command is by. gcc -ggdb -std=c99 -Wall -Werror -o test_pow02 test_pow02.c -lm

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

int main(int argc, char **argv)
{
    int my_rating = 2000;
    int opponent_rating = 2100;
    int coeff = 15;
    int score = 1;
    int new_rating;

    double Qa = pow(10, my_rating / 400); // 100000
    double Qb = pow(10, opponent_rating / 400); // 177827.941004

    double Ea = Qa / (Qa + Qb); 

    new_rating = my_rating + ( (score - Ea) * coeff );

    printf("Qa is %g\n", Qa);
    printf("Qb is %g\n", Qb);
    printf("New Rating is %d\n", new_rating);

    return 0;
}

But if I hardcoded 2100/400 into 5.25, it will work correctly. I already have -lm at the end. How do I fix this?

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

int main(int argc, char **argv)
{
    int my_rating = 2000;
    //int opponent_rating = 2100;
    int coeff = 15;
    int score = 1;
    int new_rating;

    double Qa = pow(10, my_rating / 400); // 100000
    double Qb = pow(10, 5.25); // 177827.941004

    double Ea = Qa / (Qa + Qb); 

    new_rating = my_rating + ( (score - Ea) * coeff );

    printf("Qa is %g\n", Qa);
    printf("Qb is %g\n", Qb);
    printf("New Rating is %d\n", new_rating);

    return 0;
}

Answer 1

The operation pow(10, opponent_rating / 400); does not equal to 177827.941004.

opponent_rating / 400 == 5    

Operations involving two integers will produce an integer,
in this case the result 5.25 gets clipped to 5

You should divide with a double to get a double result opponent_rating / 400.0 that is used in the pow function.