Question

i think there is some problem with global scope and local scopes in this code below. if anyone please help me, why this code is not working!!
this is a program to find 25th magical number.magical number means it has no prime factor except 2,3,5. i have started from 16 which is 12th magical number. and then checking the numbers if they are divisible with any prime number from 7. so i list the prime numbers in a global array. and check the numbers with those prime in the arrays. and if this checking is complete, the program tries to find next prime number by using the next_prime function and list them into the global array. but i am getting wrong result. it is always giving the result input+=4 which is not expected.

/*a program to find out 25th magical number. magical number
means it has no prime factor except 2,3,5*/
#include<stdio.h>
#include<math.h>
double primes[10000]={2,3,5};
int serial=3;
double next_prime(double f)//function, when called, returns the next prime number
{
    int j=0,loop_breaker=0;
    int count;
    while(j==0){
        for(count=1;primes[count]<sqrt(f)+1 &&count<serial;count++){
            if(fmod(f,primes[count])==0){
                f+=2;
                loop_breaker=1;
                break;
            }
        }
        if(loop_breaker==0){
            primes[serial]=f;
            serial++;
            j=1;
        }
    }
    return f;
}
int main()
{
    double f=7,prime_divisor,magic_serial=12,magic_number=16;
    int c,loop_breaker,is_magic;
    prime_divisor=next_prime(f);
    f+=2;//this line always comes after the previous line so that everytime the value of f gets changed
    while(magic_serial!=25){
        is_magic=0;
        loop_breaker=0;
        for(c=3;c<serial;c++){
            if(fmod(magic_number,primes[c])==0){
                loop_breaker=1;
                break;
            }
        }
        if(loop_breaker==0){
            while(prime_divisor<sqrt(magic_number)+1){
                prime_divisor=next_prime(f);
                f+=2;
                if(fmod(magic_number,prime_divisor)==0){
                    is_magic=1;
                    break;
                }
            }
        }
        if(is_magic==0){
            magic_serial++;
        }
        magic_number++;
    }
    printf("%lf",magic_number);
    return 0;

}

Answer 1

I guess the problem is about next_prime function,
maybe this one works better:

double next_prime()
{
    double f = primes[serial-1] + 2;
    int count;
    for(count=1; primes[count]<=sqrt(f) && count<serial; ++count)
    {
        if(fmod(f, primes[count])==0)
        {
            f += 2;
            count = 1;
        }
    }
    return primes[serial++] = f;
}

I also thought that there is no need for function to take a value, so I removed it
also it seems that main function does not do what you expect, so I think this one works better:

int main()
{
    double prime_divisor=next_prime(),magic_serial=12,magic_number=16;
    int c, loop_breaker;
    while(magic_serial!=25)
    {
        ++magic_number;
        loop_breaker = 1;
        while(prime_divisor<magic_number/2)
            prime_divisor = next_prime();
        for(c=3; c<serial; ++c)
        {
            if(fmod(magic_number, primes[c])==0)
            {
                loop_breaker = 0;
                break;
            }
        }
        if(loop_breaker)
            ++magic_serial;
    }
    printf("%lf\n",magic_number);
    return 0;
}

I hope this helps.