请解释这个素数生成器代码是如何工作的（在C中）

Question

int prime (int poss_prime);  

int main() {  

    int poss_prime;  

    for (poss_prime = 2; poss_prime <= MAX_INT; poss_prime++) {  
        if (prime (poss_prime)) {  
            printf("%d ", poss_prime);  
        }  
    }  

    printf("\n\n");  
    return 0;  
}  

int prime (int poss_prime) {  

    int poss_factor;  

    for (poss_factor = 2; poss_factor <= poss_prime/2; poss_factor++) {  
        if (poss_prime % poss_factor == 0) {  
            return 0;  
        }  
    }  

    return 1;

}

I don't understand why the function can be used as the condition in the if statement, and why poss_prime is divided by two in the for loop condition. Thank you.

Answer 1

the function prime() has as its return type an integer. Putting the function call in the if statement is equivalent to

int value = prime(possprime);
if(value)

It is just a neater way of doing so, if you do not subsequently require that return value. logical expressions in C also accept numerical arguments. 0 is false, and any positive integer is true. I don't know the result of a negative integer.

it would be clearer, however, if you used bool as the return type of prime, to make it clear it is a true/false situation.

As for the prime generator itself: It is checking for all factors of that number. It cannot have any factors greater than half the original value (excluding itself), which is why the for loop only goes to half of the possible primes value.

Answer 2

Basic programming.

The function prime returns 1 or 0, depending on what it finds. In C, 0 means "false" and 1 means "true".

Re the divide by 2, if there is any factor of the number, it must be half the number or less, since the smallest possible factor is 2.