c程序中的文件重定向

Question

我有一个c程序代码，它接受一个名为input.txt的文件，从文件中读取数字，并根据素数查找数字的阶乘。代码工作得很好，但我扣除了标记，因为我不理解文件重定向。 对我错过的任何解释都将不胜感激

#include<stdio.h>

int find_prime_count(int prime, int number);
int find_next_prime(int number);
int Is_Prime(int number);

int main()
{
    FILE *input;
    int factor, number, prime, prime_count, loop_counter;

    input = fopen("input.txt","r");

    fscanf(input, "%d", &number);

    while(number > 0)
    {   
        printf("\n%3d! = ", number);

        loop_counter = 0;
        //start with the smallest prime, 2
        prime = 2;
        while(prime <= number)
        {
            prime_count = 0;
            // find the number of current prime number in all the factors,
            // i.e. n, n-1, n-2, ..., 4, 3, 2

            for(factor = number; factor > 1; factor--)
            {
                if(prime <= factor)
                //if the current prime is not greater than the factor
                {
                    prime_count += find_prime_count(prime, factor);
                }
            }
            // print this set (prime, prime_count)
            if (prime == 2)
                printf("\t(%d^%d)", prime, prime_count);
            else
                printf("*(%d^%d)", prime, prime_count);
            //get the next prime number
            prime = find_next_prime(prime);
            //maintain output format
            if((++loop_counter) % 9 == 0) printf("\n       ");
        }
        printf("\n");
        fscanf(input, "%d", &number);
    }
    fclose(input);
}

/*
This function counts how many times the number
"prime" occurs in the number "number".
*/

int find_prime_count(int prime, int number)
{
    int count = 0;
    while(number % prime == 0)
    {
        count++;
        number = number / prime;
    }
    return count;
}

/*Finds the next prime number greater than the input
INPUT: a positive integer n
OUTPUT: prime number > n
*/

int find_next_prime(int number)
{
    int trial_prime;
    if(number == 1 || number == 2) return number + 1;
    //for base cases
    //choose the next odd number
    if(number % 2 == 0)
        trial_prime = number + 1;
    else
        trial_prime = number + 2;
    while( !Is_Prime (trial_prime) )
        //test the next odd number
        trial_prime += 2;
    //while loop will end only when we found a prime number
    return trial_prime;
}

/*Determines if a given number is prime or not
INPUT: a positive integer n
OUTPUT: 1, if n is prime, 0 otherwise
*/

int Is_Prime(int number)
{
    if(number % 2 == 0) return 0; //even numbers cannot be prime number
    int divisor = 3; //start with the smallest odd prime number (> 1) 3
    while(number % divisor != 0)
    {
        if(divisor * divisor > number) //"number" is a prime number
            return 1;
        divisor +=2; //checks for the next odd divisor
    }
    return 0;
}