c中的luhns算法

Question

I've looked around the internet and I've seen a lot of other luhns algorithms for the same class I'm taking but it's hard to gauge what the difference in their code is vs mine. right now I have this:

#include <stdio.h>
#include <cs50.h>
#include <math.h>
#include <string.h>

int main(void)
{
    long input;
    do
    {
        input = get_long("Number: ");
    }
    while (input < 0);

    long secondToLast, last;
    int i;
    long length = 0;
    long sum1 = 0;
    long sum2 = 0;
    long firstNum, secondNum, totalSum;

    //Finds the length of the input
    while (input > 0)
    {
        input/=10;
        length++;
    }
    printf("%li\n", length);
    //For loop goes through every 2 digits in
    //input and takes out the ones we want.
    for (i = 0; i <= length; i += 2)
    {
        secondToLast = input / (long)pow(10, i+1);
        secondToLast = secondToLast % 10;
        last = input / (long)pow(10, i);
        last = last % 10;
        printf("Second to last: %li\n", secondToLast);
        printf("Last: %li\n", last);

        //This is to separate the digits into individual digits.
        if (secondToLast * 2 > 10)
        {
            firstNum = secondToLast * 2 / 10;
            secondNum = secondToLast * 2 % 10;
            sum1 = sum1 + firstNum + secondNum - (secondToLast * 2);
        }
        //Add the digits we get
        sum1 = sum1 + secondToLast *  2;
        sum2 = sum2 + last;
        printf("Sum1: %li\n", sum1);
        printf("Sum2: %li\n", sum2);
    }

    totalSum = sum1 + sum2;
    printf("Total Sum: %li\n", totalSum);
    //Final step of checksum to see if the last digit is 0 or not.
    if (totalSum % 10 == 0)
    {
        printf("Valid \n");
    }
        else
    {
        printf("Invalid \n";)
    }

my code seems to be incorrect as when i print out the sums to check what the code is doing, they are all 0 and never get changed. it worked before, but i don't recall ever changing the code to the point where it doesn't work. if anyone could please take a look and show me where i could be messing up would be appreciated.

Answer 1

In the first part of your algorithm, where you are trying to get the length of your number:

    //Finds the length of the input (and destroy input forever)
    while (input > 0)
    {
        input/=10;
        length++;
    }
    /* here you can ensure that input is always zero,
     * because of the two clauses stated below, continue
     * reading... */

You get out of the loop only when your input variable reaches 0, so after that all your algorithm deals with a 0 in input , and no way of knowing the digits anymore. You have destroyed your input.

Before that, you did

while (input < 0);

so in case your input was negative, your program would block in a loop (with an empty body) in which nothing is done to input so you will never get out of there.