在C语言中实现Luhn算法遇问题：存在bug的代码

Hey there! Let's work through this Luhn algorithm issue for your CS50x 2018 pset1 "credit" task— I remember struggling with this exact part when I took the course, so I feel your pain. Let's break this down step by step.

First, Let's Lock Down the Full Luhn Steps (to avoid misalignment) #

You mentioned part of the algorithm, but let's recap the complete process to make sure we're on the same page:

• Start from the second-to-last digit of the card number (counting from the right, this is the "even position" if you start at 1 for the last digit)
• Multiply every other digit (those even positions from the right) by 2
• For any product that's 10 or higher, add the individual digits together (or just subtract 9— it's a shortcut that gives the same result, e.g., 14 → 1+4=5 or 14-9=5)
• Sum all these processed doubled digits
• Add that sum to the total of the remaining digits (the ones you didn't multiply by 2, the odd positions from the right)
• If the final total is a multiple of 10, the number passes the Luhn check; if not, it's invalid

Common Mistakes That Trip Up CS50x Students #

Chances are your bug is one of these super common pitfalls:

• Position mix-up: If you're treating "even positions" as counting from the left instead of the right, that's a guaranteed error. Always start counting from the last digit (rightmost) when applying Luhn.
• Botching two-digit products: Forgetting to split numbers like 18 into 1+8 and instead adding 18 directly will throw off your sum drastically.
• Incorrect digit extraction: When working with the card number as a long long, make sure you're pulling digits from right to left using temp % 10 (to get the last digit) then temp /= 10 (to chop it off). Going left to right is way harder here.
• Merging sums too early: Keep separate variables for the sum of doubled digits and the sum of original digits— mixing them up mid-loop leads to messy, hard-to-debug calculations.

Example Working Snippet (C, for CS50x) #

Here's a clean, commented implementation you can compare against your code:

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

int main(void) {
    // Get the card number from user
    long long card_num = get_long_long("Enter credit card number: ");
    long long temp = card_num;
    int sum_doubled = 0;
    int sum_original = 0;
    bool double_next = false; // Start with false: last digit doesn't get doubled

    while (temp > 0) {
        int current_digit = temp % 10;

        if (double_next) {
            int doubled = current_digit * 2;
            // Handle two-digit products: add digits or subtract 9
            sum_doubled += (doubled > 9) ? (doubled - 9) : doubled;
        } else {
            sum_original += current_digit;
        }

        // Flip the flag for the next digit (moving left)
        double_next = !double_next;
        // Chop off the last digit we just processed
        temp /= 10;
    }

    int total_sum = sum_doubled + sum_original;
    if (total_sum % 10 == 0) {
        // Note: You'll still need to add card issuer checks (Amex, Visa, MasterCard) here
        printf("VALID\n");
    } else {
        printf("INVALID\n");
    }
    return 0;
}

Quick Debugging Tips #

• Test with a known valid number: Try 4003600000000014 (a valid Visa). Manually calculate the sum:
  • Doubled digits (right-to-left even positions): 12=2, 02=0, 02=0, 62=12→3, 02=0, 02=0, 4*2=8 → total 13
  • Original digits: 4+0+0+0+0+0+3 → total 7
  • Final sum 13+7=20, which is divisible by 10. If your code doesn't return VALID for this, you know exactly where to dig.
• Add print statements: Print current_digit, sum_doubled, and sum_original in each loop iteration. Compare the values to your manual calculation to spot where things go wrong.

内容的提问来源于stack exchange，提问作者PY98