c - 如何在 C 中添加产品数字而不是产品本身？

Question

我正在尝试用 C 完成 CS50 类(class)的作业，在该类(class)中我必须实现 Luhn 算法来验证信用卡号。这是一个详细说明的快速示例:

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信用卡号:4003600000000014。
现在对于每隔一个数字，从数字的倒数第二个数字开始:

1-0-0-0-0-6-0-4

让我们将每个数字乘以 2:

1•2 + 0•2 + 0•2 + 0•2 + 0•2 + 6•2 + 0•2 + 4•2

这给了我们:

2 + 0 + 0 + 0 + 0 + 12 + 0 + 8

现在让我们将这些产品的数字(即，不是产品本身)加在一起:

2 + 0 + 0 + 0 + 0 + 1 + 2 + 0 + 8 = 13

现在让我们将总和 (13) 添加到未乘以 2 的数字总和(从末尾开始):

13 + 4 + 0 + 0 + 0 + 0 + 0 + 3 + 0 = 20

是的，该和 (20) 的最后一位是 0，因此该数字有效。

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我想出了如何分别提取信用卡中的每个数字(我知道我的方法很无聊而且可能不实用)，所以下一步是将每个其他数字乘以 2 并相加(乘积的数字，不是数字本身)，这就是我需要帮助的方法吗？ 我的代码:

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

int main(void)
{
    long credit_card_number;
    do
    {
        credit_card_number = get_long("Enter your credit card number: ");

    }
    while (credit_card_number < 1 || credit_card_number > 9999999999999999);

    //American Express uses 15-digit numbers. American Express numbers start with 34 or 37
    //MasterCard uses 16-digit numbers. MasterCard numbers start with 51, 52, 53, 54, or 55.
    //Visa uses 13- and 16-digit numbers. Visa numbers start with 4.
    // checksum

    long last_number;
    long credit_card_without_last_number;
    long second_to_last_number;
    long credit_card_without_second_number;
    long third_number;
    long credit_card_without_third_number;
    long fourth_number;
    long credit_card_without_fourth_number;
    long fifth_number;
    long credit_card_without_fifth_number;
    long sixth_number;
    long credit_card_without_sixth_number;
    long seventh_number;
    long credit_card_without_seventh_number;
    long eighth_number;
    long credit_card_without_eighth_number;
    long ninth_number;
    long credit_card_without_ninth_number;
    long tenth_number;
    long credit_card_without_tenth_number;
    long eleventh_number;
    long credit_card_without_eleventh_number;
    long twelfth_number;
    long credit_card_without_twelfth_number;
    long thirteenth_number;
    long credit_card_without_thirteenth_number;
    long fourteenth_number;
    long credit_card_without_fourteenth_number;
    long fifteenth_number;
    long credit_card_without_fifteenth_number;
    long sixteenth_number;

    long multiply_digits;

    //separating each number starting from the last (right)in its own variable.
    last_number = credit_card_number % 10;
    credit_card_without_last_number = credit_card_number / 10;

    second_to_last_number = credit_card_without_last_number % 10;
    credit_card_without_second_number = credit_card_without_last_number / 10;

    third_number = credit_card_without_second_number % 10;
    credit_card_without_third_number = credit_card_without_second_number / 10;

    fourth_number = credit_card_without_third_number % 10;
    credit_card_without_fourth_number = credit_card_without_third_number / 10;

    fifth_number = credit_card_without_fourth_number % 10;
    credit_card_without_fifth_number = credit_card_without_fourth_number / 10;

    sixth_number = credit_card_without_fifth_number % 10;
    credit_card_without_sixth_number = credit_card_without_fifth_number / 10;

    seventh_number = credit_card_without_sixth_number % 10;
    credit_card_without_seventh_number = credit_card_without_sixth_number / 10;

    eighth_number = credit_card_without_seventh_number % 10;
    credit_card_without_eighth_number = credit_card_without_seventh_number / 10;

    ninth_number = credit_card_without_eighth_number % 10;
    credit_card_without_ninth_number = credit_card_without_eighth_number / 10;

    tenth_number = credit_card_without_ninth_number % 10;
    credit_card_without_tenth_number = credit_card_without_ninth_number / 10;

    eleventh_number = credit_card_without_tenth_number % 10;
    credit_card_without_eleventh_number = credit_card_without_tenth_number / 10;

    twelfth_number = credit_card_without_eleventh_number % 10;
    credit_card_without_twelfth_number = credit_card_without_eleventh_number / 10;

    thirteenth_number = credit_card_without_twelfth_number % 10;
    credit_card_without_thirteenth_number = credit_card_without_twelfth_number / 10;

    fourteenth_number = credit_card_without_thirteenth_number % 10;
    credit_card_without_fourteenth_number = credit_card_without_thirteenth_number / 10;

    fifteenth_number = credit_card_without_fourteenth_number % 10;
    credit_card_without_fifteenth_number = credit_card_without_fourteenth_number / 10;

    sixteenth_number = credit_card_without_fifteenth_number % 10;

    //Here I need the help to multiply these numbers by two and then add each product's
    //digits to the rest of the unused numbers.
    multiply_digits = (second_to_last_number*2)+(fourth_number*2)+(sixth_number*2)+(eighth_number*2)+(tenth_number*2)+(twelfth_number*2)+(fourteenth_number*2)+(sixteenth_number*2);

}

Answer 1

尝试这样做

int main(){
    long cNo = 4003600000000014;
    int arr[16];

    for(int i=0; i<16; i++){
        arr[15-i] = cNo % 10;
        cNo /= 10;
    }

    int multipliedSum = 0;
    for(int i=0; i<16; i++){
        if(i%2==1)
            multipliedSum += arr[i];
        else{
            if(arr[i]*2<10){
                multipliedSum += (arr[i]*2);
            }else{
                int num = arr[i]*2;
                while(num){
                    multipliedSum += num%10;
                    num/=10;
                }
            } 
        }
    }

    printf("valid = %s\n",multipliedSum%10==0?" True": " False");
}

你会得到以下内容

valid = True