c - 控制可能会达到非无效功能的终点。 C

Question

我在此代码的返回行上收到上述错误。

// Finds the length of the card number 
int find_length(long long n)
{
    int len;
    for (len = 0; n != 0; n /= 10, len++)
    return len;
}

完整代码:

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



void print_credit_card_brand(long long ccn);
bool check_validity(long long credit_card_number);
int find_length(long long n);
bool checksum(long long ccn);



int main(void)
{
    long long credit_card_number;
    do
    {
        credit_card_number = get_long_long("Enter valid credit card number without spaces or hyphens: ");
    } while (credit_card_number < 0);

    if (check_validity(credit_card_number) == true)
        print_credit_card_brand(credit_card_number);
    else
        printf("INVALID\n");
}



// Checks validity of credit card
bool check_validity(long long credit_card_number)
{
    int len = find_length(credit_card_number);
    return (len == 13 || len == 15 || len == 16) && checksum(credit_card_number);
}



// Finds the length of the card number 
int find_length(long long n)
{
    int len;
    for (len = 0; n != 0; n /= 10, len++)
    return len;
}



// Determines validity using Luhn’s Algorithm
bool checksum(long long ccn)
{
    int sum = 0;
    for (int i =0; ccn != 0; i++, ccn /=  10)
    {
        if (i % 2 == 0)
             sum += ccn % 10;
        else
        {
            int digit = 2 * (ccn % 10);
            sum += digit / 10 + digit % 10;
        }
    }
    return (sum % 10) == 0;
}



// Prints results besed on length of credit card and 1st 2 digits
void print_credit_card_brand(long long ccn)
{
    if ( (ccn >= 34e13 && ccn < 35e13) || (ccn >= 37e13 && ccn < 38e13) )
        printf("AMEX\n");
    else if (ccn >= 51e14 && ccn < 56e14)
        printf("MASTERCARD\n");
    else if ( (ccn >= 4e12 && ccn < 5e12) || (ccn >= 4e15 && ccn < 5e15) )
        printf("VISA\n");
    else
        printf("INVALID\n");
}

Answer 1

您忘记在for循环后放置分号。所以用这种错字功能看起来像

int find_length(long long n)
{
    int len;
    for (len = 0; n != 0; n /= 10, len++)
    {
        return len;
    }
}

该函数可以例如通过以下方式查找

unsigned int find_length( unsigned long long n )
{
    const unsigned long long Base = 10;
    unsigned int len = 0;

    for ( ; n != 0; n /= Base )
    {
        ++len;
    }

    return len;
}

请注意，您应该使用无符号整数类型。否则，用户可以输入一个负数，并且您的程序将产生错误的结果。