Luhn Algorithm

#include <iostream>
#include <string>
#include <cctype> // For isdigit function

bool isValid(const std::string& number) {
    int sum = 0;
    bool isOddIndex = false;
    
    // Check if the input contains only digits
    for (char c : number) {
        if (!std::isdigit(c)) {
            return false; // Return false if any non-digit character is found
        }
    }

    for (int i = number.size() - 1; i >= 0; i--) {
        int digit = number[i] - '0';

        if (isOddIndex) {
            digit *= 2;
            if (digit > 9) {
                digit -= 9;
            }
        }

        sum += digit;
        isOddIndex = !isOddIndex;
    }

    return (sum % 10 == 0);
}

int main() {
    std::string cardNumber;
    std::cout << "Enter the card number: ";
    std::cin >> cardNumber;

    if (isValid(cardNumber)) {
        std::cout << "The card number is valid." << std::endl;
    } else {
        std::cout << "The card number is invalid." << std::endl;
    }

    return 0;
}

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import java.util.Scanner;

public class LuhnAlgorithm {
    
    public static boolean isValid(String number) {
        int sum = 0;
        boolean isOddIndex = false;
        
        // Check if the input contains only digits
        for (char c : number.toCharArray()) {
            if (!Character.isDigit(c)) {
                return false; // Return false if any non-digit character is found
            }
        }

        for (int i = number.length() - 1; i >= 0; i--) {
            int digit = number.charAt(i) - '0';
            
            if (isOddIndex) {
                digit *= 2;
                if (digit > 9) {
                    digit -= 9;
                }
            }
            
            sum += digit;
            isOddIndex = !isOddIndex;
        }
        
        return (sum % 10 == 0);
    }

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        System.out.print("Enter the card number: ");
        String cardNumber = scanner.nextLine();
        
        if (isValid(cardNumber)) {
            System.out.println("The card number is valid.");
        } else {
            System.out.println("The card number is invalid.");
        }
        
        scanner.close();
    }
}

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def is_valid(number: str) -> bool:
    if not number.isdigit():
        return False

    sum = 0
    is_odd_index = False
    
    for digit in reversed(number):
        digit = int(digit)
        
        if is_odd_index:
            digit *= 2
            if digit > 9:
                digit -= 9
        
        sum += digit
        is_odd_index = not is_odd_index
    
    return sum % 10 == 0

# Example usage
card_number = input("Enter the card number: ")
if is_valid(card_number):
    print("The card number is valid.")
else:
    print("The card number is invalid.")

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