Pontus Pontus - 1 month ago 13
Java Question

How do I implement the Luhn algorithm?

I am trying to create a program to validate 10 to 12 digit long number sequences based on the luhn algorithm, but my program keeps on telling me that every number is invalid even though they're not.

This number should be valid, but my code doesn't think so:

8112189876


This number should not be valid, which my program agrees with, as it thinks every number is invalid:
8112189875


Here is my code:

static void luhn(){
System.out.print("Enter number to validate:\n");
String pnr = input.nextLine();
int length = pnr.length();
int sum = 0;
for (int i = 1, pos = length - 1; i < 10; i++, pos--){
char tmp = pnr.charAt(pos);
int num = tmp - 0
int product;
if (i % 2 != 0){
product = num * 1;
}
else{
product = num * 2;
}
if (product > 9)
product -= 9;
sum+= product;
boolean valid = (sum % 10 == 0);
if (valid){
System.out.print("Valid!\r");
}
else{
System.out.print("Invalid!");
}
}
}

Answer

The first thing I see is that you have:

int num = tmp - 0

You should instead have:

int num = tmp - '0';

Secondly, you should be validating your sum outside of the for loop, because you only care about the sum after processing all the digits.

Thirdly, you are starting from the end of the number, and you are not including the first number of your string. Why not use i for both tasks?

Resulting (working) method:

static void luhn(){
  System.out.print("Enter number to validate:\n");
  String pnr = input.nextLine();
  // this only works if you are certain all input will be at least 10 characters
  int extraChars = pnr.length() - 10;
  if (extraChars < 0) {
    throw new IllegalArgumentException("Number length must be at least 10 characters!");
  }
  pnr = pnr.substring(extraChars, 10 + extraChars);
  int sum = 0;
  // #3: removed pos
  for (int i = 0; i < pnr.length(); i++){
    char tmp = pnr.charAt(i);
    // #1: fixed the '0' problem
    int num = tmp - '0';
    int product;
    if (i % 2 != 0){
      product = num * 1;
    }
    else{
      product = num * 2;
    }
    if (product > 9)
      product -= 9;
    sum+= product;              
  }
  // #2: moved check outside for loop
  boolean valid = (sum % 10 == 0);
  if (valid){
    System.out.print("Valid!\r");
  }
  else{
    System.out.print("Invalid!");
  }
}

Stylistically, this method would be more useful if, instead of method signature

static void luhn() {

it instead had method signature

static boolean luhn(String input) {

This easily allows your code to get the String from ANY source (a file, hardcoded, etc.) and do anything with the result (print a message as yours does, or do something else). Obviously you would move the System.out.print, input.nextLine(), and if(valid) bits of code outside of this method.

Full refactored program:

import java.util.Scanner;

public class Luhn {
  private static Scanner input;

  public static void main(String... args) {
    input = new Scanner(System.in);
    System.out.print("Enter number to validate:\n");
    String pnr = input.nextLine();
    boolean result = luhn(pnr);
    printMessage(result);
    input.close();
  }

  static boolean luhn(String pnr){
    // this only works if you are certain all input will be at least 10 characters
    int extraChars = pnr.length() - 10;
    if (extraChars < 0) {
      throw new IllegalArgumentException("Number length must be at least 10 characters!");
    }
    pnr = pnr.substring(extraChars, 10 + extraChars);
    int sum = 0;
    for (int i = 0; i < pnr.length(); i++){
      char tmp = pnr.charAt(i);
      int num = tmp - '0';
      int product;
      if (i % 2 != 0){
        product = num * 1;
      }
      else{
        product = num * 2;
      }
      if (product > 9)
        product -= 9;
      sum+= product;              
    }
    return (sum % 10 == 0);
  }

  private static void printMessage(boolean valid) {
    if (valid){
      System.out.print("Valid!\r");
    }
    else{
      System.out.print("Invalid!");
    }
  }
}
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