How can I replace duplicate numbers in an array with a different number?

Question

for a school project I have created a program that prints the number of lottery tickets the user requests. It works fine, but there is one problem: there are duplicate numbers in the arrays. I have been instructed by my professor that I need to make sure there are no duplicates within the first 5 numbers of the array. How can I iterate through the array, check for duplicate numbers, and then replace each duplicate number with a different random number? For example, if the array is {27, 22, 34, 15, 15} then the program should change the duplicate number to {27, 22, 34, 15, 12}. Please help.

    import java.util.Random;
    import java.util.Scanner;

    /**
    * SuperLotto program prompts user for number
    * of lottery tickets, printing them as requested
    * 
    * @author j
    */
    public class SuperLotto {   

   /**
   * This method declares an array numberList
   * containing 6 elements
   */
   public static void quickPick() {
    int numberList[] = new int[6];
    
    //calls the printTicket method for the array
    printTicket(numberList);
}


/**
 * This method iterates through array to print the elements
 * 
 * @param numberList - this parameter accepts the array
 */
public static void printTicket(int[] numberList) {
    Random rnd = new Random();
    
    /* method assigns random numbers to first 
     * 5 elements in the array and prints them
     */
    for (int i = 0; i < 5; i++) {
    System.out.print(rnd.nextInt(((27-1) + 1) + 1) + " ");
    }
    
    /* method assigns random number to last element
     * in array and prints it in special format
     */
    for (int i = 0; i < 1; i++) {
        System.out.print("(MEGA: " + rnd.nextInt(((27-1) + 1) + 1) + ")");
    }
    
}


//main method
public static void main(String[] args) {
    Scanner scnr = new Scanner(System.in);
    String name;
    int numberOfTickets;
    String decide;
    
    //Program prompts the user for their name
    System.out.println("Welcome to the Quicky Mart, what is your name?");
    //User inputs their name
    name = scnr.next();
    
    //new line
    System.out.println();
    
    //Program prompts the user for number of tickets
    System.out.println("Hi " + name + ", how many Super Lotto "
            + "tickets would you like to purchase?");
    //User inputs the number of tickets
    numberOfTickets = scnr.nextInt();
    
    //new line
    System.out.println();
    
    /* Program outputs the number
     * of tickets entered by user
     */
    System.out.println("Super Lotto Ticket(s): ");
    for (int i = 0; i < numberOfTickets; i++) {
        quickPick();
        System.out.println();
    }       
    
    //new line
    System.out.println();
    
    /* System prompts user if they would
     * like to run the program again
     */
    System.out.println("Good luck! "
            + "Would you like to run the program again?");
    
    //User enters their decision
    decide = scnr.next();
    
    /* If user decides yes,
     * main program runs again
     */
    while (decide.equalsIgnoreCase("yes")) {
        System.out.println();
        main(null);
        break;
    }
    
    /* If user decides no,
     * program terminates with a message
     */
    if (decide.equalsIgnoreCase("no")) {
        System.out.println();
        System.out.println("Goodbye!");
    }
    
  }
}

Don´t want to spoil all the fun, but have a look at java stream api. Java already provides you with the functionalities you need. — ILikeTrains, Sep 18 '22 at 00:03
Does this answer your question? [Generating Unique Random Numbers in Java](https://stackoverflow.com/questions/8115722/generating-unique-random-numbers-in-java) — Henry Twist, Sep 18 '22 at 00:10
calling `main(null);` is a kind of a bad solution, a `do { all the code in main } while (user selects yes)` loop would make more sense. You're using recursion here. — zapl, Sep 18 '22 at 02:19

score 0 · Answer 1 · answered Sep 18 '22 at 01:49

you could use an array list to hold generated numbers, and use the list.contains() method to stop duplicates from getting in. since you print the tickets to the console, your quickpick method doesnt actually do anything, so you can delete it and just call print ticket directly. and since print ticket doesnt actually iterate over or use that array its passed, no need to pass that either. you could rewrite printTicket as i have below and you shouldnt get any duplicates.

public static void printTicket(){
Random rnd = new Random();
List<Integer> list = new ArrayList<>();
while(list.size() <= 6) {
int n = rnd.nextInt(((27-1) + 1) + 1);
if(!list.contains(n){
list.add(n);
}
}
for(int i = 0; i < 4; i++) {
System.out.print(list.get(i) + " ");
}
System.out.print("(Mega: " + list.get(list.size()-1) + ")");
}

score 0 · Answer 2 · answered Sep 18 '22 at 02:20

In pseudo code (not wanting you to miss out on coding practice):

create a List<Integer> numbers of all possible numbers (eg 1 to 27)
use Collections.shuffle() to put the numbers in random order
use List#subList() to get the winning numbers (eg the first 5 elements of the list)

How can I replace duplicate numbers in an array with a different number?

2 Answers2