java random numbers must be different

Question

I'm creating a small program that extracts 3 random numbers, the only condition is that all three numbers must be different from each other, for example: 3,9,3 is not acceptable.

This i my code, i tried it several times and occasionally appear numbers equal to each other.

What is wrong in my code ?

public class Premi {
public static void main(String[] args) {
    int num = (int) (Math.random() *10) + 1;
    int num2 = (int) (Math.random() *10) + 1;
    int num3 = (int) (Math.random() *10) + 1;
    boolean first = true;
    boolean second = true;
    boolean third = true;
    while(first) {
        if (num!=num2) {
            first=false;
        } else if (num==num2) {
            num = (int) (Math.random() *10) + 1;
        }   
    }
    while(second) {
        if (num!=num3) {
            second=false;
        } else if (num==num3) {
            num = (int) (Math.random() *10) + 1;
        }   
    }
    while(third) {
        if (num2!=num3) {
            third=false;
        } else if (num2==num3) {
            num2 = (int) (Math.random() *10) + 1;
        }   
    }
    System.out.println(num + "," + num2 + "," + num3);
}
}

Thank you.

Create a list. While this list's length is `< 3`, generate a random integer. If it's in the list, keep iterating. If it's not, add it to the list. Why all the checks like this? — Nepho, Dec 15 '17 at 13:51
If you change `num2` in the last while loop, you might make it match `num` and you don't check this — doctorlove, Dec 15 '17 at 13:51
yes but i can't use this code because, i'm studing java for first time ad i'm at chapter 5 of my book and until now on the book are explained the cycle and the if-else iteration — Fetex, Dec 15 '17 at 13:54
By the way: You don't need `if else` in your code. If `(num!=num2)` is `false` then `(num==num2)` must be `true`. — Würgspaß, Dec 15 '17 at 13:59

Alex Savitsky · Accepted Answer · 2017-12-15T14:07:53.023

1

Set<Integer> numbers = new HashSet<>();
while (numbers.size() < 3)
    numbers.add(((int) (Math.random() *10) + 1));

Using Set here will guarantee that the numbers are not repeating.

And if you're not allowed to use any collections or arrays (I'll reserve my opinion of courses that forbid students to use valid techniques) you can use:

int n1 = (int) (Math.random() * 10) + 1;
int n2;
do {
    n2 = (int) (Math.random() * 10) + 1;
} while (n2 == n1);
int n3;
do {
    n3 = (int) (Math.random() * 10) + 1;
} while (n3 == n2 || n3 == n1);

The do loop is guaranteed to run at least once.

edited Dec 15 '17 at 14:07

answered Dec 15 '17 at 13:51

Alex Savitsky

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yes but i can't use this code because, i'm studing java for first time ad i'm at chapter 5 of my book and until now on the book are explained the cycle and the if-else iteration – Fetex Dec 15 '17 at 13:53
So you're not allowed to use `Collection`s at all? What about arrays? – Alex Savitsky Dec 15 '17 at 13:59
no, only cycle or if-else if it's needed – Fetex Dec 15 '17 at 14:03
Ok, edited my answer to account for your course's quirks... – Alex Savitsky Dec 15 '17 at 14:08
Using set for generating unique numbers is inefficient. Preferred way to generate unique numbers is Fisher-Yates algorithm. – matoni Dec 15 '17 at 14:38

java random numbers must be different

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