如何在0-9范围内生成三个随机数而无需从Android中的arraylist重复

Question

I store my numbers like that :

ArrayList<Integer> nr=new ArrayList<Integer>();

    nr.add(0);
    nr.add(1);
    nr.add(2);
    nr.add(3);
    nr.add(4);
    nr.add(5);
    nr.add(6);
    nr.add(7);
    nr.add(8);
    nr.add(9);

    Random r1 = new Random();
    Random r2 = new Random();
    Random r3 = new Random();

   int rnd1 = r1.nextInt(nr.size());
    nr.remove(rnd1);
   int rnd2 = r2.nextInt(nr.size());
    nr.remove(rnd2);
   int rnd3 = r2.nextInt(nr.size());
    nr.remove(rnd3);

I try to remove the generated numbers but it keeps repeating after some tries

Answer 1

The idea here is that you are generating a random number in the range 0..9. Then you remove one element and you generate another random number in the range 0..8. You finish with generating a third number in the range 0..7.

The correct way would be to use the numbers that you generated as indexes for the elements. You take an element with this index: this is your "random" number. Then you remove this element so that you couldn't take it again (in my case, I obtain and remove the element using one operation: "remove").

ArrayList<Integer> nr = new ArrayList<Integer>();

nr.add(0);
nr.add(1);
nr.add(2);
nr.add(3);
nr.add(4);
nr.add(5);
nr.add(6);
nr.add(7);
nr.add(8);
nr.add(9);

Random r = new Random();

int rndPos1 = r.nextInt(nr.size());
int rnd1 = nr.remove(rndPos1);

int rndPos2 = r.nextInt(nr.size());
int rnd2 = nr.remove(rndPos2);

int rndPos3 = r.nextInt(nr.size());
int rnd3 = nr.remove(rndPos3);

System.out.println(rnd1 + ", " + rnd2 + ", " + rnd3);

Answer 2

You could shuffle your ArrayList and choose the first 3 items from it like so:

import java.util.Arrays;
import java.util.ArrayList;
import java.util.Collections;

class Main {
  public static void main(String[] args) {
    ArrayList<Integer> nr =new ArrayList<Integer>();
    nr.add(0);
    nr.add(1);
    nr.add(2);
    nr.add(3);
    nr.add(4);
    nr.add(5);
    nr.add(6);
    nr.add(7);
    nr.add(8);
    nr.add(9);
    System.out.println("The ArrayList looks like: " + Arrays.toString(nr.toArray()));
    System.out.println("Three unique random numbers from it are:");
    //shuffle the list
    Collections.shuffle(nr);
    for(int i=0; i<3; i++) {
      System.out.println(nr.get(i));
      nr.remove(i);
    }
    System.out.println("The ArrayList now looks like: " + Arrays.toString(nr.toArray()));
    Collections.sort(nr);
    System.out.println("Sorted: " + Arrays.toString(nr.toArray()));
  }
}

Example Usage:

The ArrayList looks like: [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
Three unique random numbers from it are:
7
8
5
The ArrayList now looks like: [6, 2, 3, 9, 4, 0, 1]
Sorted: [0, 1, 2, 3, 4, 6, 9]

Try it here!

Answer 3

There are multiple things wrong with your code:
1. You don't need to create new random everytime.
2. Because you get all the option before hand you might run in to Array out of bounds exception.

In a loop get random number and check till you have 3 unique numbers.
Your code will look like this :

ArrayList<Integer> nr=new ArrayList<Integer>();
nr.add(0);
nr.add(1);
nr.add(2);
nr.add(3);
nr.add(4);
nr.add(5);
nr.add(6);
nr.add(7);
nr.add(8);
nr.add(9);

Random r1 = new Random();
for(nr.size != 7) {
  nr.remove(r1.nextInt(nr.size()));
}

Another good way of doing this would be :

Collections.shuffle(nr);
nr.remove(0);
nr.remove(1);
nr.remove(2);

Answer 4

Random generates random numbers by a seed. If 2 Random objects have the same seed, the random numbers they generate will be the same. By default, this seed is the current system time.

Here, you created three Random objects:

Random r1 = new Random();
Random r2 = new Random();
Random r3 = new Random();

Since code runs very fast, they are created in the same millisecond so their seeds are the same. As a result, they generate same random numbers.

The first step to fix your code is to create just one Random :

Random r = new Random();

The next step is to fix the way you select the random numbers from the array list. You should get the item at the index of rnd1 , rnd2 , rnd3

int rnd1 = r.nextInt(nr.size());
int result1 = nr.get(rnd1);
nr.remove(rnd1);
int rnd2 = r.nextInt(nr.size());
int result2 = nr.get(rnd2);
nr.remove(rnd2);
int rnd3 = r.nextInt(nr.size());
int result3 = nr.get(rnd3);
nr.remove(rnd3);

result1-3 are your non-repeating random numbers!

I have helped you cleaned up your code:

int index = r.nextInt(nr.size());
int result1 = nr.get(rnd1);
nr.remove(index);
index = r.nextInt(nr.size());
int result2 = nr.get(rnd2);
nr.remove(index);
index = r.nextInt(nr.size());
int result3 = nr.get(rnd3);
nr.remove(index);