randomly remove element from array java

Question

I am removing elements from an array in the below code. In this particular code I am removing the element at pos 2. How would I go about removing a random element in this array?

public class QuestionOneA2 {

public static void main(String[] args) {
    int size = 5;
    int pos = 2;

    String[] countries = {"Brazil", "France", "Germany", "Canada", "Italy", "England"};

        for (int i = 0; i < size; i++) {
            if(i == pos) {
                countries[i] = countries[size];
            }
            System.out.println(countries[i]);
        }   
    }
}

use e.g. int size = countries.length - 1 or just countries.length - 1 instead of a fixed size; otherwise you're doing it wrong. — user719662
– user719662, Commented Feb 7, 2016 at 13:53

Diogo Rocha · Accepted Answer · 2016-02-07 13:27:38Z

1

Random r = new Random();
int result = r.nextInt(size);
//and select/remove countries[result]

This give you a pseudo-random number between 0 and 5 (exclusive). Be careful with your size variable, I think it's not well defined.

answered Feb 7, 2016 at 13:27

Diogo Rocha

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Idos · Accepted Answer · 2016-02-07 14:22:25Z

1

Remove this element:

int randomLocation = new Random().nextInt(countries.length);
// countries[randomLocation]  <--- this is the "random" element.

Or in 1 line:

countries[(new Random()).nextInt(countries.length)];

So in order to actually remove the element you can use ArrayUtils: First import these

import java.util.Arrays;
import org.apache.commons.lang.ArrayUtils;

And then:

countries = ArrayUtils.removeElement(countries, countries[(new Random()).nextInt(countries.length)]);

If you really don't want to use ArrayUtils then you can use:

List<String> list = new ArrayList<String>(Arrays.asList(countries));
list.removeAll(Arrays.asList(countries[(new Random()).nextInt(countries.length)]));
countries = list.toArray(countries);

edited Feb 7, 2016 at 14:22

answered Feb 7, 2016 at 13:27

Idos

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7 Comments

StudentCoder Over a year ago

What do I put inside my for loop if I do this?

Idos Over a year ago

@StudentCoder I edited my answer with a clearer command to use (delete your for loop)

StudentCoder Over a year ago

Thanks, I am actually trying to solve this question using a for loop though

Idos Over a year ago

There is no use for a for loop in this program it's a waste of space and time.

StudentCoder Over a year ago

This is the first part of a 2 part question I am trying to do which will then ask the user to input the missing country and I was instructed to use a for loop, am I looking at his completely the wrong way?

|

ungive · Accepted Answer · 2021-03-09 16:04:42Z

0

In case you do not mind the order of the elements, you can also achieve this behaviour in constant time:

public <E> E removeRandom(List<E> list, Random random) {
    if (list.isEmpty())
        throw new IllegalArgumentException();

    int index = random.nextInt(list.size());
    int lastIndex = list.size() - 1;

    E element = list.get(index);
    list.set(index, list.get(lastIndex));
    list.remove(lastIndex);

    return element;
}

answered Mar 9, 2021 at 16:04

ungive

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