Randomly select n items from a map

Question

I am trying to randomly generate 'n' number of items from a HashMap where 'n' is determined by the user.

Here is what I have so far:

public static void main(String []args){
    int numColors = 3;
    HashMap<String, String> map = new HashMap<String, String>();
    map.put("White","FFFFFF");
    map.put("Blank","000000");
    map.put("Red","ED0A15");
    map.put("Green","06F76C");
    map.put("Blue","0689FF");
    map.put("Sky Blue","00C2FC");
    map.put("Light Blue","08F0FC");
    map.put("Silver","C0BFC5");
    map.put("Mint","ABD3CA");
    map.put("Off White","FFEFF0");
    map.put("Purple","736FFA");
    map.put("Lavendar","DEBEEF");
    map.put("Hot Pink","F5159A");
    map.put("Pink","DB39CC");
    map.put("Light Pink","F5C2E3");
    map.put("Blush","C95FA7");
    map.put("Orange","D4361B");
    map.put("Yellow","DEF231");
    map.put("Warm White","F3E4C3");
    map.put("Turquoise","01DCA4");

    List<String> valuesList = new ArrayList<String>(map.values());
    int randomIndex = new Random().nextInt(valuesList.size());
    String randomValue = valuesList.get(randomIndex);

    System.out.printf(randomValue);
}

It prints 1 random color for me (in hex) which I want, however I am unsure of how/which loop to use in order to generate say 3 random hex colors from the map. I declared numColors as 3 just to try and test this out.

Here is what I ended up going with:

public static void main(String []args){
    int numColors = 3;
    HashMap<String, String> map = new HashMap<String, String>();
    map.put("White","FFFFFF");
    map.put("Blank","000000");
    map.put("Red","ED0A15");
    map.put("Green","06F76C");
    map.put("Blue","0689FF");
    map.put("Sky Blue","00C2FC");
    map.put("Light Blue","08F0FC");
    map.put("Silver","C0BFC5");
    map.put("Mint","ABD3CA");
    map.put("Off White","FFEFF0");
    map.put("Purple","736FFA");
    map.put("Lavendar","DEBEEF");
    map.put("Hot Pink","F5159A");
    map.put("Pink","DB39CC");
    map.put("Light Pink","F5C2E3");
    map.put("Blush","C95FA7");
    map.put("Orange","D4361B");
    map.put("Yellow","DEF231");
    map.put("Warm White","F3E4C3");
    map.put("Turquoise","01DCA4");

    List<String> keys = new ArrayList<String>(map.keySet());
    Random rand = new Random();
    for (int i = 0; i < numColors; i++) {
        String key = keys.get(rand.nextInt(keys.size()));
        System.out.println(map.get(key));
        }

 }

Make a single instance of `Random`, and then use a for loop to run the random selection multiple times. — Mike Precup, Aug 06 '14 at 18:45
I want 3 random colors, so it could duplicate, it might not. — user3130128, Aug 06 '14 at 18:46
I am having trouble trying to comprehend how the syntax would be. As a for-loop is setup like for ( {initialization}; {exit condition}; {incrementor} ) code_block; — user3130128, Aug 06 '14 at 18:49
You basically want to select a random subset of `map.values()`. So the answer should be covered over here: http://stackoverflow.com/questions/136474/best-way-to-pick-a-random-subset-from-a-collection — aioobe, Aug 06 '14 at 18:58

Malt · Answer 1 · 2014-08-06T20:41:46.703

2

A simple solution is to shuffle the entire map using Collections.shuffle(map). Then just iterating over it and picking the first n elements.

Of course this doesn't make sense if the map is huge and you only need a couple of elements.

Edit:

Naturally, with this solution you won't get any duplicate entries

edited Aug 06 '14 at 20:41

answered Aug 06 '14 at 18:54

Malt

28,965
9
65
105

In my code example, that is all the map elements so it is not very large. – user3130128 Aug 06 '14 at 18:55
It'd probably be good to note that this prevents duplicate results. – Mike Precup Aug 06 '14 at 18:56

score 1 · Accepted Answer · answered Aug 06 '14 at 18:51

1

If I understand your question, you could do it with

List<String> keys = new ArrayList<String>(map.keySet());
Random rand = new Random();
for (int i = 0; i < numColors; i++) {
    String key = keys.get(rand.nextInt(keys.size()));
    System.out.println(map.get(key));
}

answered Aug 06 '14 at 18:51

Elliott Frisch

198,278
20
158
249

Thank you Elliott that worked perfectly. Just a side question, this solution allows for duplicate colors correct? So FFFFFF could show up twice theoretically. – user3130128 Aug 06 '14 at 19:03
Answered my own question, it does allow duplicates. – user3130128 Aug 06 '14 at 19:05

sujithvm · Answer 3 · 2014-08-06T19:04:36.680

Changes are mentioned in comments

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Random;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {

        int numColors = 3;
        HashMap<String, String> map = new HashMap<String, String>();
        map.put("White", "FFFFFF");
        map.put("Blank", "000000");
        map.put("Red", "ED0A15");
        map.put("Green", "06F76C");
        map.put("Blue", "0689FF");
        map.put("Sky Blue", "00C2FC");
        map.put("Light Blue", "08F0FC");
        map.put("Silver", "C0BFC5");
        map.put("Mint", "ABD3CA");
        map.put("Off White", "FFEFF0");
        map.put("Purple", "736FFA");
        map.put("Lavendar", "DEBEEF");
        map.put("Hot Pink", "F5159A");
        map.put("Pink", "DB39CC");
        map.put("Light Pink", "F5C2E3");
        map.put("Blush", "C95FA7");
        map.put("Orange", "D4361B");
        map.put("Yellow", "DEF231");
        map.put("Warm White", "F3E4C3");
        map.put("Turquoise", "01DCA4");

        // scanner for accepting values
        Scanner scan = new Scanner(System.in);
        System.out.println("Enter number");
        int N = scan.nextInt();

        // random object for generating random values
        Random rand = new Random();

        // converting map values to list
        List<String> valuesList = new ArrayList<String>(map.values());

        for (int i = 1; i <= N; i++) {
            // choose random value
            int randomIndex = rand.nextInt(valuesList.size());
            // get value
            String randomValue = valuesList.get(randomIndex);
            // printing
            System.out.println("Random value " + i + " : " + randomValue);
        }
    }
}

To prevent duplicates you can do something like this :

// random object for generating random values
Random rand = new Random();

// converting map values to list
List<String> valuesList = new ArrayList<String>(map.values());

Set<String> set = new HashSet<String>();

while (set.size() != N) {
    int randomIndex = rand.nextInt(valuesList.size());
    String randomValue = valuesList.get(randomIndex);
    set.add(randomValue);
}

System.out.println(set);

score 0 · Answer 4 · answered Aug 06 '14 at 19:31

0

As Malt suggested, to prevent duplicates and keep code clean:

List<String> list = new ArrayList(map.values() );
Collections.shuffle(list);

answered Aug 06 '14 at 19:31

Jama Djafarov

358
3
11

Randomly select n items from a map

4 Answers4