Select multiple/random elements from an arraylist

Question

How would I select random elements from an array list, except for one element? Here's my arraylist:

ArrayList <String> provinces = new ArrayList();

Collections.addAll(
    provinces, 
    "New Brunswick", 
    "Saskatchewan", 
    "Ontario", 
    "Nova Scotia", 
    "Quebec", 
    "Alberta");

For this example, I would like to select at random the other elements, except for Saskatchewan.

I've tried doing:

 for(int i == provinces.get (0); i < provinces.get(1); i > provinces.get(2); i < provinces.get(5)) {

     int getPossibleAnswers = (int) Math.floor(Math.random()*i);               
     String displayPossibleAnswers = provinces.get(getPossibleAnswers);                
     outputAnswers.append(displayPossibleAnswers + "\n");
     }

Obviously, this code doesn't work, and I don't know what to do. Thanks in advance!

why don't you simply create a list without Saskatchewan and apply Collections.shuffle on it — Pavneet_Singh
– Pavneet_Singh, Commented Jun 8, 2017 at 17:14

Andreas · Accepted Answer · 2017-06-08 17:18:36Z

4

Build a list of all the index values, except for the index to Saskatchewan, then shuffle that.

List<String> provinces = Arrays.asList("New Brunswick", "Saskatchewan", "Ontario", "Nova Scotia", "Quebec", "Alberta");

List<Integer> indexes = new ArrayList<>();
for (int i = 0; i < provinces.size(); i++)
    if (! provinces.get(i).equals("Saskatchewan"))
        indexes.add(i);
Collections.shuffle(indexes);

StringBuilder outputAnswers = new StringBuilder();
for (int i : indexes)
    outputAnswers.append(provinces.get(i) + "\n");

System.out.print(outputAnswers);

Sample Output

Nova Scotia
Quebec
Ontario
Alberta
New Brunswick

edited Jun 8, 2017 at 17:18

answered Jun 8, 2017 at 17:14

Andreas

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3 Comments

Pendle S Over a year ago

Sorry, sort of new to Java, what does the colon (:) do?

Andreas Over a year ago

@PendleS That is the enhanced for loop, also known as the foreach loop. See: What is the syntax of enhanced for loop in Java?

Pendle S Over a year ago

Thanks you for the answers

Pavneet_Singh · Accepted Answer · 2017-06-08 17:35:17Z

To keep it concise , As mentioned Simply create a list without Saskatchewan

    ArrayList <String> provinces = new ArrayList<>(Arrays.asList("New Brunswick",
                                     "Ontario", "Nova Scotia",
                                     "Quebec",   "Alberta"));

then shuffle the list

Collections.shuffle(provinces);

Demo

ArrayList <String> provinces = new ArrayList<>(Arrays.asList("New Brunswick",
                                     "Ontario", "Nova Scotia",
                                     "Quebec",   "Alberta"));
Collections.shuffle(provinces);
StringBuilder builder = new StringBuilder();
for (String s:provinces) {
    builder.append(s+"\n");
}
System.out.println(builder);

Output :

Quebec
New Brunswick
Alberta
Ontario
Nova Scotia

Stanislav Bashkyrtsev · Accepted Answer · 2017-06-08 18:17:20Z

0

If you need randomization often (e.g. for randomized testing), you can add a Qala Datagen dependency and then get a random element like this:

String e = sample(provinces);

Though you'd still need to remove extra element from the collection.

Disclaimer: I'm the author of the lib.

answered Jun 8, 2017 at 18:17

Stanislav Bashkyrtsev

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Comments

Blasanka · Accepted Answer · 2017-08-16 13:18:55Z

ArrayList <String> provinces = new ArrayList<>();
String removeValue = null;
Collections.addAll(
    provinces, 
    "New Brunswick", 
    "Saskatchewan", 
    "Ontario", 
    "Nova Scotia", 
    "Quebec", 
    "Alberta");

for (Iterator<String> iterator = provinces.iterator(); iterator.hasNext(); ) {
    String value = iterator.next();
    if (value.equals("Saskatchewan")) {
        removeValue = value;
        iterator.remove();
    }
}

Collections.shuffle(provinces);

for(String p: provinces){
    System.out.println(p);
}

provinces.add(removeValue);

I used Iterator because enhanced for loop did not work with remove() but without Iterator you can also loop using indexes(numbers).

Without Iterator:

for (int j = 0; j<provinces.size(); j++) {
    if(provinces.get(j).equals("Saskatchewan")){
        removeValue = provinces.get(j);
        provinces.remove(provinces.get(j));
    }
}

Also without using Collections.shuffle() you can create your own method. To do that have a look at this question.

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