select randomly from string array without repetitions

Question

Good day I have some problem regarding selecting a random string from my string array I am currently developing a guessing word game. this is my string array:

 string[] movie = {"deadpool", "batmanvssuperman", "findingdory", "titanic", "suicidesquad", "lordoftherings", "harrypotter", "jurassicpark", "hungergames", "despicableme" };

while this is the process in selecting a random string to my array, what should i do next, because I want to select the string not repeated. e.g when the program starts it will select a string then when i select random string again i want to not select the previous word that i've already selected previously.

string word = movie[r.Next(0, movie.Length)].ToUpper();

Thank you for response! Have a nice day.

The classical approach is to create a shuffled array, rather than pick at random from the original array., — unwind
– unwind, Commented Sep 22, 2016 at 14:36

Fabjan · Accepted Answer · 2016-09-23 08:44:28Z

2

Well, simply convert your array to list and shuffle it in random order :

        var rand = new Random();
        string[] movies = { "deadpool", "batmanvssuperman", "findingdory", "titanic", "suicidesquad", "lordoftherings", "harrypotter", "jurassicpark", "hungergames", "despicableme" };
        List<string> randomMovies = movies.ToList();

        for (int i = 0; i < movies.Length / 2; i++)
        {
            var randNum = rand.Next(i, randomMovies.Count);
            var temp = randomMovies[randNum];
            randomMovies[randNum] = randomMovies[i];
            randomMovies[i] = temp;
        }

Then you can just take random elements by :

var randomMovie = randomMovies.First(); 
randomMovies.Remove(randomMovie); // either remove it or use loop to iterate through the list

I sort of like to use Queue collection here :

var moviesQueue = new Queue<string>(randomMovies);    

while (moviewQueue.Count > 0)
{
    Console.WriteLine(moviewQueue.Dequeue());
}

P.S. As suggested you don't really need to delete elements from randomMovie, you can save last used index in some field and use it later;

var lastIndex = 0;
var randomMovie = randomMovies[lastIndex++];

edited Sep 23, 2016 at 8:44

answered Sep 22, 2016 at 14:46

Fabjan

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3 Comments

Nicholaus Lawson Over a year ago

This has the side effect or destroying your list once you are finished.

Fabjan Over a year ago

But you can always copy list if you need with help of randomMovies .ToList() method

u8it Over a year ago

Best answer, but you don't need to remove elements. You could just keep an index pointer around, something like int CurrentMovieIdx.

Brandon · Accepted Answer · 2016-09-22 14:35:15Z

1

Just loop if it's been selected. This is untested code:

private string _last;
private string GetNonRepeatedMovie()
{
    string selected = "";
    do
    {
        selected = movie[r.Next(0, movie.Length)].ToUpper();
    }
    while (selected == this._last);

    this._last = selected;
    return selected;    
}

This should work to select the initial string as well when the application starts.

answered Sep 22, 2016 at 14:35

Brandon

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6 Comments

Riley Over a year ago

With this method there is a very small chance that you could loop for an unacceptably long time before selecting something else. I wouldn't worry about that though.

Nicholaus Lawson Over a year ago

This will make sure the current choice isn't the last, but has no ability to make sure each choice is unique.

Tim Schmelter Over a year ago

Its more likely that you win a million in a lottery than that the user will notice the method-call at all. You just need to handle the case that there is none or only one movy ;-)

Tim Schmelter Over a year ago

@NicholausLawson: true, but it seems that OP only wants to avoid that a movie is repeated immediately. Then this is the most efficient and easiest approach:

Nicholaus Lawson Over a year ago

@TimSchmelter yeah, very possibly. Just saying 'previously selected' could go either way lol. Just pointing out. incase they meant selection without replacement or just avoid drawing the same back-to-back

|

Nicholaus Lawson · Accepted Answer · 2016-09-22 14:48:19Z

If you need to keep a memory, convert your list to be a class that contains the name and a field of whether it has been chosen or not.

If you go through all of them, turn this semiphor off and begin again.

class GuessingName
{
  public GuessingName(string name){Name = name;}

  public string Name;
  public bool chosen;
}

class RandomNamePicker{
 private List<GuessingName> names;

 public RandomNamePicker(){
  names = new List<GuessingName>();
  names.Add(new GuessingName("movie"));
 }

 string RandomPicker(){

  if(names.All(c=>c.chosen))
    names.ForEach(c=>c.chosen=false);

  int r1 = r.Next(0, names.Length);

  while(names[r1].chosen){
   r1= r.Next(0,names.Length);
  }
  return names[r1].Name;
 }
}

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