Displaying strings from Array without repeats

Question

I working on a project in c# asp.net. I would like to randomly display a string from either an array or list, if certain conditions are met, I would like to display another random string that has not been displayed.

EX.

 LIST<string> myString = new List<string> {"one", "two", "three", "four", "five"};

or

string[] myString = new[] {"one", "two", "three", "four", "five"};

void btnAnswer_Click(Object sender, EventArgs e)
{
string Next = myString[random.Next(myString.Length)];
if(my condition is met)
{
lbl.Text = Next;
}
}

I can randomly call a string from my list or array, but not sure how to store repeat results. I've tried using an algorithm that jumbles the array and then a counter for the index, but the counter (counter++) seems to add 1 once and then stops. I've tried using a list and removing the string I use, but it seems to repeat after all strings have been used.

If more code is needed I can provide, just need a point in the right direction.

Answer 1

Couple of options:

1. Have an array or list which is a copy of the master array, and whenever you choose one, remove it from the array. Once the array is empty, refresh it from the master array.

2. Similar, just create your array, shuffle it, and start giving out strings by order. so array[0], next would be array[1], and so on, once you reach the end, shuffle and start again.

There are probably others as well :)

---

Edit:
If I understand correctly from the comments, you are talking about the option where the input is not unique to begin with. If that is the case, you can create your list with a simple linq query to get only unique values (using distinct), guaranteeing no duplicates.

Have a look at this question for an example.

Answer 2

You can copy the indices into a list, it will save memory while allow us to track all the remaining items:

var indices = Enumerable.Range(0,myString.Count).ToList();
//define some method to get next index
public int? NextIndex(){
  if(indices.Count == 0) return null;
  int i = random.Next(indices.Count);
  int k = indices[i];
  indices.RemoveAt(i);
  return k;
}
if(my condition is met) {
 int? nextIndex = NextIndex();
 lbl.Text = nextIndex == null ? "" : myString[nextIndex.Value];
}

Note that the Text is set to empty if there won't be no more remaining string, however you can handle that case yourself in another way such as keep the text unchanged.

Answer 3

One option is to display one of the random string and store it in a ViewState.

When you come next time again, check whether new random string is already there in ViewState or not. If it is there in ViewState then get another random string.

Answer 4

You could probably do that, but why not get a unique list of strings first?

var uniqueStrings = myString.Distinct().ToList();

Then as you select strings, do a .Remove() on the last randomly selected value from uniqueStrings.

You said that:

I've tried using a list and removing the string I use, but it seems to repeat after all strings have been used.

The problem here is using the same instance of random for your series after you've run out. If you re-instantiate random = new Random(), the variable is re-seeded and you will have totally different results from what was generated before.