Saturday, 16 May 2015

How to Get Random Items from Array or List in C#

The simple way to get random item from an Array is to use the return value from, array.length) as index to get value from the array.

var randomIndex = random.Next(0, Array.Length);
The downside of the above code is it might return you item multiple times (repetition) as we don't keep track of items that we are getting from the source Array.

The easy approach is to consider Array as a deck of cards. We want the items to be 'shuffled' similar to a deck of cards, meaning avoiding any repetition. So we will use  a List<> for the source items, grab them at random and push them to a Stack<> to create the deck of items.

You can create a Stack from anything that is IEnumerable
var stack = new Stack(myList);

However, the stack constructor will be using a loop internally, you just don't see it. So for understanding the purpose I will use an example to create Stack with loop.

public static Stack CreateShuffledDeck(IEnumerable values) {

var random = new Random();  var list = new List(values); var stack = new Stack();  while (list.Count > 0) {  // Get the next item at random. var randomIndex = random .Next(0, list.Count); var randomItem = list[randomIndex];  // Remove the item from the list and push it to the top of the deck. list.RemoveAt(randomIndex); stack.Push(randomItem ); }  return stack; } 
Now we have a solution to create a Shuffled Deck. We can now get random items out using Stack.Pop method . Popping something from the stack means "taking the top 'thing'" off the stack.

public static string[] RandomArrayEntries(string[] arrayItems, int count) {
var listToReturn = new List();

if (arrayItems.Length != count) {
var deck = CreateShuffledDeck(arrayItems);

for (var i = 0; i < count; i++) {
var arrayItems= deck.Pop();
listToReturn .Add(item);

return listToReturn .ToArray();

return arrayItems;
We can execute the above code as following:

var countriesArray = new string[] { "Sweden", "Pakistan", "United Kingdom", "Denmark", "Norway", "Finland" };

var newRandomAraay = RandomArrayEntries(countriesArray, 3);