How do I use First() in LINQ but random?

Question

In a list like this:

var colors = new List<string>{"green", "red", "blue", "black","purple"};

I can get the first value like this:

var color = colors.First(c => c.StartsWidth("b")); //This will return the string with "blue"

Bot how do I do it, if I want want a random value matching the conditions? For example something like this:

Debug.log(colors.RandomFirst(c => c.StartsWidth("b"))) // Prints out black
Debug.log(colors.RandomFirst(c => c.StartsWidth("b"))) // Prints out black
Debug.log(colors.RandomFirst(c => c.StartsWidth("b"))) // Prints out blue
Debug.log(colors.RandomFirst(c => c.StartsWidth("b"))) // Prints out black

As in if there are multiple entries in the list matching the condition, i want to pull one of them randomly. It has (I need it to be) to be an inline solution. Thank you.

Answer 1

Random ordering then:

var rnd = new Random();
var color = colors.Where(c => c.StartsWith("b"))
                  .OrderBy(x => rnd.Next())
                  .First();

The above generates a random number for each element and sorts the results by that number.

You propbably won't notice a random results if you have only 2 elements matching your condition. But you can try the below sample (using the extension method below):

var colors = Enumerable.Range(0, 100).Select(i => "b" + i);

var rnd = new Random();

for (int i = 0; i < 5; i++)
{
    Console.WriteLine(colors.RandomFirst(x => x.StartsWith("b"), rnd));
}

Output:

b23
b73
b27
b11
b8

You can create an extension method out of this called RandomFirst:

public static class MyExtensions
{
    public static T RandomFirst<T>(this IEnumerable<T> source, Func<T, bool> predicate, 
                                                                                Random rnd)
    {
        return source.Where(predicate).OrderBy(i => rnd.Next()).First();
    }
}

Usage:

var rnd = new Random();
var color1 = colors.RandomFirst(x => x.StartsWith("b"), rnd);
var color2 = colors.RandomFirst(x => x.StartsWith("b"), rnd);
var color3 = colors.RandomFirst(x => x.StartsWith("b"), rnd);

Optimization:

If you're worried about performance, you can try this optimized method (cuts the time to half for large lists):

public static T RandomFirstOptimized<T>(this IEnumerable<T> source, 
                                        Func<T, bool> predicate, Random rnd)
{
    var matching = source.Where(predicate);

    int matchCount = matching.Count();
    if (matchCount == 0)
        matching.First(); // force the exception;

    return matching.ElementAt(rnd.Next(0, matchCount));
}

Answer 2

In case you have IList<T> you could also write a tiny extension method to pick a random element:

static class IListExtensions
{
   private static Random _rnd = new Random();

   public static void PickRandom<T>(this IList<T> items) =>
       return items[_rnd.Next(items.Count)];
}

and use it like this:

var color = colors.Where(c => c.StartsWith("b")).ToList().PickRandom();