Lets say I have an array of integers. I found out that I can randomize the order of the elements simply by doing:
Random rnd = new Random();
array = array.OrderBy(x => rnd.Next()).ToArray();
But lets say I want to keep the first and the last elements in their original place. Can I do it using the same approach (using OrderBy()) or do I need to re-think my situation?
You can't do that in a single expression, but maybe a List<int>
could help:
Random rnd = new Random();
var list = new List<int>();
list.Add(array[0]);
var partialArray = array.Skip(1).Take(array.Length - 2);
list.AddRange(partialArray.OrderBy(x => rnd.Next()));
list.Add(array[array.Length -1 ]);
Of course you can use the same approach, but you have to take care of the first and the last value. Just an example for your input array
:
var list = array.Skip(1).Take(array.Length - 2).OrderBy(x => rnd.Next()).ToList();
list.Insert(0, array.First());
list.Add(array.Last());
array = list.ToArray();
Dmitry's example is basically the same, but gives you the option to keep more elements.
You can do it:
int[] ints = new[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };
ints = ints.Select((x, index) => new { Value = x, Index = index })
.OrderBy(tuple => (tuple.Index >= start && tuple.Index <= stop) ? random.Next(start, stop) : tuple.Index)
.Select(tuple => tuple.Value)
.ToArray();
You can just use a standard shuffle algorithm modified to use a range, for example (using the Fisher-Yates algorithm ):
public static void Shuffle<T>(IList<T> array, Random rng, int start, int end)
{
for (int n = end+1; n > start+1;)
{
int k = rng.Next(start, n);
--n;
T temp = array[n];
array[n] = array[k];
array[k] = temp;
}
}
Then call it like this:
Random rng = new Random();
for (int i = 0; i < 100; ++i)
{
var array = Enumerable.Range(1, 12).ToArray();
Shuffle(array, rng, 3, 9);
Console.WriteLine(string.Join(", ", array));
}
A slightly different approach which doesn't involve using List....
Random rnd = new Random();
//array = array.OrderBy(x => rnd.Next()).ToArray();
int lastIndexToChange = array.Length - 1;
for (int i = 1; i < lastIndexToChange; i++)
{
var tempStore = array[i];
int newPosition = rnd.Next(1, lastIndexToChange);
array[i] = array[newPosition];
array[newPosition] = tempStore;
}
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