Prepend one value to a list 'x' number of times

Question

I want to add a value multiple times at the beginning of a list, what I have is...

internal static class Helper
{
  public static void Prepend(this IList<double> source, double value, int count)
  {
    for (var i = 0; i < count; ++i)
    {
      source.Insert( 0, value );
    }
  }
}

Then use it as follow

// create
var mylist = new List(){ ... };  // create the list
mylist.Prepend( 0, 10 );  // prepend 0 to the list

But I wonder if there isn't a more efficient way of doing the same thing?

Answer 1

Is there a reason you use IList? because List has the InsertRange method which uses Array.Copy amongst other optimizations.

If we need to support IList, we don't want to depend upon implementation details and IList.Insert sounds like your best option.

Another approach is you could check whether the actual implementation is List and use the InsertRange method or fallback to Insert in a loop.

Example of adding the items using InsertRange

var items = Enumerable.Repeat(0, 10);
source.InsertRange(0, items);

Answer 2

If you don't care about performance the most generic solution would be to use Concat and not to modify the original list at all. Usually you don't want to do it but return a new collection instead.

public static IEnumerable<T> Prepend<T>(this IEnumerable<T> source, T value, int count)
{
    return Enumerable.Repeat(value, count).Concat(source);
}

---

var mylist = new List(){ ... };  // create the list

If you really create a list and prepend more items to it right away in the next step then combinding the the constructor with the collection initializer is another option that might work for you:

var result = new List<int>(Enumerable.Repeat(0, 10)) { 2, 3, 4 };