IEnumerable.Count() O(n)

Question

I just stumbled upon this fragment of code, I was wondering why the Count is done during the loop.

  /// <summary>
  /// find the first index in a sequence to satisfy a condition
  /// </summary>
  /// <typeparam name="T">type of elements in source</typeparam>
  /// <param name="source">sequence of items</param>
  /// <param name="predicate">condition of item to find</param>
  /// <returns>the first index found, or -1 if not found</returns>
  public static int FindIndex<T>(this IEnumerable<T> source, Predicate<T> predicate)
  {
         for (int i = 0; i < source.Count(); i++)
         {
               if (predicate(source.ElementAt(i))) return i;
         }
         return -1; // Not found
  }

If the count can change, shouldn’t we just do it like this instead: for (int i = source.Count() - 1; i >= 0; i--)

Otherwise, I think we should calculate the count before the looping starts, instead of every time.

What would be the correct way to do this?

Answer 1

The correct way to write manual code for this would be to lose all that rubbish and simply iterate with foreach:

public static int FindIndex<T>(this IEnumerable<T> source, Predicate<T> predicate) 
{
    var index = 0;
    foreach(var item in source) {
        if(predicate(item)) {
            return index;
        }
        ++index;
    }

    return -1;
}