Can I somehow "instruct" LINQ to use binary search when the collection that I'm trying to search is ordered. I'm using an ObservableCollection<T>
, populated with ordered data, and I'm trying to use Enumerable.First(<Predicate>). In my predicate, I'm filtering by the value of the field my collection's sorted by.
As far as I know, it's not possible with the built-in methods. However it would be relatively easy to write an extension method that would allow you to write something like that :
var item = myCollection.BinarySearch(i => i.Id, 42);
(assuming, of course, that you collection implements IList ; there's no way to perform a binary search if you can't access the items randomly)
Here's a sample implementation :
public static T BinarySearch<T, TKey>(this IList<T> list, Func<T, TKey> keySelector, TKey key)
where TKey : IComparable<TKey>
{
if (list.Count == 0)
throw new InvalidOperationException("Item not found");
int min = 0;
int max = list.Count;
while (min < max)
{
int mid = min + ((max - min) / 2);
T midItem = list[mid];
TKey midKey = keySelector(midItem);
int comp = midKey.CompareTo(key);
if (comp < 0)
{
min = mid + 1;
}
else if (comp > 0)
{
max = mid - 1;
}
else
{
return midItem;
}
}
if (min == max &&
min < list.Count &&
keySelector(list[min]).CompareTo(key) == 0)
{
return list[min];
}
throw new InvalidOperationException("Item not found");
}
(not tested... a few adjustments might be necessary) Now tested and fixed ;)
The fact that it throws an InvalidOperationException
may seem strange, but that's what Enumerable.First
does when there's no matching item.