At the moment I use List<int> ints = tuple.Item2.Select(s => s.Value).ToList() but this looks inefficient when tuple.Item2 has 1000's of items. Any better way to achieve this? except using a for loop.
2 Answers
The built-in way to convert each element in one List<T1> and store the result in another List<T2> is List<T1>.ConvertAll.
List<int> ints = tuple.Item2.ConvertAll(s => s.Value);
Unlike .Select(...).ToList() or .Cast(...).ToList(), this method knows the list size in advance, and prevents unnecessary reallocations that .ToList() cannot avoid.
For this to work, tuple.Item2 must really be a List<int?>. It's not an extension method, it cannot work on the generic IEnumerable<int?> interface.
2 Comments
Jeppe Stig Nielsen
It could be that
s => s.GetValueOrDefault() is faster than just s => s.Value. This may seem surprising, but the reason is that .GetValueOrDefault() always and unconditionally exposes the backing field (value), while the .Value property first checks the Boolean field (hasValue) to see if it is true or false, and only then exposes that backing field (or throws if hasValue was false). The Nullable<> struct ensures that the backing field always has its default value in the case where we are modeling a "null", so that is why the behavior of .GetValueOrDefault() is safe.Jeppe Stig Nielsen
You can also write it as
s => s ?? 0.you can simply use Linq Cast<> to achieve this .
List<int> ints = tuple.Item2.Cast<int>();
but if an element cannot be cast to type TResult, this method will throw an exception.you have to consider catching exception.
7 Comments
roman
+1 but what about efficiency? You cannot do better than O(n) with cast anyway, I think. And you have to do
.Where(x => x.HasValue) to check nulls
Mike Zboray
You still need a
ToList() on the end.Mike Zboray
Also this is slower than the original code by a factor of 3 for 200000 items.
Jack
@mikez: Really? It looks better than original. I assume built in extension method would be faster. I will test this.
Behnam Esmaili
@RomanPekar & mike you are right.but i have suggested the way to deceive some one it is efficient :)
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s.HasValue == false...