Reversely query the List in C# LINQ

Question

List<List<double>> Return(List<double> vector, int Z, int firstidx)
{            
    return vector.Reverse()
                 .Skip(firstidx)
                 .Take(Z)
                 .Select(i => vector.Reverse().Select(j => j != 0? i / j : 0.0).ToList())
                 .ToList();
}

I want to reversely query the List but there is some error in the .Reverse() and it said that:

Operator '.' cannot be applied oprand of type 'void'`.

Even I create a intermediate variable List<double> Reversevector = vector.Reverse().ToList();

So what the correct way to use .Reverse() in linq?

Hari Prasad · Accepted Answer · 2016-08-23T06:23:43.113

9

Problem is you are using List.Reverse() method not the Enumerable.Reverse()

you have two options, either to call it as static method or explicit casting.

Enumerable.Reverse(vector)
          .Skip(firstidx)
           .Take(Z)
           .Select(i => Enumerable.Reverse(vector).Select(j => j != 0? i / j : 0.0).ToList())
           .ToList();

edited Aug 23 '16 at 06:23

answered Aug 23 '16 at 06:15

Hari Prasad

16,716
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score 5 · Answer 2 · edited May 23 '17 at 12:23

5

Use it like this, as List<T>.Reverse() doesn't return a new list:

vector.Reverse();
return vector.Skip(firstidx)
             .Take(Z)
             .Select(i => vector.Select(j => j != 0? i / j : 0.0).ToList())
             .ToList();

edited May 23 '17 at 12:23

Community

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answered Aug 23 '16 at 06:14

diiN__________

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2

Please correct `.Select(i => vector.Reverse().Select(j => j != 0? i / j : 0.0).ToList())`. As you said it does not returns `List`. – Adil Mammadov Aug 23 '16 at 06:16
@AdilMammadov Oh, didn't see it that he used it twice. – diiN__________ Aug 23 '16 at 06:17

score 0 · Answer 3 · answered Aug 23 '16 at 06:17

Copy the list to a local variable, or create an extension method similar to one below and modify your code.

public static IEnumerable<T> Reverse<T>(this IEnumerable<T> source)
{
   var reversedList = source.ToList();
   reversedList.Reverse();
   return reversedList;
}

Reversely query the List in C# LINQ

3 Answers3