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Accepted
J. Rodríguez
Asked: 2020-11-22 07:30:30 +0800 CST 2020-11-22 07:30:30 +0800 CST

"Ilist", "IEnumerable" does not contain a definition for "Skip"

  • 772

Recently I have found it necessary to find the missing numbers in a number sequence. Investigating this I have solved it in the following way:

 List<int> myLista = new List<int>() { 1, 2, 3, 5, 8, 9 };
 int primero = myLista.OrderBy(x => x).First();
 int ultimo = myLista.OrderBy(x => x).Last();
 var resultado = Enumerable.Range(primero, ultimo - primero + 1).Except(list);

But now I also need to know exactly where the sequence of numbers is interrupted? , I was doing some research and found the following page dealing with this topic: Find Missing Values ​​in a Sequence of Numbers and Other Sequence Related lists as IEnumerable Extensions Here's a method that does exactly what I want, it's called: SequenceReportMissingsBreakStarts.

But all the methods are creating "queries" with significant side effects, this is why I decided to take the example shown in the comments suggested by another user. But I can't get this to work for me:

private IEnumerable SequenceFindMissings(IEnumerable sequence)
{
    return sequence.Zip(sequence.Skip(1), (a, b) => Enumerable.Range(a + 1, (b - a) - 1))
                       .SelectMany(s => s);
}

private bool IsSequenceBroken(IEnumerable sequence)
{
    return sequence.Zip(sequence.Skip(1), (a, b) => b - a)
                       .Any(v => v != 1);
}

private IEnumerable SequenceReportMissingsBreakStarts(IList sequence)
{
    return sequence.Zip(sequence.Skip(1), (a, b) => new { b, a })
                       .Where(v => v.b - v.a > 1)
                       .Select(v => v.a);
}

I get the error: Ilist, IEnumerabledoes not contain a definition for Skipand I have added the references:

using System.Linq;

using System.Collections;

using System.Collections.Generic;

Why can't I use "Skip" in these methods, how should I modify them to make it work? , or any suggestions without having to use queries?

Environment: Visual Studio 2017 & .NetFramework 4.5.2

c#

1 Answers

  1. Best Answer

    In principle you must take into account that you are passing parameters to your methods through Interfaces ( IEnumerable, IList).

    The problem is that you must indicate the type of object that these "Generic Lists" will contain in order to later be able to use LinQ ( Skip()).

    In your case they would be "generic lists" of type int( IEnumerable<int>), the code would be like this:

        private IEnumerable<int> SequenceFindMissings(IEnumerable<int> sequence)
    {
        return sequence.Zip(sequence.Skip(1), (a, b) => Enumerable.Range(a + 1, (b - a) - 1))
                           .SelectMany(s => s);
    }

    private bool IsSequenceBroken(IEnumerable<int> sequence)
    {
        return sequence.Zip(sequence.Skip(1), (a, b) => b - a)
                           .Any(v => v != 1);
    }

    private IEnumerable<int> SequenceReportMissingsBreakStarts(IList<int> sequence)
    {
        return sequence.Zip(sequence.Skip(1), (a, b) => new { b, a })
                           .Where(v => v.b - v.a > 1)
                           .Select(v => v.a);
    }
    • 3