LINQ .select之后返回List &lt;{int，double}&gt;两个值，但我需要List <int> 只有一个值

Question

I have got this assignment. I need to create method which works with JSON data in this form:

On input N, what is top N of movies? The score of a movie is its average rate

So I have a JSONfile with 5 mil. movies inside. Each row looks like this:

{ Reviewer:1, Movie:1535440, Grade:1, Date:'2005-08-18'},
{ Reviewer:1, Movie:1666666, Grade:2, Date:'2006-09-20'},
{ Reviewer:2, Movie:1535440, Grade:3, Date:'2008-05-10'},
{ Reviewer:3, Movie:1535440, Grade:5, Date:'2008-05-11'},

This file is deserialized and then saved as a IEnumerable. And then I wanted to create a method, which returns List<int> where int is MovieId. Movies in the list are ordered descending and the amount of "top" movies is specified as a parameter of the method.

My method looks like this:

public List<int> GetSpecificAmountOfBestMovies(int amountOfMovies)
{
    var moviesAndAverageGradeSortedList = _deserializator.RatingCollection()
        .GroupBy(movieId => movieId.Movie)
        .Select(group => new
        {
            Key = group.Key, 
            Average = group.Average(g => g.Grade)
        })
        .OrderByDescending(a => a.Average)
        .Take(amountOfMovies)
        .ToList();

    var moviesSortedList = new List<int>();
    foreach (var movie in moviesAndAverageGradeSortedList)
    {
        var key = movie.Key;
        moviesSortedList.Add(key);
    }
    return moviesSortedList;
}

So moviesAndAverageGradeSortedList returns List<{int,double}> because of the .select method. So I could not return this value as this method is type of List<int> because I want only movieIds not their average grades.

So I created a new List<int> and then foreach loop which go through the moviesAndAverageGradeSortedList and saves only Keys from that List.

I think this solution is not correct because foreach loop can be then very slow when I put big number as a parameter. Does somebody know, how can I get "Keys" ( movieIds ) from the first list and therefore avoid creating another List<int> and foreach loop?

I will be thankful for every solution.

Answer 1

You can avoid the second list creation by just adding another .Select after the ordering. Also to make it all a bit cleaner you could:

return _deserializator.RatingCollection()
            .GroupBy(i => i.Movie)
            .OrderByDescending(g => g.Average(i => i.Grade))
            .Select(g => g.Key)
            .Take(amountOfMovies)
            .ToList();

Note that this won't really improve performance much (if at all) because even in your original implementation the creation of the second list is done only on the subset of the first n items. The expensive operations are the ordering by the averages of the group and that you want to perform on all items in the json file, regardless to the number of item you want to return

Answer 2

You could add another select after you have ordered the list by average

var moviesAndAverageGradeSortedList = _deserializator.RatingCollection()
            .GroupBy(movieId => movieId.Movie)
            .Select(group => new
            {
                Key = group.Key, 
                Average = group.Average(g => g.Grade)
            })
            .OrderByDescending(a => a.Average)
            .Take(amountOfMovies)
            .Select(s=> s.Key)
            .ToList();