使用LINQ在组内排序

Question

Given the following data:

List<Country> countries = new List<Country>();

countries.Add(new Country{ Name = "USA",     population=500000,  Year=2012 });
countries.Add(new Country{ Name = "USA",     population=300000,  Year=2002 });
countries.Add(new Country{ Name = "USA",     population=250000,  Year=1992 });
countries.Add(new Country{ Name = "USA",     population=20000,   Year=1982 });

countries.Add(new Country{ Name = "India",   population=1500000, Year=2012 });
countries.Add(new Country{ Name = "India",   population=1000000, Year=2002 });
countries.Add(new Country{ Name = "India",   population=50000,   Year=1982 });
countries.Add(new Country{ Name = "India",   population=80000,   Year=1992 });

countries.Add(new Country{ Name = "Germany", population=100000,  Year=2012 });
countries.Add(new Country{ Name = "Germany", population=400000,  Year=2002 });
countries.Add(new Country{ Name = "Germany", population=60000,   Year=1992 });
countries.Add(new Country{ Name = "Germany", population=4000,    Year=1982 });

countries.Add(new Country{ Name = "UK",      population=450000,  Year=2002 });
countries.Add(new Country{ Name = "UK",      population=50000,   Year=1992 });
countries.Add(new Country{ Name = "UK",      population=3000,    Year=1982 });  

I want to order the countries by the largest population for a given year, but then display all the years for that country before moving on to the next country.

Eg

1. 2012 - the population order would be India, USA, UK, Germany. So I would like the data to be ordered by all India data, all USA data, all UK data and then Germany.

2. 2002 - the population order would be India, USA, Germany and then UK. UK is last because it has no 2002 data.

I want to achieve this using LINQ, although I've used LINQ in the past I'm struggling to get my head around this. Any help would be much appreciated.

Answer 1

You need to group the data by country and then order by population for a particular year;

var year = 2002;    

var orderedCountries = countries
                        .GroupBy(c => c.Name)
                            .OrderByDescending(c => c.Where(y => y.Year == year).Select(p => p.population).FirstOrDefault())
                                .SelectMany(c=>c)  
                                .ToList();

The above code will work even if there is no data for a given year

That can be a bit much to read so you could separate out the logic in the orderby into a delegate eg;

var orderedCountries = countries
                            .GroupBy(c => c.Name)
                                .OrderByDescending(c => GetPop(c, year))
                                    .SelectMany(c=>c)  
                                        .ToList();

Func<IGrouping<string, Country>, int, long> GetPop = 
                                    ((cntry, yr) => (from c in cntry
                                                     where c.Year == yr
                                                     select c.population).First());

Answer 2

This is quite equivalent to saj's answer, but using query syntax:

var y = 2002;

var ordered = 
    from c in countries
    group c by c.Name into g
    select new
    {
        Name = g.Key,
        Population = (
            from x in g
            where x.Year == y
            select x.population).FirstOrDefault()
    } into s
    orderby s.Population descending
    join f in countries
    on s.Name equals f.Name
    select f;

Answer 3

There is a similar question asked here: Multiple "order by" in LINQ

Something like this would do the trick in your case. I tested it in VS and it works as expected:

var countries order= countries.OrderBy(c => c.Year).ThenBy(n => n.population);