I'm working on some LINQ GroupBy logic and I can't think how to elegantly and efficiently get this to work.
Basically, I have an IEnumerable<Thing> object (which is in the correct order!), where each Thing object has a RootId property. I want to group these objects on their RootId, which I have working:
IEnumerable<Thing> things; // Already has value assigned
var groups =
(from thing in things
group thing by thing.RootId into thingGroup
select thingGroup.ToArray())
.ToList();
groups is of type List<Thing[]>
Now here is the problem!
The above example is returning 5 items in the list. But, how would I merge 2 of the arrays into 1, leaving 4 items (again, keeping the order of course)??
The reason why is because 2 of the items has different RootId's but I want them to be treated the same i.e. grouped together.
I was going to concat and manipulate the arrays after the LINQ statement, but really it needs to be done as part of the group by/LINQ - any ideas?
Let me know if further examples or information is needed.
Thanks!
The merging criteria will be a manual process, so I was thinking of passing it into the groupby method like so:
var rootIdsToMerge = new List<Tuple<ID, ID>>
{
new Tuple<ID, ID>(rootIdOne, rootIdTwo),
new Tuple<ID, ID>(rootIdThree, rootIdFour)
};
So any group item with a RootId of rootIdOne will be merged with the group item with a RootId of rootIdTwo, and so on.
Since you are not using the grouping Key, you can associate the Item2 from the mapping to Item1 as a RootId key to group by:
var groups =
(from thing in things
group thing by rootIdsToMerge.FirstOrDefault(e => e.Item2 == thing.RootId)?.Item1 ?? thing.RootId
into thingGroup
select thingGroup.ToArray())
.ToList();
Or in pre C#6 (no .? operator):
var groups =
(from thing in things
let mergeWith = rootIdsToMerge.FirstOrDefault(e => e.Item2 == thing.RootId)
group thing by mergeWith != null ? mergeWith.Item1 : thing.RootId
into thingGroup
select thingGroup.ToArray())
.ToList();
Update: If you just want to consolidate a list of RootIds, then you can use a combination of Contains and First:
List<ID> rootIdsToMerge = ...;
var groups =
(from thing in things
group thing by rootIdsToMerge.Contains(thing.RootId) ? rootIdsToMerge.First() : thing.RootId
into thingGroup
select thingGroup.ToArray())
.ToList();
The variants with
List<List<ID>> rootIdsToMerge = ...;
are similar to the initial variant with tuples:
var groups =
(from thing in things
group thing by rootIdsToMerge.FirstOrDefault(ids => ids.Contains(thing.RootId))?.First() ?? thing.RootId
into thingGroup
select thingGroup.ToArray())
.ToList();
or
var groups =
(from thing in things
let mergeList = rootIdsToMerge.FirstOrDefault(ids => ids.Contains(thing.RootId))
group thing by mergeList != null ? mergeList.First() : thing.RootId
into thingGroup
select thingGroup.ToArray())
.ToList();
Related
I have a list of Stores (of type ObservableCollection<Store>) and the Store object has a property called Features ( of type List<Feature> ). and the Feature object has a Name property (of type string).
To recap, a list of Stores that has a list of Features
I have a second collection of DesiredFeatures (of type List<string> ).
I need to use LINQ to give me results of only the stores that have all the DesiredFeatures. So far, I've only been able to come up with a query that gives me an OR result instead of AND.
Here's what that looks like:
var q = Stores.Where(s=> s.Features.Any(f=> DesiredFeatures.Contains(f.name)));
I know Intersect can help, and here's how I've used it:
var q = Stores.Where(s => s.Features.Intersect<Feature>(DesiredFeatures));
This is where I'm stuck, Intersect wants a Feature object, what I need to intersect is on the Feature.Name.
The goal is to end up with an ObservableCollection where each Store has all of the DesiredFeatures.
Thank you!
You've almost done what you need. A small refine would be to swap DesiredFeatures and s.Features.
var q = Stores.Where(s => DesiredFeatures.All(df => s.Features.Contains(df)));
It means take only those stores where desired features are all contained in features of the store.
I need to use LINQ to give me results of only the stores that have all the DesiredFeatures.
In other words, each desired feature must have a matching store feature.
I don't see how Intersect can help in this case. The direct translation of the above criteria to LINQ is like this:
var q = Stores.Where(s =>
DesiredFeatures.All(df => s.Features.Any(f => f.Name == df))
);
A more efficient way could be to use a GroupJoin for performing the match:
var q = Stores.Where(s =>
DesiredFeatures.GroupJoin(s.Features,
df => df, sf => sf.Name, (df, sf) => sf.Any()
).All(match => match)
);
or Except to check for unmatched items:
var q = Stores.Where(s =>
!DesiredFeatures.Except(s.Features.Select(sf => sf.Name)).Any()
);
Going on your intersect idea, the only way I thought of making this work was by using Select to get the Store.Features (List<Feature>) as a list of Feature Names (List<string>) and intersect that with DesiredFeatures.
Updated Answer:
var q = Stores.Where(s => s.Features.Select(f => f.Name).Intersect(DesiredFeatures).Any());
or
var q = Stores.Where(s => DesiredFeatures.Intersect(s.Features.Select(f => f.Name)).Any());
Old Answer (if DesiredFeatures is a List<Feature>):
var q = Stores.Where(s => s.Features.Select(f => f.Name).Intersect(DesiredFeatures.Select(df => df.Name)).Any());
Two things you want your code to perform.
var q = Stores.Where(s=> s.Features.All(f=> DesiredFeatures.Contains(f.name)) &&
s.Features.Count() == DesiredFeatures.Count()); // Incude Distinct in the comparison if Features list is not unique
Ensure that every Feature is DesiredFeature
Store contains all Desired features.
Code above assumes uniqueness in Features collection as well as DesiredFeatures, modify code as stated in comment line if this is not right
I have this List:
string[] countries = {
"USA",
"CANADA"
};
When I run this query :
query = (from user in db where
user.Orders.Any(order => order.Price > 10 &&
countries.Contains(order.DestinationCountry)))
Output is a list of users that have Orders sent to "USA" OR "Canada".
but I want the list of users that have Orders sent to both "USA" AND" "CANADA".
I can do this using below code but i'm searching for a pure linq solution without any ForEach:
foreach (country in countries) {
query = (from user in query where
user.Orders.Any(order => order.Price > 10 &&
order.DestinationCountry == country));
}
Answers:
A. Using .Aggregate()
Generated query is just like For Each.
B.where countries.All(c => user.Orders.Any(o => o.Price > 10 && o.DestinationCountry == c))
When there is no element in Countries List (When I want all users based only on Price parameter), the result is not correct and other parameter is not considered!
Update 1:
I have tried .All() instead of .Contains() before posting and it returns 0 users.
Update 2:
I have updated my question to make it closer to the real problem.
lets say Country is not the only parameter.
Update 3:
Checked some answers and added the result to my question.
So you want a list of the users such that all the countries in the list are present in the set of order destinations?
Logically, that would be:
query = from user in db
where countries.All(c => user.Orders.Any(o => o.DestinationCountry == c))
select ...;
However, I'm not confident that EF will do what you want with that. It's not clear to me what the right SQL query would be to start with - in a simple way, at least.
query =
db.Users.Where(user =>
countries.All(country =>
user.Orders.Any(order =>
order.DestinationCountry == country)))
You can do it like this:
query = (from user in db where
user.Orders
.Where(o => countries.Contains(o.DestinationCountry))
.GroupBy(o => o.DestinationCountry)
.Count() == countries.Count
);
The idea is to keep only the orders going to countries of interest, then group by country, and check that the number of groups equals the number of countries.
It's possible using Enumerable.Aggregate:
query = countries.Aggregate(query,
(q, c) =>
from user in q
where user.Orders.Any(order => order.DestinationCountry == c)
select user);
but really, this is harder to understand than your foreach loop, so I'd just go with that.
Note that although I refer to a member of Enumerable, that member of Enumerable is actually building up an IQueryable<User> query chain just like your foreach loop, so this will not cause the filtering to move to the client.
I want a select the first group in the list. Is there a better way to do this.
Here is how I'm doing it:
var match = (from m in recordList select m).FirstOrDefault();
var matches = recordList.Where(d => d.DateDetails == match.DateDetails);
var lookup = matches.ToLookup(a => a.DateDetails).First();
lookaheadList = lookup.ToList();
I'm selecting the first group and pushing it into a second list called lookaheadlist.
My DATA is as follows:
DateDetails Track Details
0025 ABCD
0025 EFGH
0030 XXXXX
0030 XXXXX
There is no need for the ToLookup. The lookup groups by different DateDetails, but matches is already filtered to a single date, so there is already only one group to select.
You could skip the filter and just go with:
var match = recordList.ToLookup(a => a.DateDetails).First()
lookaheadList = match.ToList();
However, this is redundant for a couple of reasons:
If you're not storing the result of ToLookup and using it to look up other groups by date, there was no point creating the lookup object -- you could have just used GroupBy.
If you only need the first group, there is no need for any grouping at all (either by ToLookup or GroupBy).
To directly grab the items that match the first date, use:
var firstDate = recordList.First().DateDetails;
var matches = recordList.Where(d => d.DateDetails == firstDate)
lookaheadList = matches.ToList();
Assuming
group1 = "025"
and
group2= "030"
I think you're missing a "group by"
C# LINQ Query - Group By
I suggest you to use GroupBy
var match = (from m in recordList select m).FirstOrDefault();
var firstGroup= recordList.Where(d => d.DateDetails == match.DateDetails).GroupBy(x=> x.DateDetails).Select(x => x.First());
and then use firstGroup variable to filter that elements
have a read at this #Jon Skeet answer about the comparison of ILookup and IGrouping performances
Bear in mind that ToLookup is a "do it now" operation (immediate
execution) whereas a GroupBy is deferred
I am using LinqPad to learn Linq by querying the NetFlix OData source.
(BTW I know their is a similar question already on SO...didn't help me).
Here is the query I got working which is awesome.
from x in Titles
//where x.Rating=="PG"
where x.Instant.Available==true
where x.AverageRating>=4.0
//where x.Rating.StartsWith("TV")
//where x.Genres.First (g => g.Name.Contains("Family") ) //(from y in Genres where y.Name.Contains("Family") select y)
//where x.Genres.First (g => g.Name=="")
//orderby x.Name
orderby x.AverageRating descending
//select x
//)
select new {x.Name, x.Rating, x.AverageRating, x.ShortSynopsis}
(Pardon all the comments...it is a testament to the fact I am experimenting and that I will change the query for various needs).
There are two thing I cannot figure out.
First. Let's say I only want to return the first 10 results.
Second (and most importantly). I want to filter by a partial string of the genre. Each title contains a Genres collection. I want to show only Genres where the Name contains a certain string (like "Family"). Even better filter using Titles where genre.name.contains "firstFilter" AND "secondFilter".
Basically, I want to filter by genre(s) and I cannot figure out how to do it since Title contains its own Genres collection and I cannot figure out how to return only title that are in one or more genres of the collection.
Thanks for your help!
ps...it seems that Netflix OData source does not support Any operator.
Seth
To return the first 10 results, surround your code above with parentheses and put a .Take(10) on the end
var foo = ( from x in Titles... ).Take(10);
There is no way to do take using query syntax in C# currently.
As for the genre filter, as klabranche points out, oData does not support many of the same Linq constructs you can use locally with a regular IEnumerable or IQueryable.
klabranche's solution doesn't support contains. It does however make 2 round trips to the server to get results. (see my comment on his answer as to why this seems necessary)
Here is an alternative which makes one roundtrip to the server to get data, then it processes that data locally. Because some of the query runs locally, you can use string.Contains, "or" clauses, and other goodness.
The downside of this approach is it retrieves more data over the wire than is needed to answer the query. On the other hand, it's easy to understand and it works.
When I combine "Family" and "Children", it returns 21 results.
var oDataQuery = from x in Titles
where x.AverageRating >= 4
&& x.Instant.Available==true
orderby x.AverageRating descending
select new {x.Name, x.Rating, x.AverageRating, x.ShortSynopsis, x.Genres};
var localQuery = from o in oDataQuery.ToList()
where o.Genres.Any(g => g.Name.Contains("Family"))
&& o.Genres.Any(g => g.Name.Contains("Children"))
select new {o.Name, o.Rating, o.AverageRating, o.ShortSynopsis };
localQuery.Dump();
OData and the Netflix API support the Take() and Contains() methods:
from t in Titles
where t.Name.Contains("Matrix")
select t
(from t in Titles
where t.Name.Contains("Matrix")
select t).Take(10)
To get the first 10:
(from x in Titles
where x.Instant.Available==true
where x.AverageRating>=4.0
orderby x.AverageRating descending
select new {x.Name, x.Rating, x.AverageRating, x.ShortSynopsis}
).Take(10)
To filter by a single genre (Not what you want...):
from g in Genres
from t in g.Titles
where g.Name == "Horror"
where t.Instant.Available==true
where t.AverageRating >=4.0
orderby t.AverageRating descending
select new {t.Name, t.Rating, t.AverageRating, t.ShortSynopsis}
However, you wanted to have multiple genres BUT OData doesn't support Select Many queries which is why contains fails or trying to OR the Genre Name.
Below fails because contains returns many...
var q1 = from g in Genres
from t in g.Titles
where g.Name.Contains("Horror")
where t.Instant.Available==true
where t.AverageRating >=4.0
orderby t.AverageRating descending
select new {t.Name, t.Rating, t.AverageRating, t.ShortSynopsis};
To filter by multiple genres I found you can use a Concat or Union query (in LinqPad be sure to change to C# statements not expression):
var q1 = from g in Genres
from t in g.Titles
where g.Name=="Horror"
where t.Instant.Available==true
where t.AverageRating >=4.0
orderby t.AverageRating descending
select new {t.Name, t.Rating, t.AverageRating, t.ShortSynopsis};
var q2 = from g in Genres
from t in g.Titles
where g.Name=="HBO"
where t.Instant.Available==true
where t.AverageRating >=4.0
orderby t.AverageRating descending
select new {t.Name, t.Rating, t.AverageRating, t.ShortSynopsis};
var concat = q1.ToList().Concat(q2);
//var union = q1.Union(q2);
By unioning the two queries it will remove duplicates but these are what you want If I understand you correctly in that you want movies that are only in both genres?
In that case you will want to use Concat which will return all records.
Now you just need to find records that are in the query more than once and you have your results:
var results = from c in concat
group c by c.Name into grp
where grp.Count() > 1
select grp;
I'm wondering if its possible to join together IEnumerable's.
Basically I have a bunch of users and need to get their content from the database so I can search and page through it.
I'm using LINQ to SQL, my code at the moment it:
public IEnumerable<content> allcontent;
//Get users friends
IEnumerable<relationship> friends = from f in db.relationships
where f.userId == int.Parse(userId)
select f;
IEnumerable<relationship> freindData = friends.ToList();
foreach (relationship r in freindData)
{
IEnumerable<content> content = from c in db.contents
where c.userId == r.userId
orderby c.contentDate descending
select c;
// This is where I need to merge everything together
}
I hope that make some sense!
Matt
If I understand correctly what you are trying to do, why don't you try doing:
var result = from r in db.relationships
from c in db.contents
where r.userId == int.Parse(userId)
where c.userId == r.UserId
orderby c.contentDate descending
select new {
Relationship = r,
Content = c
}
This will give you an IEnumerable<T> where T is an anonymous type that has fields Relationship and Content.
If you know your users will have less than 2100 friends, you could send the keys from the data you already loaded back into the database easily:
List<int> friendIds = friendData
.Select(r => r.UserId)
.Distinct()
.ToList();
List<content> result = db.contents
.Where(c => friendIds.Contains(c.userId))
.ToList();
What happens here is that Linq translates each Id into a parameter and then builds an IN clause to do the filtering. 2100 is the maximum number of parameters that SQL server will accept... if you have more than 2100 friends, you'll have to break the ID list up and combine (Concat) the result lists.
Or, if you want a more literal answer to your question - Concat is a method that combines 2 IEnumerables together by creating a new IEnumerable which returns the items from the first and then the items from the second.
IEnumerable<content> results = Enumerable.Empty<content>();
foreach (relationship r in friendData)
{
IEnumerable<content> content = GetData(r);
results = results.Concat(content);
}
If you're doing an INNER join, look at the .Intersect() extension method.
Which things are you merging?
There are two main options you could use: .SelectMany(...) or .Concat(...)