Just a bit rusty on the old linq.
If I have 2 collections EG NewCustomerList and OldCustomerList and see if a surname exists already how would I do it in linq .I am sure there are many ways. SelectMany rings a bell but forgot how to do it!
In a forEach I would do something like that. What is the equivalent in linq?
foreach (var oldCustomer in OldCustomerList)
{
foreach (var newCustomer in NewCustomerList.Where(x => x.Surname == oldCustomer.Surname))
{
break;
}
}
Any Suggestions? Thanks a lot
So you're trying to see whether any of the old customer surnames are in the new customer list?
One simple option: do a join and see if it's empty:
if (OldCustomerList.Join(NewCustomerList, x => x.Surname, x => x.Surname,
(x, y) => null).Any())
{
...
}
(I've used a null projection because we really don't care about the join result.)
Another option:
var oldSurnames = new HashSet<string>(OldCustomrList.Select(x => x.Surname));
if (NewSurnameList.Any(x => oldSurnames.Contains(x.Surname))
{
...
}
I suspect you may find that you actually want the result in terms of which surnames are in common though... if you can give us more context, we may be able to help you more.
You can do this by:
NewCustomerList.Where(n => OldCustomerList.Any(o => o.Surname == n.Surname))
Here's another approach, which also has O(n + m) complexity + quick-pass semantics:
OldCustomerList.Select(cust => cust.Surname)
.Intersect(NewCustomerList.Select(cust => cust.Surname))
.Any();
IMO, it is more readable than an explicit Enumerable.Join: "test if the projection of surnames from the old customer-list has any elements in common with the projection of surnames from the new customer-list", which is pretty close to the problem-statement.
Related
I'm trying to add a column to the following LINQ expression. I want the column to contain a string concatenation of a text value in a many table called WasteItems. The join would be on "Waste.WasteId = WasteItem.WasteId". My problem is I need to display in a single dynamic column a string such as "EW (5); EX (3)" if there was 8 records in WasteItem and the column containing the 2 character string was called WasteItem.EWC. Hope that makes sense, there must be an efficient way since I realise LINQ is very powerfull. I'm new to it and not sure how to start or go about this:
return from waste in this._db.Wastes
where (from u in _db.UsersToSites.Where(p => p.UserId == userId && p.SystemTypeId == SystemType.W)
select u.SiteId)
.Contains(waste.SiteId)
orderby waste.Entered descending select waste;
THANKS IN ADVANCE
Something like this should do:
wastes.GroupJoin(db.WasteItems, w => w.WastId, wi => wi.WasteId, (w,wi) => new { w, wi })
.AsEnumerable()
.Select(x => new
{
x.w.Name,
Items = string.Join(", ", x.wi.GroupBy(wi => wi.EWC).Select(g => string.Format("{0} ({1})", g.Key, g.Count())))
})
Where wastes is the result from your query. The AsEnumerable() is necessary because Entity Framework can not handle string.Join, so that part must be dealt with in memory.
I could not check the syntax, obviously, but at least it may show you the way to go.
I have a dictionary of type
Dictionary<DateTime,double> dictionary
How can I retrive a minimum value and key coresponding to this value from this dictionary using linq ?
var min = dictionary.OrderBy(kvp => kvp.Value).First();
var minKey = min.Key;
var minValue = min.Value;
This is not very efficient though; you might want to consider MoreLinq's MinBy extension method.
If you are performing this query very often, you might want to consider a different data-structure.
Aggregate
var minPair = dictionary.Aggregate((p1, p2) => (p1.Value < p2.Value) ? p1 : p2);
Using the mighty Aggregate method.
I know that MinBy is cleaner in this case, but with Aggregate you have more power and its built-in. ;)
Dictionary<DateTime, double> dictionary;
//...
double min = dictionary.Min(x => x.Value);
var minMatchingKVPs = dictionary.Where(x => x.Value == min);
You could combine it of course if you really felt like doing it on one line, but I think the above is easier to read.
var minMatchingKVPs = dictionary.Where(x => x.Value == dictionary.Min(y => y.Value));
You can't easily do this efficiently in normal LINQ - you can get the minimal value easily, but finding the key requires another scan through. If you can afford that, use Jess's answer.
However, you might want to have a look at MinBy in MoreLINQ which would let you write:
var pair = dictionary.MinBy(x => x.Value);
You'd then have the pair with both the key and the value in, after just a single scan.
EDIT: As Nappy says, MinBy is also in System.Interactive in Reactive Extensions.
What does it mean and why (if at all) is it important?
It means you can add additional "operators" to a query. It's important because you can do it extremely efficiently.
For example, let's say you have a method that returns a list (enumerable) of employees:
var employees = GetEmployees();
and another method that uses that one to return all managers:
IEnumerable<Employee> GetManagers()
{
return GetEmployees().Where(e => e.IsManager);
}
You can call that function to get managers that are approaching retirement and send them an email like this:
foreach (var manager in GetManagers().Where(m => m.Age >= 65) )
{
SendPreRetirementMessage(manager);
}
Pop quiz: How many times will that iterate over your employees list? The answer is exactly once; the entire operation is still just O(n)!
Also, I don't need to have separate methods for this. I could compose a query with these steps all in one place:
var retiringManagers = GetEmployees();
retiringManagers = retiringManagers.Where(e => e.IsManager);
retiringManagers = retiringManagers.Where(m => m.Age >= 65);
foreach (var manager in retiringMangers)
{
SendPreRetirementMessage();
}
One cool thing about this is that I can change is at run time, such that I can include or not include one part of the composition inside an if block, such that the decision to use a specific filter is made at run time, and everything still comes out nice and pretty.
I think it means that you can daisy chain your queries, like this
var peterJacksonsTotalBoxOffice
= movies.Where(movie => movie.Director == "Peter Jackson")
.Sum(movie => movie.BoxOffice);
I want to do something like DELETE FROM TABLE WHERE ID NOT IN (1,2,3) AND PAGEID = 9
I have a List of IDS but that could be changed if needs be. I can't work out how to get a boolean result for the LINQ parser.
Here is what Subsonic expects I think.
db.Delete(content => content.PageID == ID).Execute();
I can't work out how to do the NOT IN statement. I've tried the List.Contains method but something not quite right.
UPDATE: One alternative is to do:
var items = TABLE.Find(x => x.PageID == ID)'
foreach(var item in items)
{
item.Delete();
}
This hits the database a lot more though
When you say "something not quite right" what exactly do you mean?
I'd expect to write:
List<int> excluded = new List<int> { 1, 2, 3 };
db.Delete(content => !excluded.Contains(content.PageID)).Execute();
Note that you need to call Contains on the array of excluded values, not on your candidate. In other words, instead of saying "item not in collection" you're saying "collection doesn't contain item."
Try .Contains:
db.Delete(content => content.PageID.Contains(<Array containing ID's>).Execute();
(the above is just an example, might need some polishing for your specific situation)
I have found that this works but its not via LINQ
var table = new WebPageContentTable(_db.DataProvider);
var g = new SubSonic.Query.Delete<WebPageContent(_db.DataProvider)
.From(table)
.Where(table.ID)
.NotIn(usedID)
.Execute();
I have found that this does work and via LINQ - however it hits the database multiple times.
var f = WebPageContent.Find(x => !usedID.Any(e => e == x.ID));
if (f.Count > 0)
{
var repo = WebPageContent.GetRepo();
repo.Delete(f);
}
This I imagine would work in one hit to the database but I get an exception thrown in QueryVisitor::VisitUnary
WebPageContent.Delete(x => !usedID.Any(e => e == x.ID));
I felt like the following should be possible I'm just not sure what approach to take.
What I'd like to do is use the include method to shape my results, ie define how far along the object graph to traverse. but... I'd like that traversal to be conditional.
something like...
dealerships
.include( d => d.parts.where(p => p.price < 100.00))
.include( d => d.parts.suppliers.where(s => s.country == "brazil"));
I understand that this is not valid linq, in fact, that it is horribly wrong, but essentially I'm looking for some way to build an expression tree that will return shaped results, equivalent to...
select *
from dealerships as d
outer join parts as p on d.dealerid = p.dealerid
and p.price < 100.00
outer join suppliers as s on p.partid = s.partid
and s.country = 'brazil'
with an emphasis on the join conditions.
I feel like this would be fairly straight forward with esql but my preference would be to build expression trees on the fly.
as always, grateful for any advice or guidance
This should do the trick:
using (TestEntities db = new TestEntities())
{
var query = from d in db.Dealership
select new
{
Dealer = d,
Parts = d.Part.Where
(
p => p.Price < 100.0
&& p.Supplier.Country == "Brazil"
),
Suppliers = d.Part.Select(p => p.Supplier)
};
var dealers = query.ToArray().Select(o => o.Dealer);
foreach (var dealer in dealers)
{
Console.WriteLine(dealer.Name);
foreach (var part in dealer.Part)
{
Console.WriteLine(" " + part.PartId + ", " + part.Price);
Console.WriteLine
(
" "
+ part.Supplier.Name
+ ", "
+ part.Supplier.Country
);
}
}
}
This code will give you a list of Dealerships each containing a filtered list of parts. Each part references a Supplier. The interesting part is that you have to create the anonymous types in the select in the way shown. Otherwise the Part property of the Dealership objects will be empty.
Also, you have to execute the SQL statement before selecting the dealers from the query. Otherwise the Part property of the dealers will again be empty. That is why I put the ToArray() call in the following line:
var dealers = query.ToArray().Select(o => o.Dealer);
But I agree with Darren that this may not be what the users of your library are expecting.
Are you sure this is what you want? The only reason I ask is, once you add the filter on Parts off of Dealerships, your results are no longer Dealerships. You're dealing in special objects that are, for the most part, very close to Dealerships (with the same properties), but the meaning of the "Parts" property is different. Instead of being a relationship between Dealerships and Parts, it's a filtered relationship.
Or to put it another way, if I pull a dealership out of your results and passed to a method I wrote, and then in my method I call:
var count = dealership.Parts.Count();
I'm expecting to get the parts, not the filtered parts from Brazil where the price is less than $100.
If you don't use the dealership object to pass the filtered data, it becomes very easy. It becomes as simple as:
var query = from d in dealerships
select new { DealershipName = d.Name,
CheapBrazilProducts = dealership.Parts.Where(d => d.parts.Any(p => p.price < 100.00) || d.parts.suppliers.Any(s => s.country == "brazil")) };
If I just had to get the filtered sets like you asked, I'd probably use the technique I mentioned above, and then use a tool like Automapper to copy the filtered results from my anonymous class to the real class. It's not incredibly elegant, but it should work.
I hope that helps! It was an interesting problem.
I know this can work with one single Include. Never test with two includes, but worth the try:
dealerships
.Include( d => d.parts)
.Include( d => d.parts.suppliers)
.Where(d => d.parts.All(p => p.price < 100.00) && d.parts.suppliers.All(s => s.country == "brazil"))
Am I missing something, or aren't you just looking for the Any keyword?
var query = dealerships.Where(d => d.parts.Any(p => p.price < 100.00) ||
d.parts.suppliers.Any(s => s.country == "brazil"));
Yes that's what I wanted to do I think the next realease of Data Services will have the possiblity to do just that LINQ to REST queries that would be great in the mean time I just switched to load the inverse and Include the related entity that will be loaded multiple times but in theory it just have to load once in the first Include like in this code
return this.Context.SearchHistories.Include("Handle")
.Where(sh => sh.SearchTerm.Contains(searchTerm) && sh.Timestamp > minDate && sh.Timestamp < maxDate);
before I tried to load for any Handle the searchHistories that matched the logic but don't know how using the Include logic you posted so in the mean time I think a reverse lookup would be a not so dirty solution