I need to filter a list, by removing all items have the same language code as the item before (the items are orderd by time). This way, I want to detect all border crossings.
Is there a way to do this with one line of linq?
var test = new List<Sample> { new Sample("AT", "test1"),
new Sample("AT", "test2") ,
new Sample("AT", "test3") ,
new Sample("DE", "test4") ,
new Sample("DE", "test5") ,
new Sample("DE", "test6") ,
new Sample("AT", "test7") ,
new Sample("AT", "test8") ,
new Sample("AT", "test9")
};
var borderChanges = new List<Sample>();
var lastCountry = "";
foreach (var sample in test)
{
if (sample.country != lastCountry)
{
lastCountry = sample.country;
borderChanges.Add(sample);
}
}
I think this is what are you looking for:
test.Where((x,idx) => idx == 0 || x.Country != test[idx - 1].Country));
In case someone else is curious to see, how this problem can be solved with Aggregate(), here is the answer:
var borderChanges = test.Take(1).ToList();
test.Aggregate((last, curr) =>
{
if (last.Code != curr.Code)
{
borderChanges.Add(curr);
}
return curr;
});
Aggregate() executes the lambda expression for every element in the list, except the first. So we initialize the list with the first item.
In the lamda we add all items where the current item doesnt equal the aggregated value. In our case the aggredated value is always the previously checked item.
In the foreach iteration, access to neighboring elements of the sequence is impossible. Perhaps your goal can be achieved with the Aggregate extension method for IEnumerable<T>, where you can compare neighboring elements of the sequence.
Related
I made a new query to select from Article Class with where clause for each item selected. However, it keeps getting the whole list every time although there are selected fields!
Here is my code:
ParseQuery<Article> query = new ParseQuery<Article>();
if (souCategorie.SelectedIndex >= 0)
{
query.WhereEqualTo("idSCategorie", listeSouCategorie.ElementAt(souCategorie.SelectedIndex));
}
if(motcle.Text.Length > 0)
{
query.WhereContains("nom", motcle.Text);
// query.WhereContains("description", motcle.Text);
}
if(distance.Text.Length>0)
if (Convert.ToDouble(distance.Text) > 0)
{
Debug.WriteLine(distance.Text);
ParseGeoPoint geo = new ParseGeoPoint();
geo.Latitude = geoposition.Coordinate.Latitude;
geo.Longitude = geoposition.Coordinate.Longitude;
query.WhereWithinDistance("coordonnees", geo, ParseGeoDistance.FromKilometers(Convert.ToDouble(distance.Text)));
}
IEnumerable<Article> lst = await query.FindAsync();
rechercheResult.DataContext = lst.ToList();
What could possibly be wrong?
I know that queries can do funky stuff when you start trying to use GeoPoint stuff. I would try setting up two queries, one that just queries for objects within a distance, then pass that query into the second query that has the whereEqualTo and whereContains calls.
I am slow today
There is a request
"Take random child and put it into another garden."
I changed the code, but error in the last line of code "Does not contain a definition…and no extension method":
var query = db.Child.Where(x => x.Garden != null);
int count = query.Count();
int index = new Random().Next(count);
var ch = db.Child.OrderBy(x => query.Skip(index).FirstOrDefault());
ch.Garden_Id = "1";
What am I doing wrong?
It's hard to tell what you're doing wrong, because you didn't say why the results you're getting does not satisfy you.
But I can see two possible mistakes.
You're counting items with x.Garden != null condition, but taking from all children.
Take returns IEnumerable<T> even when you specify it to return only 1 item, you should probably use First instead.
I think your k should be
var k = db.Child.Where(x => x.Garden != null).Skip(rnd.Next(0,q)).First();
I have a long query that returns
Item {
DateTime entryDate
.....
}
I like to combine the result of this query with another table
Value {
DateTime date,
double value
}
such that if entryDate >= CUTOFF, then take the value on CUTOFF, else take the value on entryDate. In other words, I'd want to achieve:
SELECT Item.*, Value.value WHERE
MIN( Item.entryDate, CUTOFF ) == Value.date
Excuse my syntax, but that's the idea.
EDIT: After some trial and error, I came up with this linq-to-sql query:
from iValue in Values
join iItem in ... (long query)
let targetDate = iItem.EntryDate > CUTOFF ? iItem.EntryDate : CUTOFF
where iValue.Date == targetDate
select new
{
iItem,
targetDate,
iValue
}
Thanks for your help.
yourLongQuery.Where(y => y.Item.entryDate == Value.date || CUTOFF == Value.date)
.Select(x => new {
entrydate = (x.Item.entryDate < CUTOFF ? x.Item.entryDate : CUTOFF),
/*rest of x.Item properties here */ ,
x.Value.date,
x.Value.value
});
Filter the query, Combine the two items into one item and modify the first item
Given that you've returned your data from your Item query and that the Value table is relatively small then this is a nice way to go:
var lookup = values.ToLookup(v => v.date, v => v.value);
var query =
from i in items
let c = i.entryDate < CUTOFF ? i.entryDate : CUTOFF
let v = lookup[c].FirstOrDefault()
select new
{
Item = i,
Value = v,
};
The ToLookup extension is very useful and often overlooked.
such that if entryDate >= CUTOFF, then take the value on CUTOFF, else
take the value on entryDate. In other words, I'd want to achieve:
SELECT Item.*, Value.value WHERE MAX( Item.entryDate, CUTOFF ) ==
Value.date
This is contradictory - if entryDate >= CUTOFF then take the value on CUTOFF imples that you want MIN(Item.entryDate, CUTOFF), not MAX.
Having said that, you just want to select the value.Value that matches each item of your query. Each item should look up the relevant value which matches your MAX (or, I believe, MIN) statement.
query.Select(item =>
{
var matchingValue = Values.Single(v =>
v.date == Min(item.entryDate, CUTOFF));
return new { item, matchingValue.value };
});
This will return an IQueryable of anonymous { Item, double } objects.
If you require this to be a executed as a single SQL statement you'll need to do some refactoring. A good start is to swap matchingValue into a single statement.
query.Select(item => new { item, context.Values.Single(v =>
v.date == Min(item.entryDate, CUTOFF)).value });
I don't have a Visual Studio in front of me to confirm, but I am not sure that the Math.Min function is mapped in LINQ-to-SQL. Let's assume it's not.
query.Select(item => new { item, context.Values.Single(v =>
v.date == (item.entryDate < CUTOFF ? item.entryDate : CUTOFF)).value });
I believe that will resolve to a single query if you execute it with a .ToList() but can't confirm until I have some tools in front of me. Test it with SQL profiler to be sure.
I have a linq query like this:
var accounts =
from account in context.Accounts
from guranteer in account.Gurantors
where guranteer.GuarantorRegistryId == guranteerRegistryId
select new AccountsReport
{
recordIndex = ?
CreditRegistryId = account.CreditRegistryId,
AccountNumber = account.AccountNo,
}
I want to populate recordIndex with the value of current row number in collection returned by the LINQ. How can I get row number ?
Row number is not supported in linq-to-entities. You must first retrieve records from database without row number and then add row number by linq-to-objects. Something like:
var accounts =
(from account in context.Accounts
from guranteer in account.Gurantors
where guranteer.GuarantorRegistryId == guranteerRegistryId
select new
{
CreditRegistryId = account.CreditRegistryId,
AccountNumber = account.AccountNo,
})
.AsEnumerable() // Moving to linq-to-objects
.Select((r, i) => new AccountReport
{
RecordIndex = i,
CreditRegistryId = r.CreditRegistryId,
AccountNumber = r.AccountNo,
});
LINQ to objects has this builtin for any enumerator:
http://weblogs.asp.net/fmarguerie/archive/2008/11/10/using-the-select-linq-query-operator-with-indexes.aspx
Edit: Although IQueryable supports it too (here and here) it has been mentioned that this does unfortunately not work for LINQ to SQL/Entities.
new []{"aap", "noot", "mies"}
.Select( (element, index) => new { element, index });
Will result in:
{ { element = aap, index = 0 },
{ element = noot, index = 1 },
{ element = mies, index = 2 } }
There are other LINQ Extension methods (like .Where) with the extra index parameter overload
Try using let like this:
int[] ints = new[] { 1, 2, 3, 4, 5 };
int counter = 0;
var result = from i in ints
where i % 2 == 0
let number = ++counter
select new { I = i, Number = number };
foreach (var r in result)
{
Console.WriteLine(r.Number + ": " + r.I);
}
I cannot test it with actual LINQ to SQL or Entity Framework right now. Note that the above code will retain the value of the counter between multiple executions of the query.
If this is not supported with your specific provider you can always foreach (thus forcing the execution of the query) and assign the number manually in code.
Because the query inside the question filters by a single id, I think the answers given wont help out. Ofcourse you can do it all in memory client side, but depending how large the dataset is, and whether network is involved, this could be an issue.
If you need a SQL ROW_NUMBER [..] OVER [..] equivalent, the only way I know is to create a view in your SQL server and query against that.
This Tested and Works:
Amend your code as follows:
int counter = 0;
var accounts =
from account in context.Accounts
from guranteer in account.Gurantors
where guranteer.GuarantorRegistryId == guranteerRegistryId
select new AccountsReport
{
recordIndex = counter++
CreditRegistryId = account.CreditRegistryId,
AccountNumber = account.AccountNo,
}
Hope this helps.. Though its late:)
I have this class
public class Line
{
public string ConnectionsIndex{get;set;}
}
my Linq problem is that I have to aggregate these Lines
var l1 = new Line{ ConnectionsIndex="01,02"};
var l2 = new Line{ ConnectionsIndex="02,03"};
var l3 = new Line{ ConnectionsIndex="01,03"};
into this
var l4 = new Line{ ConnectionsIndex="01,02,03"};
It's possible to do with Linq?
DETAIL:
The thing is more complicate (at least for me) when I add the other items that I have in my collection.
var l5 = new Line (ConnectionsIndex = "02,04");
var l6 = new Line (ConnectionsIndex = "03,06");
because do not exist other lines with the pairs 03,04 , 01,04 , 01,06 and 02,06
I do not know if I have explained it well ...
in practice, imagine you have all the points of a polygon, I want to get a line of all the items from the query by giving a list of connections between all points of each polygon.
(my list contains more than one polygon)
One point should not be included in result if not connected to all others.
This is an example of my list content:
ConnectionsIndex="166,171"
ConnectionsIndex="166,174"
ConnectionsIndex="166,333"
ConnectionsIndex="169,170"
ConnectionsIndex="171,175"
ConnectionsIndex="171,334"
ConnectionsIndex="167,174"
ConnectionsIndex="172,174"
ConnectionsIndex="174,335"
ConnectionsIndex="177,341"
ConnectionsIndex="180,200"
ConnectionsIndex="181,183"
ConnectionsIndex="182,199"
ConnectionsIndex="184,185"
ConnectionsIndex="186,188"
ConnectionsIndex="189,192"
ConnectionsIndex="190,230"
ConnectionsIndex="191,375"
In this List you have for example a triangle between 166, 171 and 334
More detail:
var group = lines.Where(x => x.ConnectionsIndex.Split(',').Contains(line. ConnectionsIndex.Split(',')[0]) ||
x. ConnectionsIndex.Split(',').Contains(line. ConnectionsIndex.Split(',')[1])).ToList();
if (group.Count()==1)
{
straight_lines.Add(line);
}
else
{
//Here I have a "group" with all the lines between point.. I want to get distinc points
}
Something like:
var connections = (from line in lines
from connection in line.Split(',')
select connection).Distinct()
.ToArray();
Line line = new Line { ConnectionsIndex = string.Join(",", connections) };
This doesn't order the connections, but you can easily add that if you need it.
This would all be cleaner if you were happy to have ConnectionsIndex as a collection of strings instead of a single delimited string, of course :)
I used this:
var l4 = new Line{
ConnectionsIndex =
string.Join(",", (lines.SelectMany(x => x.ConnectionsIndex.Split(','))
.Distinct()
.OrderBy(s => s)).ToArray())
};
This is the bad way I have found... and it works!
var l = linee.Distinct(
(a, b) => a.ConnectionsIndex == b.ConnectionsIndex,x=>x.ConnectionsIndex.GetHashCode())
.ToList();
var single_lines = new List<Linea>();
var multiple_lines = new List<Linea>();
foreach (var linea in l)
{
var group = l
.Where(x => x.ConnectionsIndex.Split(',').Contains(linea.ConnectionsIndex.Split(',')[0]) ||
x.ConnectionsIndex.Split(',').Contains(linea.ConnectionsIndex.Split(',')[1])).ToList();
if (group.Count()==1)
{
single_lines.Add(linea);
}
else
{
var indexes = new List<string>();
var dist = group.Select(x => new {Index = x.ConnectionsIndex.Split(',').ToList()}).ToList();
foreach (var linea1 in dist)
{
indexes=indexes.Concat(linea1.Index).ToList();
}
var indexstring = new StringBuilder();
foreach (var s in indexes.Distinct().OrderBy(x=>Convert.ToInt32(x)))
{
indexstring.Append(s).Append(',');
}
indexstring.Remove(indexstring.Length - 1, 1);
multiple_lines.Add(new Linea() {ConnectionsIndex = indexstring.ToString()});
}
}
var multi_distinct=multiple_lines.Distinct(
(a, b) => a.ConnectionsIndex == b.ConnectionsIndex, x => x.ConnectionsIndex.GetHashCode())
.ToList();
linee = single_lines.Concat(multi_distinct).ToList();
If you find or known better solutions, you are welcome!
Just a note that I think what you're asking for is a way to find maximal cliques (a concept from graph theory). This is known to be an NP-Hard problem. I think your version will work sometimes, and hopeflly for those cases you're interested in. But, not for complicated cases where anything may be connected to anything else. Indeed, if you have a lot of nodes, those cases aren't feasible, even with large CPU cycle budgets (regardless of LINQ).