I'm using Linq To Nhibernate, and with a HQL statement I can do something like this:
string hql = "from Entity e order by rand()";
Andi t will be ordered so random, and I'd link to know How can I do the same statement with Linq to Nhibernate ?
I try this:
var result = from e in Session.Linq<Entity>
orderby new Random().Next(0,100)
select e;
but it throws a exception and doesn't work...
is there any other way or solution?
Thanks
Cheers
I guess Linq to NHibernate is unable to convert the Random.Next call to SQL...
An option would be to sort the results after you retrieve them from the DB :
var rand = new Random();
var query = from e in Session.Linq<Entity>
select e;
var result = from e in query.AsEnumerable()
orderby rand.Next(0,100)
select e;
Note that you need to use a single instance of Random, because the seed is based on the current number of ticks ; if you create several instances of Random with very short interval, they will have the same seed, and generate the same sequence.
Anyway, sorting a collection based on a random number is not such a good idea, because the sort won't be stable and could theoretically last forever. If you need to shuffle the results, you can use the Fisher-Yates algorithm :
var result = Session.Linq<Entity>().ToArray();
var rand = new Random();
for(int i = result.Length - 1; i > 0; i--)
{
int swapIndex = rand.Next(i + 1);
var tmp = result[i];
result[i] = result[swapIndex];
result[swapIndex] = tmp;
}
Related
If I wrote :
for (int i = 0; i < Strutture.Count(); i++)
{
}
and Strutture is an IEnumerable with 200 elements, IIS crash. That's because I see every time I do Strutture.Count() it executes all LINQ queries linked with that IEnumerable.
So, how can I get the "current" number of elements? I need a list?
"That's because I see every time I do Strutture.Count() it executes all LINQ queries linked with that IEnumerable."
Without doing such, how is it going to know how many elements there are?
For example:
Enumerable.Range(0,1000).Where(i => i % 2==0).Skip(100).Take(5).Count();
Without executing the LINQ, how could you know how many elements there are?
If you want to know how many elements there are in the source (e.g. Enumerable.Range) then I suggest you use a reference to that source and query it directly. E.g.
var numbers = Enumerable.Range(0,1000);
numbers.Count();
Also keep in mind some data sources don't really have a concept of 'Count' or if they do it involves going through every single item and counting them.
Lastly, if you're using .Count() repetitively [and you don't expect the value to actually change] it can be a good idea to cache:
var count = numbers.Count();
for (int i =0; i<count; i++) // Do Something
Supplemental:
"At first Count(), LINQ queries are executes. Than, for the next, it just "check" the value :) Not "execute the LINQ query again..." :)" - Markzzz
Then why don't we do that?
var query = Enumerable.Range(0,1000).Where(i => i % 2==0).Skip(100).Take(5).Count();
var result = query.ToArray() //Gets and stores the result!
result.Length;
:)
"But when I do the first "count", it should store (after the LINQ queries) the new IEnumerable (the state is changed). If I do again .Count(), why LINQ need to execute again ALL queries." - Markzzz
Because you're creating a query that gets compiled down into X,Y,Z. You're running the same query twice however the result may vary.
For example, check this out:
static void Main(string[] args)
{
var dataSource = Enumerable.Range(0, 100).ToList();
var query = dataSource.Where(i => i % 2 == 0);
//Run the query once and return the count:
Console.WriteLine(query.Count()); //50
//Now lets modify the datasource - remembering this could be a table in a db etc.
dataSource.AddRange(Enumerable.Range(100, 100));
//Run the query again and return the count:
Console.WriteLine(query.Count()); //100
Console.ReadLine();
}
This is why I recommended storing the results of the query above!
Materialize the number:
int number = Strutture.Count();
for (int i = 0; i < number; i++)
{
}
or materialize the list:
var list = Strutture.ToList();
for (int i = 0; i < list.Count; i++)
{
}
or use a foreach
foreach(var item in Strutture)
{
}
I want to extract part of a collection to another collection.
I can easily do the same using a for loop, but my linq query is not working for the same.
I am a neophyte in Linq, so please help me correcting the query (if possible with explanation / beginners tutorial link)
Legacy way of doing :
Collection<string> testColl1 = new Collection<string> {"t1", "t2", "t3", "t4"};
Collection<string> testColl2 = new Collection<string>();
for (int i = 0; i < newLength; i++)
{
testColl2.Add(testColl1[i]);
}
Where testColl1 is the source & testColl2 is the desired truncated collection of count = newLength.
I have used the following linq queries, but none of them are working ...
var result = from t in testColl1 where t.Count() <= newLength select t;
var res = testColl1.Where(t => t.Count() <= newLength);
Use Enumerable.Take:
var testColl2 = testColl1.Take(newLength).ToList();
Note that there's a semantic difference between your for loop and the version using Take. The for loop will throw with IndexOutOfRangeException exception if there are less than newLength items in testColl1, whereas the Take version will silently ignore this fact and just return as many items up to newLength items.
The correct way is by using Take:
var result = testColl1.Take(newLength);
An equivalent way using Where is:
var result = testColl1.Where((i, item) => i < newLength);
These expressions will produce an IEnumerable, so you might also want to attach a .ToList() or .ToArray() at the end.
Both ways return one less item than your original implementation does because it is more natural (e.g. if newLength == 0 no items should be returned).
You could convert to for loop to something like this:
testColl1.Take(newLength)
Use Take:
var result = testColl1.Take(newLength);
This extension method returns the first N elements from the collection where N is the parameter you pass, in this case newLength.
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:)
from f in db.Table1
orderby Guid.NewGuid()
select f
this doesn't seem to work. how can i randomize results?
How about
SELECT TOP 1 column FROM table ORDER BY NEWID and skip the linq :)
Or try this:
var t = (from row in db.Table1 order by table1.random()
select row).FirstOrDefault();
Maybe something like this works (not tested):
(from f in db.Table1 select new { f, r = Guid.NewGuid()}).OrderBy(x => x.r)
Randomize whole list
db.Table1.OrderBy(x => Guid.NewGuid())
Get single Random
db.Table1.OrderBy(x => Guid.NewGuid()).FirstOrDefault();
I like to write an extension method for this.
IEnumerable<T> Randomize(this IEnumerable<T> list)
{
T[] result = list.ToArray();
Random random = new Random();
for(int i = result.Length; i > 0; i--)
{
result[i] = random.Next(i);
}
return (result);
}
Let's ay I have this query:
var results = from row in db.Table select row;
How can I access this:
string name = results[0]["columnName"];
if you really want a particular index you can use the Skip() method with First().
var rowOffset = 0;
var results = (from row in db.Table
select row).Skip(rowOffset).First()["columnName"];
But unless you are using a Where clause I would really recommend using the indexer. The indexer is pretty much a direct reference while the LINQ statement would be using the objects iterator.
Also don't forget you can do much more advanced stuff with LINQ.
var rowOffset = 0;
var pageLength = 10;
var results = (from row in db.Table
let colValue = row["columnname"]
where colValue != null
select colValue.ToString()
).Skip(rowOffset)
.Take(pageLength)
.ToArray();
var commaString = string.Join(", ", results);
If you specifically just want the zeroth element, you can use results.First()
results is a IEnumerable list of Rows. So you can get it with a simple foreach.
foreach(var row in results)
{
string name = row["columnName"];
}
(from row in db.Table select row).First().columnName