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C# sort object list with start position and loop

I have a strange question :)
I have a object list looking like this:
var list = new []
{
new { Id = 1, Name = "Marcus" },
new { Id = 2, Name = "Mattias" },
new { Id = 3, Name = "Patric" },
new { Id = 4, Name = "Theodor" },
};
I would like to sort the list providing a "start id"
For example, if I provide "start id" 3, the result should look like this:
Id
Name
3
Patric
4
Theodor
1
Marcus
2
Mattias
I have no idea where to start, so I really need some help from you coding gods
The list is from a sql table, but it does not matter for me where the sort take place (in sql query or in c# code)

Try this:
var list = new []
{
new { Id = 1, Name = "Marcus" },
new { Id = 2, Name = "Mattias" },
new { Id = 3, Name = "Patric" },
new { Id = 4, Name = "Theodor" },
};
var start_id = 3;
var max_id = list.Max(y => y.Id);
var result =
from x in list
orderby (x.Id + max_id - start_id) % max_id
select x;
I get:

With LINQ to objects you can do something like that:
var list = new []
{
new { Id = 1, Name = "Marcus" },
new { Id = 2, Name = "Mattias" },
new { Id = 3, Name = "Patric" },
new { Id = 4, Name = "Theodor" },
};
var startId = 3;
var result = list
.GroupBy(i => i.Id >= startId ? 1 : 0) // split in two groups
.OrderByDescending(g => g.Key) // sort to have the group with startId first
.Select(g => g.OrderBy(i => i.Id)) // sort each group
.SelectMany(i => i) // combine result
.ToList();
Console.WriteLine(string.Join(", ", result.Select(i => i.Id))); // prints "3, 4, 1, 2"

You require 2 criteria to apply:
Order ascending by Id.
Return the Ids greater than threshold before the Ids less than threshold.
You can try:
var offset = 3;
var sorted1 = list
.OrderBy(item => item.Id < offset)
.ThenBy(item => item.Id);
The OrderBy condition yields true if Id is less than offset and false otherwise.
true is greater than false and therefore is returned later
A dirty way could also be:
var offset = 3;
var sorted2 = list
.OrderBy(item => unchecked((uint)(item.Id - offset)));
Here the offset is subtracted from Id and the result converted to unsigned int to make the negative values become very large positive ones. A little hacky. Might not work with queries against SQL providers.

Here's a toy Non-Linq Version
object[] ShiftList(int id)
{
var list = new dynamic[]
{
new { Id = 1, Name = "Marcus" },
new { Id = 2, Name = "Mattias" },
new { Id = 3, Name = "Patric" },
new { Id = 4, Name = "Theodor" },
};
Span<dynamic> listSpan = list;
int indexFound = -1;
for (int i = 0; i < list.Length; i++)
{
if (listSpan[i].Id == id)
{
indexFound = i;
}
}
if (indexFound is -1)
{
return list;
}
var left = listSpan.Slice(0, indexFound);
var right = listSpan[indexFound..];
object[] objs = new object[list.Length];
Span<object> objSpan = objs;
right.CopyTo(objSpan);
left.CopyTo(objSpan[right.Length..]);
return objs;
}

Try using foreach and iterate over each object in your list:
foreach (var item in list)
{
}
from here you should be able to use some of the collection methods for a list to reorder your list.

Where expression with negative values returns incorrect results

When using SQLite as a database for EFCore, the following expression doesn't work:
Where(x => x.Price > -1)
This expression will return items where Price is -5. One workaround is to use Decimal.Negate and reverse the expression, but that shouldn't be needed. Both methods work fine with using SQL as the database. Am I missing something, or is this really a bug with SQLite?
Repro below:
Given Db Model Item:
class Item
{
public long Id { get; set; }
public string Name { get; set; }
public decimal Price { get; set; }
}
and db context
class TestDbContext : DbContext
{
public TestDbContext(DbContextOptions options) : base(options) { }
public DbSet<Item> Items { get; set; }
}
and the following repo:
static async Task Main()
{
var connection = new SqliteConnection("DataSource=:memory:");
connection.Open();
var options = new DbContextOptionsBuilder<TestDbContext>()
.UseSqlite(connection)
.Options;
var context = new TestDbContext(options);
context.Database.EnsureCreated();
var items = new List<Item>
{
new Item{Id = 1, Name = "Item 1", Price = 100},
new Item{Id = 2, Name = "Item 2", Price = 10},
new Item{Id = 3, Name = "Item 3", Price = 0},
new Item{Id = 4, Name = "Item 4", Price = -1},
new Item{Id = 5, Name = "Item 5", Price = -5}
};
context.Items.AddRange(items);
context.SaveChanges();
var query1 = context.Items.Where(x => x.Price > 0).Select(x => x.Name);
var results1 = await query1.ToArrayAsync();
Console.WriteLine($"Items with price greater than 0 - {string.Join(',', results1)}");
var query2 = context.Items.Where(x => x.Price > -1m).Select(x => x.Name);
var results2 = await query2.ToArrayAsync();
Console.WriteLine($"Items with price greater than -1 - {string.Join(',', results2)}");
var query3 = context.Items.Where(x => decimal.Negate(x.Price) < 1).Select(x => x.Name);
var results3 = await query3.ToArrayAsync();
Console.WriteLine($"Items with price greater than -1 - {string.Join(',', results3)}");
Console.ReadLine();
}
The result of query2 produces Item 1, 2, 3 and 5, where only Item 1, 2, 3 are expected.

How to group by in LINQ?

I need to return the last 30 days of a speciefic user daily appointments and check if the user made at least 8 hours of appointments for each day.
in sql i can do that with this command:
select IDAppointment,IDUser, SUM(DurationInHours) from Note where AppointmentDate > *lastmonth and IDUser = #userID group by IDUser,IDAppointment,AppointmentDate
and after that i get the result and validate the DurationInHours(double type).
Is it possible to do it using LINQ?
Get the list of the last month user appointments and validate it day by day
Thanks!

This should be roughly there although this is off the top of my head as not at an IDE.
var result = context.Notes
.Where(n => [Your where clause])
.GroupBy(n => new { n.IDUser, n.IDAppointment, n.AppointmentDate})
.Select(g => new {
g.Key.IDAppointment,
g.Key.IDUser,
g.Sum(n => n.DurationInHours)});
UPDATE:
For reference your where clause will be something like this... (again off the top of my head)
DateTime lastMonth = DateTime.Today.AddMonths(-1);
int userId = 1 // TODO: FIX
var result = context.Notes.Where(n => n.AppointmentDate > lastMonth
&& n.IDUser = userId)
Resulting in....
DateTime lastMonth = DateTime.Today.AddMonths(-1);
int userId = 1 // TODO: FIX
var result = context.Notes
.Where(n => n.AppointmentDate > lastMonth
&& n.IDUser = userId)
.GroupBy(n => new { n.IDUser, n.IDAppointment, n.AppointmentDate})
.Select(g => new {
g.Key.IDAppointment,
g.Key.IDUser,
g.Sum(n => n.DurationInHours)});

Here is a solution which I tested.
DateTime lastMonth = DateTime.Today.AddMonths(-1);
int selectedUserId = 2;
var notes = new List<Note>(
new Note[] {
new Note() {
AppointmentDate = new DateTime(2013,7,30){},
IDAppointment = 1, IDUser = 1, DurationInHours = 1
},
new Note() {
AppointmentDate = new DateTime(2013,7,30){},
IDAppointment = 1, IDUser = 1, DurationInHours = 2
},
new Note() {
AppointmentDate = new DateTime(2013,7,30){},
IDAppointment = 1, IDUser = 1, DurationInHours = 3
},
new Note() {
AppointmentDate = new DateTime(2013,7,28){},
IDAppointment = 2, IDUser = 2, DurationInHours = 2
},
new Note() {
AppointmentDate = new DateTime(2013,7,28){},
IDAppointment = 2, IDUser = 2, DurationInHours = 3
},
new Note() {
AppointmentDate = new DateTime(2013,7,27){},
IDAppointment = 2, IDUser = 2, DurationI nHours = 4
},
new Note() {
AppointmentDate = new DateTime(2013,7,26){},
IDAppointment = 3, IDUser = 3, DurationInHours = 3
},
new Note() {
AppointmentDate = new DateTime(2013,7,25){},
IDAppointment = 3, IDUser = 3, DurationInHours = 4
},
new Note() {
AppointmentDate = new DateTime(2013,7,24){},
IDAppointment = 3, IDUser = 3, DurationInHours = 5
}
}
);
var results = from n in notes
group n by new {n.IDUser, n.IDAppointment, n.AppointmentDate}
into g
where g.Key.AppointmentDate > lastMonth &&
g.Key.IDUser == selectedUserId
select new {
g.Key.IDAppointment,
g.Key.IDUser,
TotalHours = g.Sum(n => n.DurationInHours)
};
The summation property needed to be given a name explicitly (i.e. TotalHours) or else you get error CS0746: Invalid anonymous type member declarator. Anonymous type members must be declared with a member assignment, simple name or member access.

Linq query to group by field1, count field2 and filter by count between values of joined collection

I'm having trouble with getting a my linq query correct. I've been resisting doing this with foreach loops because I'm trying to better understand linq.
I have following data in LinqPad.
void Main()
{
var events = new[] {
new {ID = 1, EventLevel = 1, PatientID = "1", CodeID = "2", Occurences = 0 },
new {ID = 2, EventLevel = 2, PatientID = "1", CodeID = "2", Occurences = 0 },
new {ID = 3, EventLevel = 1, PatientID = "2", CodeID = "1", Occurences = 0 },
new {ID = 4, EventLevel = 3, PatientID = "2", CodeID = "2", Occurences = 0 },
new {ID = 5, EventLevel = 1, PatientID = "3", CodeID = "3", Occurences = 0 },
new {ID = 6, EventLevel = 3, PatientID = "1", CodeID = "4", Occurences = 0 }
};
var filter = new FilterCriterion();
var searches = new List<FilterCriterion.Occurence>();
searches.Add(new FilterCriterion.Occurence() { CodeID = "1", MinOccurences = 2, MaxOccurences = 3 });
searches.Add(new FilterCriterion.Occurence() { CodeID = "2", MinOccurences = 2, MaxOccurences = 3 });
filter.Searches = searches;
var summary = from e in events
let de = new
{
PatientID = e.PatientID,
CodeID = e.CodeID
}
group e by de into t
select new
{
PatientID = t.Key.PatientID,
CodeID = t.Key.CodeID,
Occurences = t.Count(d => t.Key.CodeID == d.CodeID)
};
var allCodes = filter.Searches.Select(i => i.CodeID);
summary = summary.Where(e => allCodes.Contains(e.CodeID));
// How do I find the original ID property from the "events" collection and how do I
// eliminate the instances where the Occurences is not between MinOccurences and MaxOccurences.
foreach (var item in summary)
Console.WriteLine(item);
}
public class FilterCriterion
{
public IEnumerable<Occurence> Searches { get; set; }
public class Occurence
{
public string CodeID { get; set; }
public int? MinOccurences { get; set; }
public int? MaxOccurences { get; set; }
}
}
The problem I have is that need to filter the results by the MinOccurences and MaxOccurences filter property and in the end I want the "events" objects where the IDs are 1,2,3 and 4.
Thanks in advance if you can provide help.

To access event.ID at the end of processing you need to pass it with your first query. Alter select to this:
// ...
group e by de into t
select new
{
PatientID = t.Key.PatientID,
CodeID = t.Key.CodeID,
Occurences = t.Count(d => t.Key.CodeID == d.CodeID),
// taking original items with us
Items = t
};
Having done that, your final query (including occurrences filter) might look like this:
var result = summary
// get all necessary data, including filter that matched given item
.Select(Item => new
{
Item,
Filter = searches.FirstOrDefault(f => f.CodeID == Item.CodeID)
})
// get rid of those without matching filter
.Where(i => i.Filter != null)
// this is your occurrences filtering
.Where(i => i.Item.Occurences >= i.Filter.MinOccurences
&& i.Item.Occurences <= i.Filter.MaxOccurences)
// and finally extract original events IDs
.SelectMany(i => i.Item.Items)
.Select(i => i.ID);
This produces 1, 2 as result. 3 and 4 are left out as they don't get past occurrences filtering.

I have run your program in linqpad.
My understanding is that you want to filter using filter.MinOccurences and filter.MaxOccurences on Occurences count of result data set.
You can add additional filters using Where clause.
if (filter.MinOccurences.HasValue)
summary = summary.Where (x=> x.Occurences >= filter.MinOccurences);
if (filter.MaxOccurences.HasValue)
summary = summary.Where (x=> x.Occurences <= filter.MaxOccurences);

Help with this Linq query (many-to-many join)

I have three domain objects:
Child, Classroom and ChildClassroom. Here are lists of each:
var childrens = new List<Child>() {
new Child() { ChildId = 1, FirstName = "Chris" },
new Child() { ChildId = 2, FirstName = "Jenny" },
new Child() { ChildId = 3, FirstName = "Dave" },
};
var classrooms = new List<Classroom>() {
new Classroom() { ClassroomId = 1, FullName = "Kindergarten" },
new Classroom() { ClassroomId = 2, FullName = "Elementary" },
new Classroom() { ClassroomId = 3, FullName = "Secondary" },
};
var childclassrooms = new List<ChildClassroom>() {
new ChildClassroom() { ClassroomId = 1, ChildId = 1 },
new ChildClassroom() { ClassroomId = 2, ChildId = 1 },
new ChildClassroom() { ClassroomId = 3, ChildId = 2 },
};
What I want is:
var childClassroomRelationships = new object[] {
new {
childid = 1,
classrooms = new object[] {
new { classroomId = 1, occupied = true },
new { classroomId = 2, occupied = true },
new { classroomId = 3, occupied = false }
},
...
};
What's the way to go about this in Linq?

You could do this:
var childClassroomRelationships = (
from child in children
select {
childid = child.ChildId,
classrooms = (
from classroom in classrooms
select new {
classroomId = classroom.ClassroomId,
occupied = childclassrooms.Any(
cc => cc.ChildId == child.ChildId),
// Since you wanted an array.
}).ToArray()
// Since you wanted an array.
}).ToArray();
What's very important here is that a join should not be used here, if it was, you would get inner join semantics, which would cause children who are not in any classrooms to not show up (which it seems you don't want from the example you gave).
Note that this will materialize all sequences because of the calls to ToArray.
Also, it is slightly inefficient, in that to check the occupancy, it has to reiterate the entire childclassroms sequence every time.
This can be improved by "indexing" the childclassrooms map for efficient lookup, like so:
IDictionary<int, HashSet<int>> classroommap = (
from mapping in childclassrooms
group mapping.ClassroomId by mapping.ChildId into g
select g).ToDictionary(g => g.Key, g => new HashSet<int>(g));
This will give you a map of HashSet<int> instances which you can look up the child in once you know the classroom. With that, the first query becomes:
var childClassroomRelationships = (
from child in children
select {
childid = child.ChildId,
classrooms = (
from classroom in classrooms
select new {
classroomId = classroom.ClassroomId,
occupied = classroommap.ContainsKey(child.ChildId) &&
classroommap[child.ChildId].
Contains(classroom.ClassroomId),
// Since you wanted an array.
}).ToArray()
// Since you wanted an array.
}).ToArray();

var kidsInClass = (
from kid in childrens
from c in classrooms
select new {
ChildID = kid.ChildId,
classrooms = (
from cc in childclassrooms
select new {
ClassroomID = c.ClassroomId,
Occupied = cc.ChildId == kid.ChildId
}).ToArray()
}).ToArray();