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Counting number of items in ObservableCollection where it equals 1 - C#

I made an SQL query and filled the data to an ObservableCollection. The database contains many columns so I want to count how many instances where a specific column = 1, then return that number to an int.
The query:
var test = from x in m_dcSQL_Connection.Testheaders
where dtStartTime <= x.StartTime && dtEndtime >= x.StartTime
select new {
x.N,
x.StartTime,
x.TestTime,
x.TestStatus,
x.Operator,
x.Login,
x.DUT_id,
x.Tester_id,
x.PrintID
};
Then I add the data pulled from the database to an Observable Collection via:
lstTestData.Add(new clsTestNrData(item.N.ToString(),
item.StartTime.ToString(),
item.TestTime.ToString()
etc.....
I want to count how many times TestStatus = 1.
I have read about the .Count property but I do not fully understand how it works on ObservableCollections.
Any help?

The standard ObservableCollection<T>.Count property will give you the number of items in the collection.
What you are looking for is this:
testStatusOneItemCount = lstTestData.Where(item => item.TestStatus == 1).Count()
...which uses IEnumerable<T>.Count() method which is part of LINQ.

To elaborate a bit, Count will simply count the objects in your collection.
I suggest having a quick look at linq 101. Very good examples.
Here's an example:
// Assuming you have :
var list = new List<int>{1,2,3,4,5,6 };
var items_in_list = list.Count(); // = 6;
Using linq's Where, you're basically filtering out items, creating a new list. So, the following will give you the count of all the numbers which are pair:
var pair = list.Where(item => item%2 ==0);
var pair_count = pair.Count; // = 3
You can combine this without the temp variables:
var total = Enumerable.Range(1,6).Where(x => x % 2 ==0).Count(); // total = 6;
Or you can then select something else:
var squares_of_pairs = Enumerable.Range(1,6)
.Where(x => x % 2 ==0).
.Select( pair => pair*pair);
// squares_of_pairs = {4,16, 36}. You can count them, but still get 3 :)

C# List grouping and assigning a value

I have a list of Orders. This list contains multiple orders for the same item, see the table below.
I then want to assign each item that is the same (i.e. ABC) the same block ID. So ABC would have a block ID of 1 & each GHJ would have a block ID of 2 etc. What is the best way of doing this?
Currently I order the list by Order ID and then have a for loop and check if the current Order ID is equal to the next Order ID if so assign the two the same block ID. Is there a better way of doing this using linq or any other approach?
Order ID Block ID
ABC
ABC
ABC
GHJ
GHJ
GHJ
MNO
MNO

You can do this that way, it will assign same blockid for same orderid
var ordered = listOrder.GroupBy(x => x.OrderId).ToList();
for (int i = 0; i < ordered.Count(); i++)
{
ordered[i].ForEach(x=>x.BlockId=i+1);
}
it will group orders by orderid then assign each group next blockid. Note that it won't be done fully in linq, because linq is for querying not changing data.

Always depends of what better means for you in this context.
There are a bunch of possible solutions to this trivial problem.
On top of my head, I could think of:
var blockId = 1;
foreach(var grp in yourOrders.GroupBy(o => o.OrderId))
{
foreach(var order in grp)
{
order.BlockId = blockId;
}
blockId++;
}
or (be more "linqy"):
foreach(var t in yourOrders.GroupBy(o => o.OrderId).Zip(Enumerable.Range(1, Int32.MaxValue), (grp, bid) => new {grp, bid}))
{
foreach(var order in t.grp)
{
order.BlockId = t.bid;
}
}
or (can you still follow the code?):
var orders = yourOrders.GroupBy(o => o.OrderId)
.Zip(Enumerable.Range(1, Int16.MaxValue), (grp, id) => new {orders = grp, id})
.SelectMany(grp => grp.orders, (grp, order) => new {order, grp.id});
foreach(var item in orders)
{
item.order.BlockId = item.id;
}
or (probably the closest to a simple for loop):
Order prev = null;
blockId = 1;
foreach (var order in yourOrders.OrderBy(o => o.OrderId))
{
order.BlockId = (prev == null || prev.OrderId == order.OrderId) ?
blockId :
++blockId;
prev = order;
}
Linq? Yes.
Better than a simple loop? Uhmmmm....
Using Linq will not magically make your code better. Surely, it can make it often more declarative/readable/faster (in terms of lazy evaluation), but sure enough you can make otherwise fine imperative loops unreadable if you try to force the use of Linq just because Linq.
As a side note:
if you want to have feedback on working code, you can ask at codereview.stackexchange.com

Use LINQ to get datatable row numbers meeting certain conditions

How can I get an array of datatable row numbers which meet a certain criteria? For example I have a datatable with a column "DateTime". I want to retrieve the row numbers of the datatable where "DateTime" equals the variable startTime.
I know how to retrieve the actual row, but not the number of the row in the datatable.
Any help will be appreciated :)

int count = tblData.AsEnumerable()
.Count(row => row.Field<DateTime>("DateTime").Equals(startTime));
or as query:
int count = (from row in tblData.AsEnumerable()
where row.Field<DateTime>("DateTime").Equals(startTime)
select row).Count();

If I am reading the question right, using the overload of Select that allows a second input for the index may work for you. Something like
var indices =
table.AsEnumerable()
.Select((row, index) => new { row, index })
.Where(item => item.row.Field<DateTime?>("DateTime") == startTime)
.Select(item => item.index)
.ToArray();
If that date matches on the first, third, and sixth rows, the array will contain indices 0, 2, and 5. You can, of course, add 1 to each index in the query if you would like row numbers to start at 1. (ex: .Select(item => item.index + 1))

This is not possible. Note that with SQL (I assume you use SQL), the row order returned is not guaranteed. Your rows are ordered physically according to the primary key. So if you want a reliable row identifier, you must use your primary key number/id.

grouping and counting in linq-to-sql

I have the following query that receives a list of IDs and I want to do a count. There's also an object model CountModel that holds the counts with each property defined as an int.
public class GetCountByStatus(List<int> TheIDs)
{
...using MyDC...
var CountData = (from d in MyDC.Data
where TheIDs.Contains(d.ID)
group d by d.Status into statusgroup
select new CountModel()
{
CountStatus1 = (from g in statusgroup
where g.Status == 1
select g).Count(),
CountStatus2 = (from g in statusgroup
where g.Status == 2
select g).Count(),
CountStatusN = ....
}).Single();
If for instance there are no elements with status N, will this code crash or will the count be 0 for CountStatusN ? Is this the best way to do what I want?
Thanks.

I would go for a dictionary instead, try something like this:
var countData = MyDC.Data.Where(y => TheIDs.Contains(y.ID))
.GroupBy(y => y.Status).ToDictionary(y => y.Key, y => y.Count());
I haven't tried it my self and not written the code in VS, but I think that is almost how you do can do it. That will give you a dictionary where the key is the status and the value is the count of that status.
Defining a model with properties named SomethingX is not very flexible. That means you have to go in an change the model when there is a new status. Keeping the data in the dictionary instead will save you from that.

Count() will always return an integer, which is 0 if there are no elements with the given status. Therefore CountStatusN will always be an integer as well.

How can I subsample data from a time series with LINQ to SQL?

I have a database table full of time points and experimental values at those time points. I need to retrieve the values for an experiment and create a thumbnail image showing an XY plot of its data. Because the actual data set for each experiment is potentially 100,000 data points and my image is only 100 pixels wide, I want to sample the data by taking every nth time point for my image and ignoring the rest.
My current query (which retrieves all the data without sampling) is something simple like this:
var points = from p in db.DataPoints
where p.ExperimentId == myExperimentId
orderby p.Time
select new {
X = p.Time,
Y = p.Value
}
So, how can I best take every nth point from my result set in a LINQ to SQL query?

This will do every nth element:
int nth = 100;
var points = db.DataPoints
.Where(p => p.ExperimentId == myExperimentId)
.OrderBy(p => p.Time)
.Where( (p, index) => index % nth == 0 )
.Select( p => new { X = p.Time, Y = p.Value } );
It works by using the Queryable.Where overload which provides an index in the sequence, so you can filter based off the index.

.Skip(n).Take(1)
Will return one sample point. Call it repeatedly to get more points.
http://msdn.microsoft.com/en-us/library/bb386988.aspx
If performance becomes an issue, and you have a primary identity key of type int containing consecutive values, you can try returning all records that will evenly divide the primary key by your n.
.Where(x => x.PK % n == 0)

You should you
.Skip(n).Take(100)
It skips how many every record you want it to skip and Takes 100 records.
HTH