What is the best way to format code with chained methods? Especially if it goes on for a long time? If you have a chain of three or so, you can put it on one line, but it gets cumbersome after you have a lot and it makes debugging difficult.
FYI, I'm talking about this: http://en.wikipedia.org/wiki/Method_chaining
Sometimes I write code like this (in Java):
DetachedCriteria criteria = DetachedCriteria.forClass(Taskdsr.class);
criteria=criteria.add(someRestriction);
criteria=criteria.add(someOtherRestriction);
criteria=criteria.setFetchMode(Criteria.DISTINCT_ROOT_ENTITY);
in place of:
DetachedCriteria criteria = DetachedCriteria.forClass(Taskdsr.class).add(someRestriction).add(someOtherRestriction).setFetchMode(Criteria.DISTINCT_ROOT_ENTITY);
You can format it across multiple lines:
DetachedCriteria criteria = DetachedCriteria.forClass(Taskdsr.class)
.add(someRestriction)
.add(someOtherRestriction)
.setFetchMode(Criteria.DISTINCT_ROOT_ENTITY);
Related
This works:
connection = get_connection()
cursor=connection.cursor()
cursor.execute('show application_name')
application_name_of_connection=cursor.fetchone()[0]
But why four lines? Is there no way to get this in one line?
No, there is not a way to do this in one line. Languages such as Python are designed to describe a set of instructions. There are four instructions here. There may even be helper methods or clever/inefficient arrangements of these statements that could lower this to three lines, but you are best to keep it how it is.
For example, if you are for some reason using this code frequently, you would encapsulate it in its own method
def get_application_name_of_connection()
connection = get_connection()
cursor=connection.cursor()
cursor.execute('show application_name')
return cursor.fetchone()[0]
And then simply:
get_application_name_of_connection()
This is how it works. You want less code? Hide the functionality.
It can be done with comprehension but on sqlite:
results = [row[0] for row in get_connection().cursor().execute('select * from mytable')]
If you are using postgresql then it doesn't work, you could try this for a more compact form:
with get_connection().cursor() as cur:
cur.execute('show all')
print cur.fetchone()
But only if you need the cursor thrown away after the block execution.
I didn't tried on other db managers, results may vary.
I'm just looking to be able to sort the results of a BatchedJoinBlock (http://msdn.microsoft.com/en-us/library/hh194683.aspx) so that the different results of the different targets stay together. I will explain! Example in some pseudo-code:
Dim batchedJoin = New BatchedJoinBlock(Of String, object)(4)
batchedJoin.Target1.Post("String1Target1")
batchedJoin.Target2.Post(CType(BuildIt, StringBuilder1))
batchedJoin.Target1.Post("String1Target2")
batchedJoin.Target2.Post(CType(BuildIt, StringBuilder2))
Dim results = batchedJoin.Receive()
'This sorts one result...
Dim SortByResult = results.Item1.OrderBy(Function(item) item.ToString, New NaturalStringComparer)
Basically I've got a string and an object, the SortByResult variable above sorts the strings exactly as I'd like them to sort. I'm looking for a way to get the objects that used to be at the same index number in target2 into the same order. e.g. if "String1Target1" changes order I'd like to somehow reliably refer to/pair it together with "StringBuilder1". The actual end result just needs to be that the objects (target2) are sorted in the order that is dictated by the strings being sorted (target1). Something like:
Dim EndResult = results.Item2.OrderBy(strings in target1)
but I'll gladly take an intermediate solution! I've also tried using a dictionary (results.Item2.ToDictionary) with the string as a key (which would also be a fine solution) but it's a bit beyond my ken using lamba expressions in the proper context. I can realistically do this in several steps with a list or something, but I'm trying to get something more efficient/learn something, and it seems like there's a lot of default options with the results of the jointblock that I'm just not experienced enough to use. Thanks in advance for any help you can provide!
To me, it looks like you don't actually want BatchedJoinBlock, because the two pieces of data always come together. A better option for that would be a BatchBlock of Tuple<string, object>. When you have that, you can then use LINQ directly to sort each batch:
results.OrderBy(Function(tuple) tuple.Item1)
Using VB.Net, I'm looking for best practices in dealing with the following idiom:
For i as Integer = 0 To o1.Count - 1
o1(i).x = o2(i).x
Next
What I really want is something VB.Net doesn't offer -- additional and simultaneous iterators on For Each statements.
For Each m1 As c1 In o1, m2 As c2 In o2
m1.x = m2.x
Next
I'm interested in both Linq and non-Linq recommendations, comments about Copy method design, comparisions to C# or other languages.
I'm sure your actual real world situation is more complicated than the example you have shown, but I thought it was worth mentioning that, if all you are doing is trying to copy an entire list of items from one list to another, you can simply do something like this:
Dim list2 = New List(Of Object)(list1)
I'm not entirely sure what you are trying to do with this. We could probably help you better if you elaborate a little more on your problem.
If both lists are of the same Type you could append one list to the other and then use a for each on just one list.
I submitted a request to extend the For Each statement to implement additional and simultaneous iterators.
http://visualstudio.uservoice.com/forums/121579-visual-studio/suggestions/2923571-allow-additional-clauses-on-for-each-similar-to-c-
I'm still interested in other solutions but until then, this is the answer.
Given a datatable containing two columns like this:
Private Function CreateDataTable() As DataTable
Dim customerTable As New DataTable("Customers")
customerTable.Columns.Add(New DataColumn("Id", GetType(System.Int32)))
customerTable.Columns.Add(New DataColumn("Name", GetType(System.String)))
Dim row1 = customerTable.NewRow()
row1.Item("Id") = 1
row1.Item("Name") = "Customer 1"
customerTable.Rows.Add(row1)
Dim row2 = customerTable.NewRow()
row2.Item("Id") = 2
row2.Item("Name") = "Customer 2"
customerTable.Rows.Add(row2)
Dim row3 = customerTable.NewRow()
row3.Item("Id") = 3
row3.Item("Name") = "Customer 3"
customerTable.Rows.Add(row3)
Return customerTable
End Function
Would you use this snippet to retrieve a List(Of Integer) containing all Id's:
Dim table = CreateDataTable()
Dim list1 As New List(Of Integer)
For i As Integer = 0 To table.Rows.Count - 1
list1.Add(CType(table.Rows(i)("Id"), Integer))
Next
Or rather this one:
Dim list2 = (From r In table.AsEnumerable _
Select r.Field(Of Integer)("Id")).ToList()
This is not a question about whether to type cast the Id column to Integer by using .Field(Of Integer), CType, CInt, DirectCast or whatever but generally about whether or not you choose Linq over forloops as the subject implies.
For those who are interested: I ran some iterations with both versions which resulted in the following performance graph:
graph http://dnlmpq.blu.livefilestore.com/y1pOeqhqQ5neNRMs8YpLRlb_l8IS_sQYswJkg17q8i1K3SjTjgsE4O97Re_idshf2BxhpGdgHTD2aWNKjyVKWrQmB0J1FffQoWh/analysis.png?psid=1
The vertical axis shows the milliseconds it took the code to convert the rows' ids into a generic list with the number of rows shown on the horizontal axis. The blue line resulted from the imperative approach (forloop), the red line from the declarative code (linq).
Whatever way you generally choose: Why do you go that way and not the other?
Whenever possible I favor the declarative way of programming instead of imperative. When you use a declarative approach the CLR can optimize the code based on the characteristics of the machine. For example if it has multiple cores it could parallelize the execution while if you use an imperative for loop you are basically locking this possibility. Today maybe there's no big difference but I think that in the future more and more extensions like PLINQ will appear allowing better optimization.
I avoid linq unless it helps readability a lot, because it completely destroys edit-and-continue.
When they fix that, I will probably start using it more, because I do like the syntax a lot for some things.
For almost everything I've done I've come to the conclusion that LINQ is optimized enough. If I handcrafted a for loop it would have better performance, but in the grand scheme of things we are usually talking milliseconds. Since I rarely have a situation where those milliseconds will make any kind of impact, I find it's much more important to have readable code with clear intentions. I would much rather have a call that is 50ms slower than have someone come along and break it altogether!
Resharper has a cool feature that will flag and convert loops into Linq expressions. I will flip it to the Linq version and see if that hurts or helps readability. If the Linq expression more clearly communicates the intent of the code, I will go with that. If the Linq expression is unreadable, I will flip back to the foreach version.
Most of the performance issues don't really compare with readability for me.
Clarity trumps cleverness.
In the above example, I would go with the the Linq version since it clearly explains the intent and also locks out people accidently adding side effects in the loop.
I recently found myself wondering whether I've been totally spoiled by LINQ. Yes, I now use it all the time to pick all sort of things out from all sort of collections.
I started to, but found out in some cases, I saved time by using this approach:
for (var i = 0, len = list.Count; i < len; i++) { .. }
Not necessarily in all cases, but some. Most extension methods use the foreach approach of querying.
I try to follow these rules:
Whenever I'm just querying (filtering, projecting, ...) collections, use LINQ.
As soon as I'm actually 'doing' something with the result (i.e, introduce side effects), I'll use a for loop.
So in this example, I'll use LINQ.
Also, I always try to split up the 'query definition' from the 'query evaluation':
Dim query = From r In table.AsEnumerable()
Select r.Field(Of Integer)("Id")
Dim result = query.ToList()
This makes it clear when that (in this case in-memory) query will be evaluated.
I'm working on my first project using LINQ (in mvc), so there is probably something very simple that I missed. However, a day of searching and experimenting has not turned up anything that works, hence the post.
I'm trying to write a LINQ query (Linq to SQL) that will contain a multiple number of conditions in the where statement separated by an OR or an AND. We don't know how many conditions are going to be in the query until runtime. This is for a search filter control, where the user can select multiple criteria to filter by.
select * from table
where table.col = 1
OR table.col = 2
OR table.col = 7
.... 'number of other conditions
Before I would just construct the SQL query as a string while looping over all conditions. However, it seems like there should be a nice way of doing this in LINQ.
I have tried looking using expression trees, but they seem a bit over my head for the moment. Another idea was to execute a lambda function inside the where statement, like so:
For Each value In values
matchingRows = matchingRows.Where(Function(row) row.col = value)
However, this only works for AND conditions. How do I do ORs?
I would use PredicateBuilder for this. It makes dynamic WHERE clauses very easy.
AND is easy - you can just call Where in a loop. OR is much trickier. You mention SQL, so I'm assuming this is something like LINQ-to-SQL, in which case one way I've found to do this involves building custom Expression trees at runtime - like so (the example is C#, but let me know if you need help translating it to VB; my VB isn't fantastic any more, so I'll let you try first... you can probably read C# better than I can write VB).
Unfortunately, this won't work with EF in 3.5SP1 (due to the Expression.Invoke), but I believe this is fixed in 4.0.
Something like this should work (forgive my VB):
Expression(Of Func(Of Something, Boolean)) filter = Nothing
ParameterExpression rowParam = Expression.Parameter("row", CType(Something))
For Each value In values
filterPart = Expression.Equal( _
Expression.Property(rowParam, "col"), _
Expression.Constant(value)))
If filter Is Nothing Then
filter = filterPart
Else
filter = Expression.OrElse(filter, filterPart)
End If
Next
If newPredicate IsNot Nothing Then
matchingRows = matchingRows.Where( _
Expression.Lambda(Of Func(Of SomeType, Boolean))(filter, rowParam))
End If
No guarantees, however, my VB is a little rusty :-)
But PredicateBuilder might be a better solution if you want to do more complicated stuff than just Ands and Ors.