I have a Dictionary of objects I have created in smalltalk, which I am iterating over by enumerating it based on the key/value pairs.
For value object in the dictionary, I am calling a method on that object. Based on certain conditions, I would like for this object to be able to add a new member to dictionary, and possibly delete another one.
I've been looking at the 'Perform' and 'Messages' facilities in Smalltalk, but I'm not sure if it is even possible to do what I'm trying to do - is it possible to return a message (or multiple messages), which another object can process and perform?
For example, could my method return 'removeKey: 19' and 'add object' at the same time?
I am using GNU Smalltalk, if it matters.
When you iterate over the collection, pass the collection as part of the argument:
aCollection copy do: [:each | each doSomethingOn: aCollection]
The copy ensures that the #doSomethingOn: can alter the original collection without messing up the iteration.
A Smalltalk method can't return multiple values, but it can return a Collection containing those values:
foo
^ Array with: 1 with: 2.
So you return a Collection with multiple methods, and just iterate over it, sending the messages in the Collection.
The class Message can do what you want:
(Message selector: #raisedTo: argument: 2) sendTo: 3
That produces "9" when evaluated.
Note, adding or removing things from a collection while iterating over it generally is not a good idea. Try copying the collection first, iterating over the copy and modifying the original from within the block being used to iterate over the copy.
As you iterate through, add the items you want to add to aDictionary, to aTemporaryDictionaryOfAdds, and the items you want to delete to aTemporaryDictionaryOfDeletes
Then iterate through each of those, adding to and removing from aDictionary as you do.
If you add the deletes to aDictionaryOfThingsDeletedFromADictionary, you have a history, too.
Related
I would like to populate a listbox with a list of installed printers in VB.net.
This works:
Dim printerList As System.Drawing.Printing.PrinterSettings.StringCollection
printerList = System.Drawing.Printing.PrinterSettings.InstalledPrinters
For Each printerName In printerList
ListBox1.Items.Add(printerName)
Next
This does not work:
ListBox1.Items.AddRange(printerList)
...because of the following type-conversion error:
Public Sub AddRange (value As
System.Windows.Forms.ListBox.ObjectCollection)': Value of type
'System.Drawing.Printing.PrinterSettings.StringCollection' cannot be
converted to 'System.Windows.Forms.ListBox.ObjectCollection'.
Is it possible to directly cast one to the other for use in AddRange() as shown? Or is the loop the only (or most efficient) way?
Well, you're dealing with 2 collections that were created before the more modern generic lists and enumerables, so their use is less fluid.
In this case, the AddRange method accepts either another ObjectCollection instance (not your case), or an array of Objects. If you want to benefit from the latter, you'll need to transform the StringCollection instance to an array of Objects. Here is how this can be done:
ListBox1.Items.AddRange(printerList.Cast(Of Object)().ToArray())
That said, I would stick with your current For Each loop. It is very readable, and doesn't create an intermediate array. But, I doubt either choice will make much difference, so pick your favorite.
I understand arrays. I know some java from 15 years ago, and I know about classes, objects, instances, variables, static variables and constructors. Not so familiar with these things in VB.
I don't understand object collections..
Suppose I draw a listbox, and name it lstbox1
I see that I can say lstbox1.items.item(0) or lstbox1.items(0)
The fact that I can say listbox1.items(0) puzzles me a little bit. If an object collection is not an array, then it's clearly not an object either.
This link https://msdn.microsoft.com/en-us/library/yb7y698k(v=vs.90).aspx says Collection is an object.
But then what is items(0) items is not a class so that can't be calling a constructor... and items is not a method, it's a property that is an instance of object collection, so I can't see how the (0) works.. I know what it refers to the first object, the element with index 0, but I don't understand how that can work. I know blah(0) would work if blah was an array. And I am sure lstbox1.items is not an array of object collections it's just 1 object collection.
Is it a data structure like an Array, that has its own syntax.. for example one can say dim blah as Integer() or dim blah() as Integer and thus define it without even stating the class Array. Is ObjectCollection a bit like that? It does seem to allow (index) after an instance of it.
VB has a concept called Default Properties. In the case of an ItemCollection type (and a number of other types, as well), the Item property is the default property for the collection. This allows you to use the shorthand from the question.
It's basically just a bit of syntactical sugar. When you say, lstbox1.items(0), it's just shorthand for lstbox1.items.item(0).
Also, don't mistake the various collection types for simple arrays. They will have similar syntax, but every collection type has it's own quirks and use cases, and it's generally worth your time to look at the documentation for the specific type you're working with. Don't assume something is an array, just because you can access the items by index.
Think about this just as a "language feature". This is what really called indexer property. It is implemented as default property in vb.net. In c# implementation is different. The data structure behind it could be anything you want - array, list, dictionary, hashtable. The fact is - it lets you access something by supplying a parameter without calling property syntax. myParentObject(1) instead of myParentObject.GetChildObject(1)
In VB, default property must be indexed.
listbox1 . Items . item(0) --> listbox1 - main object that has property Items , which is a collection. This collection has property item, which is default property or indexer. Item is a property exposing single object from underlying collection.
Let's say we have a Class and we instantiate it, creating an Instance of that Class. This Instance has a number of (instance)variables, defined by the class, that I need to use. I'd like to get all these (instance)variables in an Array or some Collection so I can iterate through them and set them to some value, not nil.
How can I do this?
I would like to build up on #Uko's answer because there is a more direct way to implement his idea.
The message instSize sent to a Class will answer the number of named instance variables of its instances. This, of course, would include instance variables defined in superclasses.
For instance, RemoteTempVectorNode instSize answers with 17 (wow!). Therefore you could do:
fields := (1 to: anObject class instSize) collect: [:i | anObject instVarAt: i]
and then, change them with:
values withIndexDo: [:v :i | anObject instVarAt: i put: v]
where values is the array of new values you want to inject into the object.
So, why I'm suggesting this instead of instVarNamed:? Because the latter is indirect. If you take a look at its implementation you will see that it has to first find out the name of the i-th ivar by sending instVarIndexFor:ifAbsent: to the object's class. In other words, if you need the ivar names, follow #Uko's suggestion; otherwise don't bring them into the equation because they will only add CPU cycles to your program.
One more thing. As #Sean DeNegris wisely raised in his comment to your question, it would be beneficial if you elaborated a little bit more on why you need such an unusual maneuver.
EDIT:
Now that Pharo has Flexible Object Layouts the mapping between inst var names and the class instSize is no longer valid (in classes that use the new capability.) So, the simpler approach of using just indexes would not work with generality. In fact, under the new "taxonomy" the instSize (number of fields) of an object may be different from the #numberOfInstanceVariables. I guess that the added flexibility has its costs and benefits.
You can send #allInstVarNames to a class (Behavior) to get names of all instance variables defined by it and by superclasses. If you need without superclass variables, you can use #instVarNames
Let's say that var is your variable that you need to work with. Then you can get the collection of instance variable names and iterate them.
You can use #instVarNamed:put: to set instance variable by name, and #instVarNamed: to get the value by name (in case you need).
I think that something like this may help you:
var class allInstVarNames do: [ :instVarName |
var instVarNamed: instVarName put: <yourValue>
I am passing a few optional arguments to a function as a tuple, since all of these have to be passed together or not at all. I would like to be able to iterate over the elements of the tuple numerically, and perform an operation on each item. For example:
Public Function myFunction(Optional t As Tuple(Of Integer, String, SomeType) = Nothing) As Integer
For i = 0 to 2
someCollection(i).someMethod(t(i)) 'Pseudocode for accessing ith item in tuple
Next
End Function
One way to resolve the problem would be to use a list, but then I lose the ability to enforce the number of members (which will always be fixed) and the types of each member. Another way would be to write out the statement three times with t.Item1, t.Item2 etc, but this is ugly.
Is there any way to access the nth item in a tuple?
Note: I would like to accomplish this with a tuple if at all possible, even though I am aware I could create alternate method signatures.
(Sure, I’ll turn this into an answer!)
You can put the items into an array for convenience; maintaining the type isn’t really an issue at that point, since if you’re doing the same thing with all of them they need to have some sort of common base class or interface.
Dim a() As Object = {t.Item1, t.Item2, t.Item3}
Then just iterate over that.
I am trying to build a createEntity OEntity for an object that has multiple child collections within it.
I have looked over many of the example projects, but they all seem to assume that you have a known number of child objects in a collection so that you can use .inLine(“ObjectName”, ObjectOEntity1, ObjecteOEntity2…)
I have tried looking at the documentation and have not identified anything that leads me to think I can create a collection of OEntity objects that can then be added to my parent object with the inline.
The closest I found was the example listed on:
http://code.google.com/p/odata4j/source/browse/odata4j-fit/src/test/java/org/odata4j/producer/jpa/northwind/test/CreateTest.java?name=0.6
Has anyone else run into this problem?
If so how did you get around it?
You can pass in an array of OEntity objects. The core4j library that is used by odata4j contains some helper methods that can - for example - be used to get an array from an Iterable:
OEntity[] entitiesArray = Enumerable.create(entitiesIterable)
.toArray(OEntity.class);
But as there are also two variants of the properties method...
OCreateRequest<T> properties(OProperty<?>... props);
OCreateRequest<T> properties(Iterable<OProperty<?>> props);
... it might make sense to add an inline method that directly takes an Iterable<OEntity>.