I have been trying to figure this out for some time now and can't seem to figure out an answer to it. I don't see why this would be impossible. I am coding in VB.NET.
Here is my problem:
I need to dynamically create variables and be able to reference them later on in the code.
More Details:
The number of variables comes from some math run against user defined values. In this specific case I would like to just create integers, although I foresee down the road needing to be able to do this with any type of variable. It seems that my biggest problem is being able to name them in a unique way so that I would be able to reference them later on.
Simple Example:
Let's say I have a value of 10, of which I need to make variables for. I would like to run a loop to create these 10 integers. Later on in the code I will be referencing these 10 integers.
It seems simple to me, and yet I can't figure it out. Any help would be greatly appreciated. Thanks in advance.
The best way to do something like this is with the Dictionary(T) class. It is generic, so you can use it to store any type of objects. It allows you to easily store and retrieve code/value pairs. In your case, the "key" would be the variable name and the "value" would be the variable value. So for instance:
Dim variables As New Dictionary(Of String, Integer)()
variables("MyDynamicVariable") = 10 ' Set the value of the "variable"
Dim value As Integer = variables("MyDynamicVariable") ' Retrieve the value of the variable
You want to use a List
Dim Numbers As New List(Of Integer)
For i As Integer = 0 To 9
Numbers.Add(0)
Next
The idea of creating a bunch of named variables on the fly is not something you are likely to see in any VB.Net program. If you have multiple items, you just store them in a list, array, or some other type of collection.
'Dim an Array
Dim xCount as Integer
Dim myVar(xCount) as String
AddButton Event . . .
xCount += 1
myVar(xCount) = "String Value"
'You will have to keep Track of what xCount Value is equal to to use.
'Typically could be an ID in A DataTable, with a string Meaning
Related
Here is what i have tried so far , but no new shot variable is being declared
Module Module1
Dim shotlist As New List(Of Boolean)
Dim shono As Integer = 0
Dim shonos As String
Dim shotname As String
Dim fshot As Boolean
Dim shots As String
Sub Main()
For i As Integer = 0 To 1000
Dim ("shots" & i) as String = "shots" & i
fshot = Convert.ToBoolean(shots)
Next
End Sub
End Module
You can't do things this way. The variable names you see when you write a program are lost, turned into memory addresses by the compiler when it compiles your program. You cannot store any information using a variable's name - the name is just a reference for you, the programmer, during the time you write a program (design time)
Think of variables like buckets, with labels on the outside. Buckets only hold certain kinds of data inside
Dim shotname as String
This declares a bucket labelled shotname on the outside and it can hold a string inside. You can put a string inside it:
shotname = "Shot 1"
You can put any string you like inside this bucket, and thus anything you can reasonably represent as a string can also be put inside this bucket:
shotname = DateTime.Now.ToString()
This takes the current time and turns it into a string that looks like a date/time, and puts it in the bucket. The thing in the bucket is always a string, and lots of things (nearly anything actually) can be represented as a string, but we don't type all our buckets as strings because it isn't very useful - if you have two buckets that hold numbers for example, you can multiply them:
Dim a as Integer
a=2
Dim b as Integer
b=3
Dim c as Integer
c=a*b
But you can't multiply strings, even if they're strings trust look like numbers; strings would have to be converted to numbers first. Storing everything as a string and converting it to some other type before working on it then converting it back to a string to store it would be very wearisome
So that's all well and good and solves the problem of storing varying information in the computer memory and giving it a name that you can reference it by, but it means the developer has to know all the info that will ever be entered into the program. shotname is just one bucket storing one bit of info. Sure you could have
Dim shotname1 as String
Dim shotname2 as String
Dim shotname3 as String
But it would be quite tedious copy paste exercise to do this for a thousand shotname, and after you've done it you have to refer to all of them individually using their full name. There isn't a way to dynamically refer to bucket labels when you write a program, so this won't work:
For i = 0 To 1000
shotname&i = "This is shotname number " & i
Next i
Remember, shotname is lost when compiling, and i is too, so shotname&i just flat out cannot work. Both these things become, in essence, memory addresses that mean something to the compiler and you can't join two memory addresses together to get a third memory address that stores some data. They were only ever nice names for you to help you understand how to write the program, pick good names and and not get confused about what is what
Instead you need the mechanism that was invented to allow varying amounts of data not known at the design-time of the program - arrays
At their heart, arrays are what you're trying to do. You're trying to have load of buckets with a name you can vary and form the name programmatically.
Array naming is simple; an array has a name like any other variable (what I've been calling a bucket up to now - going to switch to calling them variables because everyone else does) and the name refers to the array as a whole, but the array is divided up into multiple slots all of which store the same type of data and have a unique index. Though the name of the array itself is fixed, the index can be varied; together they form a reference to a slot within the array and provide a mechanism for having names that can be generated dynamically
Dim shotnames(999) as String
This here is an array of 1000 strings, and it's called shotnames. It's good to use a plural name because it helps remind you that it's an array, holding multiple something. The 999 defines the last valid slot in the array - arrays start at 0, so with this one running 0 to 999 means there are 1000 entries in it
And critically, though the shotnames part of the variable name remains fixed and must be what you use to refer to the array itself(if you want to do something with the entire thing, like pass it to a function), referring to an individual element/slot in the array is done by tacking a number onto the end within brackets:
shotnames(587) = "The 588th shotname"
Keep in mind that "starts at 0 thing"
This 587 can come from anything that supplies a number; it doesn't have to be hard coded by you when you write the program:
For i = 0 to 999
shotnames(i) = "The " & (i+1) & "th shotname"
Next i
Or things that generate numbers:
shotnames(DateTime.Now.Minute) = "X"
If it's 12:59 when this code runs, then shotnames(59), the sixtieth slot in the array, will become filled with X
There are other kinds of varying storage; a list or a dictionary are commonly used examples, but they follow these same notions - you'll get a variable where part of the name is fixed and part of the name is an index you can vary.
At their heart they are just arrays too- if you looked inside a List you'd find an array with some extra code surrounding it that, when the array gets full, it makes a new array of twice the size and copies everything over. This way it provides "an array that expands" type functionality - something that arrays don't do natively.
Dictionaries are worth mentioning because they provide a way to index by other things than a number and can achieve storage of a great number of things not known at design time as a result:
Dim x as New Dictionary(Of String, Object)
This creates a dictionary indexed by a string that stores objects (I.e. anything - dates, strings, numbers, people..)
You could do like:
x("name") = "John"
x("age") = 32
You could even realize what you were trying to do earlier:
For i = 0 to 999
x("shotname"&i) = "The " & (i+1) & "th shotname"
Next i
(Though you'd probably do this in a Dictionary(Of string, string), ie a dictionary that is both indexed by string and stores strings.. and you probably wouldn't go to this level of effort to have a dictionary that stores x("shotname587") when it's simpler to declare an array shotname(587))
But the original premise remains true: your variable has a fixed part of the name (ie x) and a changeable part of the name (ie the string in the brackets) that is used as an index.
And there is no magic to the indexing by string either. If you opened up a dictionary and looked inside it, you'd find an array, together with some bits of code that take the string you passed in as an index, and turn it into a number like 587, and store the info you want in index 587. And there is a routine to deal with the case where two different strings both become 587 when converted, but other than that it's all just "if you want a variable where part of the name is changeable/formable programmatically rather than by the developer, it's an array"
I want to be able to allow the user to input a string and then use that string to create a different variable. For example:
Dim UserInput As String
UserInput = Console.Readline()
Dim (UserInput) As String
So if the user input "Hello" a new variable called 'Hello' would be made.
Is this possible and if not, are there any alternatives?
The names of variables must be known at compile-time. What you're asking for here is to wait until run-time to know the name. That just doesn't work
What you can do is use a Dictionary(Of String, String) to hold your data.
This code works:
Dim UserInput As String
UserInput = Console.Readline()
Dim UserInputData As New Dictionary(Of String, String)
UserInputData(UserInput) = "Some Value"
Console.WriteLine(UserInputData(UserInput))
So if you enter Bar then the dictionary has this value in it:
The bottom-line is that everything is known at compile-time, but you can store data based on the user input at run-time.
It is currently not possible to dynamically create single variables in VB.net. However, you can use lists or dictionaries (source: Dynamically create variables in VB.NET)
The name of the variable does not matter to the user because they will never see it. They will only ever see the data it holds. There really is no reason for this
Dim policy_key() As RenewalClaim.PolicyKeyType
policy_key(0).policyEffectiveDt = date_format_string(ld_EffectiveDate)
Getting error at Line2.
An Error occured - Object reference not set to an instance of an object.
Each element of object arrays also needs to be declared as a new object too.
Dim policy_key() As RenewalClaim.PolicyKeyType
Redim policy_key(0)
policy_Key(0) = new RenewalClaim.PolicyKeyType
policy_key(0).policyEffectiveDt = date_format_string(ld_EffectiveDate)
QUICK TIP: When declaring classes structures etc, it is useful to name them so you can see what type they are....
e.g.
cls_Policy_Key for a class
str_Policy_Key for a structure etc.
When you come back to your code after a year.. you will thank yourself for doing so.
Dim policy_key() As RenewalClaim.PolicyKeyType
is part of your problem. When you are declaring policy_key() you are actually declaring it as an array with no elements. If you don't particularly need to use an array, for example, if you don't need to add objects to a particular element number, you might be better using a list and declaring it like this
Dim policy_key As New List(Of RenewalClaim.PolicyKeyType)
This way, you can add items easily without having to resize your array each time - The code is a little longer than Trevor's answer, but less prone to errors when you extend your code -
dim newPolicy_Key as RenewalClaim.PolicyKeyType
newPolicy_Key.policyEffectiveDt = date_format_string(ld_EffectiveDate)
policy_Key.add(newPolicyKey)
I have a bunch of type Double variables in my program, say for example
Dim Area as Double = 0
Dim Perimeter as Double = 0
Somewhere in my program I want to calculate these values, so I define
Public Sub TheSquare(ByRef TheArea as Double, ByRef ThePerim as Double, ByVal TheSide as Double)
TheArea = TheSide^2
ThePerim = 4 * TheSide
End Sub
and somewhere in the program I'm collecting side lengths and calculating the area and perimeter; say
While True
S = GetSideValueFromSomewhere()
TheSquare(Area, Perimeter, S)
End
In my real program, I have, say, 20 quantities that I want to calculate. Obviously each one has a different equation. But in the end I want to output all 20 to a file, so to save typing, I create an array of the quantities, like this:
Dim TypingSaver() as Double = {Area, Perimeter}
so that I can dump values to file with a three-line for-loop instead of copying and pasting 20 variable names.
This does exactly what I want if Area and Perimeter were reference types like Objects. But since they are Doubles, TypingSaver contains only their values, not references to the actual variables. So after I use my TheSquare function the values of Area and Perimeter are correctly updated but TypingSaver just constains whatever the values of Area and Perimeter were when I declared the array.
So the question is: how can I create an array of references to doubles in VB.NET?
With approach that you are doing you can't do this, since as soon as you created array you copied all variables to the array and any changes that you are doing on variables are not reflected on array variables (like you pointed out).
What I would recommend create another class that will contain all your variables (20 variables name) as properties (get and set) and then override ToString method which will return list of all your variables. So when you need to dump those variables you will call ToString() method and it will return current values of all your parameters.
There is a nasty way to do this. All numeric types are value types, but arrays are reference types. So I can change all my Doubles to arrays of doubles, like this:
Dim Area(0) as Double
Dim Perimeter(0) as Double
So now Area and Perimeter are 1-element arrays of double. My "looping array" then becomes
Dim TypingSaver() as Array = {Area, Perimeter}
Now TypingSaver stores references to Area and Perimeter. For me this was an easy change because I could search-and-replace for the Double declaration, change the type for TypingSaver, then in two other places I had to change direct access of this form:
TypingSaver(1) = 7
to
TypingSaver(1).SetValue(7, 0)
Not pretty, but it keeps my code consistent in that I have other "looping arrays" for other objects that are all related to each other.
Although it is not clear in my question the real solution is to restructure everything so instead of storing everything in arrays I crate a class which has all the objects I need and create a single array of that, as suggested in part by Blezden.
The single-element array idea was actually terrible. The code became horrendously slow. There is another work around a friend suggested: create a wrapper class like this:
Public Class DoubleWrapper
Public Value As Double
End Class
Then when an array of DoubleWrappers is created it will be by reference, of course.
I understand this is not ideal but what I was looking for was a workaround until I have the time to rewrite the code from scratch.
Fairly similar to your most recent answer, you'll want to wrap your types as "Nullable", which basically makes it an object that could be null, but also a reference type.
Ie. Dim testDouble as Nullable(Of Double) or Dim testDouble2 as Double?
See:
Nullable(Of T) Structure - MSDN
What is the integer reference type in C#?
Dim A As Collection
Set A = New Collection
Dim Arr2(15, 5)
Arr2(1,1) = 0
' ...
A.Add (Arr2)
How can I access the Arr2 through A? For example, I want to do the following:
A.Item(1) (1,1) = 15
so the above would change the first element of the first two-dimensional array inside the collection...
Hmmm...the syntax looks legal enough without having VBA in front of me. Am I right that your problem is that your code "compiles" and executes without raising an error, but that the array in the collection never changes? If so, I think that's because your A.Item(1) might be returning a copy of the array you stored in the collection. You then access and modify the chosen element just fine, but it's not having the effect you want because it's the wrong array instance.
In general VBA Collections work best when storing objects. Then they'll work like you want because they store references. They're fine for storing values, but I think they always copy them, even "big" ones like array variants, which means you can't mutate the contents of a stored array.
Consider this answer just a speculation until someone who knows the underlying COM stuff better weighs in. Um...paging Joel Spolsky?
EDIT: After trying this out in Excel VBA, I think I'm right. Putting an array variant in a collection makes a copy, and so does getting one out. So there doesn't appear to be a direct way to code up what you have actually asked for.
It looks like what you actually want is a 3-D array, but the fact that you were tyring to use a collection for the first dimension implies that you want to be able to change it's size in that dimension. VBA will only let you change the size of the last dimension of an array (see "redim preserve" in the help). You can put your 2-D arrays inside a 1-D array that you can change the size of, though:
ReDim a(5)
Dim b(2, 2)
a(2) = b
a(2)(1, 1) = 42
ReDim Preserve a(6)
Note that a is declared with ReDim, not Dim in this case.
Finally, it's quite possible that some other approach to whatever it is you're trying to do would be better. Growable, mutable 3-D arrays are complex and error-prone, to say the least.
#jtolle is correct. If you run the code below and inspect the values (Quick Watch is Shift-F9) of Arr2 and x you will see that they are different:
Dim A As Collection
Set A = New Collection
Dim Arr2(15, 5)
Arr2(1, 1) = 99
' ...
A.Add (Arr2) ' adds a copy of Arr2 to teh collection
Arr2(1, 1) = 11 ' this alters the value in Arr2, but not the copy in the collection
Dim x As Variant
x = A.Item(1)
x(1, 1) = 15 ' this does not alter Arr2
Maybe VBA makes a copy of the array when it assigns it to the collection? VB.net does not do this. After this code, a(3,3) is 20 in vba and 5 in vb.net.
Dim c As New Collection
Dim a(10, 10) As Integer
a(3, 3) = 20
c.Add(a)
c(1)(3, 3) = 5
I recently had this exact issue. I got round it by populating an array with the item array in question, making the change to this array, deleting the item array from the collection and then adding the changed array to the collection. Not pretty but it worked....and I can't find another way.
If you want the collection to have a copy of the array, and not a reference to the array, then use the array.clone method:-
Dim myCollection As New Collection
Dim myDates() as Date
Dim i As Integer
Do
i = 0
Array.Resize(myDates, 0)
Do
Array.Resize(myDates, i + 1)
myDates(i) = Now
...
i += 1
Loop
myCollection.Add(myDates.Clone)
Loop
At the end, myCollection will contain the accumulative collection of myDates().