CODEWITHSUNDEEP
vb.net
user2622509
user2622509

Reputation: 117

How to edit a linkedlistnode's value

I am working on a program that creates a linkedlist having a structure as nodes. At a certain point I need to change some values inside of a particular node (structure). Here is some code:

Structure tp
   Dim a as double
   Dim b as integer
End Structure

Sub main()
   Dim lst as new LinkedList(of tp)
   Dim node as tp
   node.a = 10.1
   node.b = 1
   lst.addfirst(node)

   '......
   ' Now I want to change the value of first node
   lst.first.value.a = 2 
   ' But I get "Expression is a value and therefore cannot be the target of an assignment."

End Sub

How do I perform such a modification?

Upvotes: 0

Views: 184

Answers (2)

Victor Zakharov
Victor Zakharov

Reputation: 26434

The solution is simple - replace your structure with a class and adjust usage accordingly:

Class tp
  Public a As Double
  Public b As Integer
End Class

Sub main()
  Dim lst As New LinkedList(Of tp)
  Dim node As New tp
  node.a = 10.1
  node.b = 1
  lst.AddFirst(node)
  lst.First.Value.a = 2
End Sub

Problem was that structures are passed by value, unlike classes, so Value will return a copy of the object, rather than a copy of the reference to that object. In the down-on-earth terms it means you cannot change the original object. With a class, you can.

I would always recommend using classes, probably because I am more used to them, and so biased to recommend them. But if you decide to use a Structure, please make sure you are aware of the differences:

Upvotes: 2

tweellt
tweellt

Reputation: 2171

Your LinkedList is of type tp so its value is a tp type object.

In order to change the value you have to provide a new structure:

dim newtp as tp
newtp.a = 2
newtp.b = lst.first.value.b

Then AddBefore this newtp and Remove the old one.

Upvotes: 0

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