Android Question LIST2 = SHUFFELIST(LIST1) also changes LIST1, Why?

[FERNANDO SILVEIRA]

Hello Guys,

I have the following code to shuffle LIST1 into LIST2, but at the end both lists are shuffled. I want LIST1 as it was originally.

What am I doing wrong?

    Dim lstWords1, lstWords2 As List
    lstWords1 = Array("AVIÃO", "BOLA", "CASA", "COPO", "DADO", "FACA", "FLOR", "FOGO", "FOTO", "GATO", "LEÃO", "MALA", "MENINA", "MENINO", "QUEIJO", "SUCO", "TREM", "XÍCARA", "CÂMARA")
'    lstWords1 = File.ReadList(File.DirAssets, "tabpalavras.txt")
    Log("LIST1 before shuffle: " & lstWords1)
    lstWords2.Initialize2(ShuffleList(lstWords1))
    Log("LIST2  after shuffle: " & lstWords2)  
    Log("LIST1  after shuffle: " & lstWords1)


Sub ShuffleList(StList As List) As List
    Dim wElement As String
    Dim wIndex As Int
    For i = 0 To StList.size - 1
        wIndex = Rnd(i, StList.size)
        wElement = StList.get(i)
        StList.set(i, StList.get(wIndex))
        StList.set(wIndex, wElement)
    Next
    Return StList
End Sub

Regards,
Fernando

 

[Mahares]

What am I doing wrong?

This is how I would do it:

Log("LIST1  before shuffle: " &lstWords1)
    lstWords2.Initialize
    lstWords2.AddAll(lstWords1)
    lstWords2=ShuffleList(lstWords2)
    Log("LIST2  after shuffle: " &lstWords2)
    Log("LIST1  after shuffle: " & lstWords1)

 

[FERNANDO SILVEIRA]

Mahares, it worked your way. Thanx

I just don't understand the concept behind my way and yours. Maybe @Erel could provide me more details.

 

[Mahares]

When you use shufflelist(lstwords1) in your code, you are re-shulling lstwords1 again which you do not want to do.

 

[OliverA]

What am I doing wrong?

When you pass a non-primitive to a sub or when you assign it to a different variable, a copy of the reference is passed.
This means that the data itself isn't duplicated.

Source: https://www.b4x.com/android/forum/threads/variables-objects-in-basic4android.8385/#content

 

[klaus]

From the List help:
List.Initialize2
Method
Initializes a list with the given values. This method should be used to convert arrays to lists.
Note that if you pass a list to this method then both objects will share the same list,
and if you pass an array the list will be of a fixed size. Meaning that you cannot later add or remove items.

 

[LucaMs]

From the List help:
List.Initialize2
Method
Initializes a list with the given values. This method should be used to convert arrays to lists.
Note that if you pass a list to this method then both objects will share the same list,
and if you pass an array the list will be of a fixed size. Meaning that you cannot later add or remove items.

So, to be quite, use always List.Inizialize

 

[FERNANDO SILVEIRA]

Thank you all.
I was not familiar with the concept primitive/non-primitive types.

Cheers!