Explaining Lists

[pliroforikos]

Good day to all,
In this thread Erel explaining how to assign Arrays in List Items.
In the first code writes

Dim arr(3) As Int
Dim List1 As List
List1.Initialize
For i = 1 To 5
arr(0) = i * 2
arr(1) = i * 2
arr(2) = i * 2
List1.Add(arr) 'Add the whole array as a single item
Next
arr = List1.Get(0) 'get the first item from the list
Log(arr(0)) 'What will be printed here???

It is something like this and i understand why this is happens.

I don't understand the next code where he declares the same array inside for loop

Dim arr(3) As Int 'This call is redundant in this case.
Dim List1 As List
List1.Initialize
For i = 1 To 5
Dim arr(3) As Int
arr(0) = i * 2
arr(1) = i * 2
arr(2) = i * 2
List1.Add(arr) 'Add the whole array as a single item
Next
arr = List1.Get(0) 'get the first item from the list
Log(arr(0)) 'Will print 2

Why the previous arrays is not lost? If i log(Arr(0)) after for loop what are we gonna see?

 

[ilan]

For i = 1 To 5
Dim arr(3) As Int
arr(0) = i * 2
arr(1) = i * 2
arr(2) = i * 2
List1.Add(arr) 'Add the whole array as a single item
Next

you are adding on each cycle an array to the list.
if you want to set the array inside a loop then set it and after loop cycle is finish add the array to the list.

Dim arr(5) As Int
Dim List1 As List
List1.Initialize

For i = 0 To 4
    arr(i) = i * 2
Next

List1.Add(arr) 'Add the whole array as a single item

dim newarr() as int = List1.Get(0) 'get the first item from the list
Log(newarr(0)) '0

 

[ilan]

or maybe i misunderstood you, what exactly do you want to do?

you can do it also like this if you want to add on each cycle a new array to the list

    Dim arr(3) As Int 'This call is redundant in this case.
    Dim List1 As List
    List1.Initialize
    For i = 1 To 5
        Dim arr(3) As Int
        arr(0) = i * 2
        arr(1) = i * 2
        arr(2) = i * 2
        List1.Add(arr) 'Add the whole array as a single item
    Next
    Dim newarr() As Int = List1.Get(0) 'get the first item from the list
    Log(newarr(0)) 'Will print 2

 

[pliroforikos]

For i = 1 To 5
Dim arr(3) As Int
arr(0) = i * 2
arr(1) = i * 2
arr(2) = i * 2
List1.Add(arr) 'Add the whole array as a single item
Next

you are adding on each cycle an array to the list.
if you want to set the array inside a loop then set it and after loop cycle is finish add the array to the list.

Dim arr(5) As Int
Dim List1 As List
List1.Initialize

For i = 0 To 4
    arr(i) = i * 2
Next

List1.Add(arr) 'Add the whole array as a single item

dim newarr() as int = List1.Get(0) 'get the first item from the list
Log(newarr(0)) '0

So the arr Array lost after entered to the list?

 

[ilan]

I don't understand the next code where he declares the same array inside for loop

it is not the same array it is a new array. isinde the LOOP you are declaring a new array with:

 Dim arr(3) As Int

 

[ilan]

So the arr Array lost after entered to the list?

no you can access all array if you want to:

    Dim arr(3) As Int 'This call is redundant in this case.
    Dim List1 As List
    List1.Initialize
    For i = 1 To 5
        Dim arr(3) As Int
        arr(0) = i * 2
        arr(1) = i * 2
        arr(2) = i * 2
        List1.Add(arr) 'Add the whole array as a single item
    Next
    Dim arr() As Int = List1.Get(3) 'get the first item from the list
    Log(arr(0)) 'Will print 8

this will print 8

Waiting for debugger to connect...
Program started.
8

 

[ilan]

define what you want to get and i will write the code

 

[pliroforikos]

Well i don't want to create a certain code but to explain what happens with lists.

I know that lists doesn't store any value only references. So how list reference to 3 arrays with same array name? Or maybe array name is lost but data still remain in memory?

 

[ilan]

I know that lists doesn't store any value only references.

as far as i know this is not true. you create an object and put it inside a list that holds this object. now you can refer to the object that is holded in the list.

a list is not just pointing to objects that are created somewhere in the class. if it was like this then this code would never work:

    Dim l As List
    l.Initialize
    
    For i = 0 To 9
        Dim txt As String = "item" & i
        l.Add(txt)
    Next

    Log(l)

Waiting for debugger to connect...
Program started.
(ArrayList) [item0, item1, item2, item3, item4, item5, item6, item7, item8, item9]

as you can see i am declaring a new variable (string) and add it to a list with a value. now all strings are stored inside the list.

if the list was declared as a global variable i could access it from anywhere in the class so it is storing data inside and not just storing reference to some objects.
it is storing the objects themselves.

 

[Erel]

o how list reference to 3 arrays with same array name?

The variable name is not important.

For i = 1 To 5
 Dim arr(3) As Int
 arr(0) = i * 2
 arr(1) = i * 2
 arr(2) = i * 2
 List1.Add(arr) 'Add the whole array as a single item
Next

A new array is created each iteration.

 

[pliroforikos]

ok, now i understand
I misunderstand the list values. Now all explained!

Thank you

 

[pliroforikos]

So that happens?

and if yes why 1st code at my first post doesn't work?

 

[Erel]

Dim arr(3) As Int

This important line creates a new array of ints with 3 slots and assigns it to a local variable name arr.

I'm not sure that I understand the illustration.

Another example:

Dim arr() As Int = Array As Int(1, 2, 3)
Log(arr(0)) '1

Dim arr2() As Int = arr
arr2(0) = 100
Log(arr(0)) '100

Dim arr(3) As Int
arr(0) = 200
Log(arr(0)) '200
Log(arr2(0)) '100

 

[pliroforikos]

This is easy cos the different names of variables
The picture above means that list store values inside it not references .

 

[Erel]

This is easy cos the different names of variables

No. Two arrays are created in this example.
arr points to the first one and then to the second.
arr2 points to the first one.

The picture above means that list store values inside it not references .

The list stores references to arrays.

 

[pliroforikos]

Well, forgive me but i'm still confused.
So lists stores references to values and not the values and its not important the variable name of value... ?
Or it has to do with primitives and non-primitives types. Primitives types stored by value and non-primitives by reference?

 

[Sandman]

Perhaps this would be easier if we updated the example to use other variable names..?

 

[pliroforikos]

Perhaps this would be easier if we updated the example to use other variable names..?

No it's not useful to declare more variables. But ok, i think i got the idea and now i can explain it
to my students... I hope ?
Thank you all

 

[Sandman]

Other does not mean more.

 

[Erel]

I'll give another example. It is logically correct (under the hood there is special treatment for primitives but it is only done for performance reasons).

Dim x As Int = 3 'x points to an int object with the value 3. This is an immutable object. All ints are immutable objects.
x = 4 'now x points to a different int object.
Dim y As Int = x 'y points to the same object that x points to. y is not a reference or alias of x. A copy of the pointer is made.
x = 5 'y still points to 4
Dim s1 As String = "abc" 's1 points to the immutable string object with the value "abc"
Dim s2 As String = s1 's2 was assigned with a copy of the pointer to "abc"
s1 = s1 & "d" 's1 points to a new immutable string object
Log(s2) 'abc
Dim sb As StringBuilder 'sb points to a StringBuilder object. StringBuilder is a mutable object, this means that the object state can be changed.
sb.Initialize
sb.Append("abc") 'sb still points to the same object. The only way to make sb point to a different object is with the '=' operator or by calling Dim.
Dim sb2 As StringBuilder = sb 'a copy of the pointer to the single StringBuilder was made.
sb2.Appean("d")
Log(sb.ToString) 'abcd
Log(sb2.ToString) 'abcd