Structs and foreach loops

This topic contains 11 replies, has 6 voices, and was last updated by nerobot 1 year, 6 months ago.

Viewing 12 posts - 1 through 12 (of 12 total)

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Posts

June 24, 2017 at 4:39 pm #8891

Participant

I have a List<Vec2f> and I want to iterate it and change it’s contents, after a few minutes I realized that structs because are handled by value would not be mutated as referenced classes do.

If you have any workaround on this I would be interested to know.

June 24, 2017 at 8:14 pm #8893

abakobo

Participant

You may use a temporary pointer.
Local structPtr:myStruct Ptr
structPtr=varptr myStructInstance
structPtr->x=123

Never tried it though..
If you want a non temporary pointer you’ll have to keep your structs alive.

June 24, 2017 at 8:39 pm #8894

Ethernaut

Participant

Hmmm, couldn’t figure it out with List… here’s how I do it with Stacks, though.

#Import  "<std>"
Using std..

Struct MyStruct
	Field value := 0.0
End

Function Main()
	
	Local myList := New Stack< MyStruct >
	
	'Populates list
	For Local n := 0 Until 20
		myList.Add( New MyStruct )
	Next
	
	'Set value, copy struct back into list
	For Local n := 0 Until myList.Length
		Local s := myList[n]
		s.value = Rnd( 100.0 )
		myList[n] = s
	End
	
	'read values
	For Local n := 0 Until myList.Length
		Print myList[n].value
	End

End

#Import "<std>"

Using std..

Struct MyStruct

Field value := 0.0

End

Function Main()

Local myList := New Stack< MyStruct >

'Populates list

For Local n := 0 Until 20

myList.Add( New MyStruct )

'Set value, copy struct back into list

For Local n := 0 Until myList.Length

Local s := myList[n]

s.value = Rnd( 100.0 )

myList[n] = s

End

'read values

For Local n := 0 Until myList.Length

Print myList[n].value

End

June 25, 2017 at 2:49 am #8896

Danilo

Participant

Ethernaut’s code changed to use a list of Vec2f:

#Import  "<std>"

Using std..

 
Function Main()
    
    Local list := New List< Vec2f >
    
    'Populates list
    For Local n := 0 Until 20
        list.Add( New Vec2f )
    Next
    
    '
    ' Set value, copy struct back into list
    '
    Local iterator := list.All()

    While Not iterator.AtEnd
        Local item := iterator.Current

        item.x = Rnd( 100.0 )

        iterator.Current = item

        iterator.Bump()
    Wend
    
    '
    ' read values
    '
    For Local item := Eachin list
        Print item.x
    End
 
End

#Import "<std>"

Using std..

Function Main()

Local list := New List< Vec2f >

'Populates list

For Local n := 0 Until 20

list.Add( New Vec2f )

' Set value, copy struct back into list

Local iterator := list.All()

While Not iterator.AtEnd

Local item := iterator.Current

item.x = Rnd( 100.0 )

iterator.Current = item

iterator.Bump()

Wend

' read values

For Local item := Eachin list

Print item.x

End

Assign new value:

#Import  "<std>"

Using std..

 
Function Main()
    
    Local list := New List< Vec2f >
    
    'Populates list
    For Local n := 0 Until 20
        list.Add( New Vec2f )
    Next
    
    '
    ' Set new value
    '
    Local iterator := list.All()

    While Not iterator.AtEnd
        iterator.Current = New Vec2f( Rnd(100.0), Rnd(100.0) )
        iterator.Bump()
    Wend
    
    '
    ' read values
    '
    For Local item := Eachin list
        Print item.x + " : " + item.y
    End
 
End

#Import "<std>"

Using std..

Function Main()

Local list := New List< Vec2f >

'Populates list

For Local n := 0 Until 20

list.Add( New Vec2f )

' Set new value

Local iterator := list.All()

While Not iterator.AtEnd

iterator.Current = New Vec2f( Rnd(100.0), Rnd(100.0) )

iterator.Bump()

Wend

' read values

For Local item := Eachin list

Print item.x + " : " + item.y

End

June 25, 2017 at 5:01 pm #8919

cocon

Participant

Oh, I see!

By looking at Monkey’s List source code, my guessing is I see that the All() method might cooperate with the Eachin statement in the background. The All method however returns a new Iterator, not the existing (the essense of the elements) one.

I did this experiment and I see that I can’t mutate the element of the list directly, however I can mutate the local copy. Is this behavior valid or not?

#Import "<std>"
Using std..

Struct MyStruct
	Field StructValue := 0
	Method To:String()
		Return "MyStruct: " + StructValue
	End
End

Function Main()
	
	Local list := New List<MyStruct>
	For Local n := 0 Until 5
		Local s := New MyStruct
		list.Add(s)
	Next
	
	Local item := list.FirstNode().Value
	item.StructValue = 1000
	
	Print(list.FirstNode().Value)
	Print(item)
	
End

#Import "<std>"

Using std..

Struct MyStruct

Field StructValue := 0

Method To:String()

Return "MyStruct: " + StructValue

End

Function Main()

Local list := New List<MyStruct>

For Local n := 0 Until 5

Local s := New MyStruct

list.Add(s)

Local item := list.FirstNode().Value

item.StructValue = 1000

Print(list.FirstNode().Value)

Print(item)

End

June 25, 2017 at 10:04 pm #8920

Mark Sibly

Keymaster

I did this experiment and I see that I can’t mutate the element of the list directly, however I can mutate the local copy. Is this behavior valid or not?

Yes, because structs are always passed ‘by value’ (ie: copied), so this line…

Local item := list.FirstNode().Value

…copies FirstNode().Value to item.

One way to deal with thinking about this stuff is to mentally replace ‘some struct type’ with, say, ‘int’ (also a struct type) in which case you’ve simply got a list of Ints, and hopefully it’s clear that…

Local item := list.FirstNode().Value

…will set item to a *copy* of FirstNode().Value as, like all struct types, ints are copied when passed to functions, returned from functions or assigned to variables.

June 26, 2017 at 12:34 am #8923

Danilo

Participant

List extension ‘ForEach’ & ‘ForEachBackwards’ for use with structs:

#Import  "<std>"

Using std..

Class List<T> Extension

    Method ForEach:Void( func:T(item:T) )
        Local iterator := Self.All()
        While Not iterator.AtEnd
            iterator.Current = func(iterator.Current)
            iterator.Bump()
        Wend
    End

    Method ForEachBackwards:Void( func:T(item:T) )
        Local iterator := Self.Backwards()
        While Not iterator.AtEnd
            iterator.Current = func(iterator.Current)
            iterator.Bump()
        Wend
    End

End


Function Main()
    '
    ' Randomize Rnd values
    '
    Sleep( Rnd(1.0) )
    SeedRnd( Microsecs() )
    
    '
    ' Create a linked list using Vec2f type
    '
    Local lst := New List< Vec2f >
    
    '
    ' Add elememts to the list
    '
    For Local n := 0 Until 10
        lst.Add( New Vec2f )
    Next
    
    '
    ' Set values
    '
    lst.ForEach( Lambda:Vec2f( element:Vec2f )
       Global count:Int = 0
       count += 1

       element.x = count
       element.y = Rnd( 100.0 )

       Return element
    End )

    '
    ' print values
    '
    For Local item := Eachin lst
        Print item.x + "~t" + item.y
    End

    Print "----------"

    '
    ' Change all 'x' values in the list
    '
    lst.ForEachBackwards( Lambda:Vec2f( element:Vec2f )
       element.x += 100
       Return element
    End )

    '
    ' print values
    '
    For Local item := Eachin lst
        Print item.x + "~t" + item.y
    End

End

#Import "<std>"

Using std..

Class List<T> Extension

Method ForEach:Void( func:T(item:T) )

Local iterator := Self.All()

While Not iterator.AtEnd

iterator.Current = func(iterator.Current)

iterator.Bump()

Wend

End

Method ForEachBackwards:Void( func:T(item:T) )

Local iterator := Self.Backwards()

While Not iterator.AtEnd

iterator.Current = func(iterator.Current)

iterator.Bump()

Wend

End

Function Main()

' Randomize Rnd values

Sleep( Rnd(1.0) )

SeedRnd( Microsecs() )

' Create a linked list using Vec2f type

Local lst := New List< Vec2f >

' Add elememts to the list

For Local n := 0 Until 10

lst.Add( New Vec2f )

' Set values

lst.ForEach( Lambda:Vec2f( element:Vec2f )

Global count:Int = 0

count += 1

element.x = count

element.y = Rnd( 100.0 )

Return element

End )

' print values

For Local item := Eachin lst

Print item.x + "~t" + item.y

End

Print "----------"

' Change all 'x' values in the list

lst.ForEachBackwards( Lambda:Vec2f( element:Vec2f )

element.x += 100

Return element

End )

' print values

For Local item := Eachin lst

Print item.x + "~t" + item.y

End

June 26, 2017 at 1:28 am #8929

Mark Sibly

Keymaster

Nice!

September 25, 2017 at 2:39 pm #10803

cocon

Participant

Another question is how can I send the struct to a function for updating?

This currently does not work:

#Import  "<std>"
Using std..
 
Function Main()
	
	Local items := New List<Vec2f>
	
	For Local n := 0 Until 10
		items.Add(New Vec2f(Rnd(1,10), Rnd(1,10)))
	Next
	
	PrintItems(items)
	
	ResetItems(Varptr items)
	
	PrintItems(items)
 
End

Function ResetItems(items:List<Vec2f> Ptr)
	Local iter := items->All()
	While Not iter.AtEnd
		iter.Current.X = 0
		iter.Current.Y = 0
		iter.Bump()
	End
End

Function PrintItems(items:List<Vec2f>)
	Print("Items")
	For Local i := Eachin items
		Print(i)
	Next
End

#Import "<std>"

Using std..

Function Main()

Local items := New List<Vec2f>

For Local n := 0 Until 10

items.Add(New Vec2f(Rnd(1,10), Rnd(1,10)))

PrintItems(items)

ResetItems(Varptr items)

PrintItems(items)

End

Function ResetItems(items:List<Vec2f> Ptr)

Local iter := items->All()

While Not iter.AtEnd

iter.Current.X = 0

iter.Current.Y = 0

iter.Bump()

End

Function PrintItems(items:List<Vec2f>)

Print("Items")

For Local i := Eachin items

Print(i)

End

September 25, 2017 at 8:40 pm #10806

Mark Sibly

Keymaster

Same problem – Current is returning a *copy* of the iterator item. Modifying this copy has no effect.

Actually, if you just use Vec2’s ‘x’ and ‘y’ fields directly (not ‘X’ and ‘Y’ properties), eg: iter.Current.x=0, you will get a compile time error, because the compiler ‘knows’ you are trying to modify a copy. However, the compiler doesn’t know exactly what X or Y properties do – they may or may not actually modify Self – and I didn’t want to just disallow property/method calls in general on returned copies or subject people to the joys of ‘const correctness’, so it’s a bit of a half-assed check.

The solution is to get current item, modify it, and then set it back (aka read-modify-write), eg:

#Import  "<std>"
Using std..
 
Function Main()
	
	Local items := New List<Vec2f>
	
	For Local n := 0 Until 10
		items.Add(New Vec2f(Rnd(1,10), Rnd(1,10)))
	Next
	
	PrintItems(items)
	
	ResetItems(Varptr items)
	
	PrintItems(items)
 
End
 
Function ResetItems(items:List<Vec2f> Ptr)
	Local iter := items->All()
	While Not iter.AtEnd
		Local v:=iter.Current
		v.X=0
		v.Y=0
		iter.Current=v
		iter.Bump()
	End
End
 
Function PrintItems(items:List<Vec2f>)
	Print("Items")
	For Local i := Eachin items
		Print(i)
	Next
End

#Import "<std>"

Using std..

Function Main()

Local items := New List<Vec2f>

For Local n := 0 Until 10

items.Add(New Vec2f(Rnd(1,10), Rnd(1,10)))

PrintItems(items)

ResetItems(Varptr items)

PrintItems(items)

End

Function ResetItems(items:List<Vec2f> Ptr)

Local iter := items->All()

While Not iter.AtEnd

Local v:=iter.Current

v.X=0

v.Y=0

iter.Current=v

iter.Bump()

End

Function PrintItems(items:List<Vec2f>)

Print("Items")

For Local i := Eachin items

Print(i)

End

Same applies to Stacks etc.

However, you don’t need to do this with plain arrays, eg: myarray[i].X=0 will work fine, as an array element is really just a special type of variable (which is why you can Varptr it).

This means you can in fact do a little hack with stacks where you can set elements directly:

mystack.Data[i].X=0

This is because the Data property of Stack returns the underlying array. You do need to be careful ‘i’ is in range though, as array may be longer than Stack!

September 26, 2017 at 7:14 am #10814

cocon

Participant

Oh, I see! So this assignment statement is really important from what it seems.

September 26, 2017 at 7:33 am #10815

nerobot

Participant

This means you can in fact do a little hack with stacks where you can set elements directly

Hm, very interesting!

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Structs and foreach loops

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