Re: Passing a sequence by reference to a data type

[ameerwasi]

Thanks, guys, I really appreciate your help, this community, and the welcoming 
nature of this forum.

Re: Passing a sequence by reference to a data type

[solo989]

Use ref seq instead. shallow only works inside procs and not globally. Also 
it's meant for optimization only and shouldn't be relied upon to provide 
reference semantics since it may choose not to if it's more efficient not to. 
It really should only be used in the case where your copying from a var 
variable but don't plan to ever touch that var variable again so copying it by 
reference is harmless. Regardless it's probably a premature optimization in 
most cases.

Re: Passing a sequence by reference to a data type

[Stefan_Salewski]

Yes, SolitudeSF is right:


type
  lis = ref object of RootObj
v : ref seq[int]

var s : ref seq[int] = new seq[int]
s[] = @[1, 2, 3, 4, 5]
let list = lis()
list.v = s
echo list.v[]
s[].add(13)
echo list.v[]


Run

We have to call new explizit in this case. And I was a bit surprised that we 
need actually the subscript operator in the second last line, as often the 
dereference operation is not needed in Nim.

Re: Passing a sequence by reference to a data type

[SolitudeSF]

var s = new seq[int]


Run

this should work

Re: Passing a sequence by reference to a data type

[ameerwasi]

@Stefan_Salewski I don't know if I understood your intent properly but I tried 
doing this, 


type
  lis = ref object of RootObj
v : ref seq[int]


var s : ref seq[int]
s[] = @[1, 2, 3, 4, 5]
let list = lis()
list.v = s


Run

the runtime threw an illegal storage access error at line 7 where I said 


s[] = @[1, 2, 3, 4, 5]

Run

, like this 


Traceback (most recent call last)
/usercode/in.nim(7)  in
/playground/nim/lib/system/assign.nim(147) genericSeqAssign
/playground/nim/lib/system/assign.nim(111) genericAssign
/playground/nim/lib/system/assign.nim(67) genericAssignAux
/playground/nim/lib/system/gc.nim(255) unsureAsgnRef
SIGSEGV: Illegal storage access. (Attempt to read from nil?)


Run

Re: Passing a sequence by reference to a data type

[Stefan_Salewski]

I have no idea what shallowCopy(list.v, s) should do in this case.

Your v : seq[int] and your let s = @[1, 2, 3, 4, 5] both defines value types. 
Well v is a field of a reference to an object, but v is still seq, and seq are 
value types in Nim, when v get filled with data v allocates its own data buffer 
and stores its values there. I would guess what you want is v : ref seq[int]. 
Maybe with


#let s = @[1, 2, 3, 4, 5]
var s = ref seq[int]
s[].add(1) # or s[] = @[1, 2, 3, 4, 5]
list.v = s


Run

Passing a sequence by reference to a data type

[ameerwasi]

Ok, it's only been a few days of me learning nim, so this may as well be one of 
the basic things but, Idk. So my problem is that when I make a sequence and 
pass it's shallow copy to a data type and add to it, only the sequence inside 
the data type gets added to. To better illustrate this, here's a piece of code 


type
  lis = ref object of RootObj
v : seq[int]


let s = @[1, 2, 3, 4, 5]
let list = lis()
shallowCopy(list.v, s)

list.v.add(6)
echo list.v
echo s


Run

and it outputs 


@[1, 2, 3, 4, 5, 6]
@[1, 2, 3, 4, 5]


Run

but I expected it to output this 


@[1, 2, 3, 4, 5, 6]
@[1, 2, 3, 4, 5, 6]


Run

Now, if this can't be done with sequences, can you please point me to a data 
structure that allows push, pop, and insertion where this can be done. Thanks 
in advance.