REPL, Julia v1.0.1: julia> a=1; julia> while true; print(a); break; end 1 julia> while true; a=2; break; end julia> while true; a=2; print(a); break; end 2 julia> while true; print(a); a=2; break; end ERROR: UndefVarError: a not defined Stacktrace: [1] top-leve…

Yes, but you wrote if after print(a). There are two fairly simple things going on here: Having a = ... anywhere in a scope makes a local. If you access a variable before it has been assigned, it’s an undef error. Evaluation order does not matter for 1—it’s purely syntactic. Evaluation order does …

Thank you. Now I see what you mean: they are semantically equivalent for the computer. With my reply above to you “Not quite…” I meant they are not equivalent from the user perspective/expectation.

[image] Vic: I still hold 1st opinion though, quite strongly. I will probably elaborate in another post, in “Development” section. I think you’re going to find this very helpful in your research. There’s no need for duplication of that topic.

ONE MORE QUESTION HERE I’ve been told repeatedly that An explicit global is needed to change a global variable. Then why this works (again Repl, v 1.01) ? julia> v=[0 0]; julia> while true; v[1]=1; break; end julia> v 1×2 Array{Int32,2}: 1 0

global is needed to rebind a variable which is a completely different thing from mutating a container. v[1] = 1 is just pretty syntax for setindex!(v, 1, 1).

OK. But the effect is exactly the same as v=[1 0] ! I thought the intent of all these globals was to prevent modifying stuff in global scope unintentionally From https://docs.julialang.org/en/v1/manual/variables-and-scoping/: Avoiding changing the value of global variables is considered by many…

[image] Vic: But the effect is exactly the same as v=[1 0] ! No, it is definitely not. Case 1, using v[1] = 1 v = [0 0] x = v v[1] = 1 x printing x now shows [1 0]. Case 2, using v = [1 0] v = [0 0] x = v v = [1 0] x printing x now shows [0 0]. Scoping is about what value (or object) a…

I don’t understand how your explanation is relevant to what I was saying (I don’t blame you). I knew that in Julia assignment can bind several names to the same object, and mutating the object via one name results in all names pointing now to the new object But in the context I talked about, I use…

[image] Vic: With s =[0 0] , then does not s[1]=1 result in now symbol s being bound to a new object, [1 0] , as compared to [0 0] ? No, it means that the object s has been modified but it is the same object. julia> s = [0 1]; julia> pointer_from_objref(s) Ptr{Nothing} @0x000000010e6324d0 …

[image] kristoffer.carlsson: No, it means that the object s has been modified but it is the same object. That’s sounds like a logical self-contradiction to me :face_with_monocle: You are showing that with mutation, the address of the object remains the same: but then you interpret this to me…

[image] Vic: That’s sounds like a logical self-contradiction to me It isn’t though. There is a difference between repainting a house and building a new identical house with a new color. As a case where this would matter, burning down the new identical house is likely ok while burning down the…

Understanding while loop UndedVarError and scope

General Usage

Topic		Replies	Views
Global scope confusion again General Usage question , scope	52	7001	May 17, 2025
REPL and for loops (scope behavior change) General Usage	42	15355	September 16, 2020
Explain scoping confusion to a programming beginner New to Julia question	26	3358	August 11, 2020
Local variable inside a loop: why it ERRORs New to Julia question	14	442	September 27, 2025
Strange loop behavior when assigning variable within loop New to Julia question	5	570	March 8, 2019

Understanding while loop UndedVarError and scope

Related topics