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 …

julia> a=1; julia> while true; print(a); break; end 1 julia> while true; q=2; break; end julia> while true; q=2; print(q); break; end 2 julia> while true; print(q); q=2; break; end is an equivalent version of the code in OP.

I expected a to have a value for 2 reasons: a=1 present before the while loop. while loop body introducing by default a local scope is a strange (to say the least) idea to me once Julia can tell a is made local, even though a=2 occurs after print(a), somehow I expected it to also “see” the valu…

Not at all; my version illustrates the conflict between a local and global variable with same name, and user confusion it creates. Your version is not surprising at all, IMO.

The two versions are semantically equivalent. Your thread title was “Understanding while loop UndedVarError and scope”, my example was there to help you do that.

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…

Understanding while loop UndedVarError and scope

General Usage

Sukera October 22, 2018, 6:18pm 12

I think you’re going to find this very helpful in your research. There’s no need for duplication of that topic.

Topic		Replies	Views
Global scope confusion again General Usage question , scope	52	7663	May 17, 2025
REPL and for loops (scope behavior change) General Usage	42	15806	September 16, 2020
Explain scoping confusion to a programming beginner New to Julia question	26	3573	August 11, 2020
Local variable inside a loop: why it ERRORs New to Julia question	14	625	September 27, 2025
Strange loop behavior when assigning variable within loop New to Julia question	5	630	March 8, 2019

Understanding while loop UndedVarError and scope

Related topics