I have been using Julia for all my codes. I was explaining all the advantages of Julia over Python to my lab members and one of them decided to give it a try. Since I had stumbled over scoping rules when I started, I warned about using global in for loop by showing a code which went something like t…

Here’s a simplified version of your original code: x = 0 v = [0] let global x = 1 # global needed here v[1] = 1 # no global needed here end So the question is why is global needed to assign to x but not to assign to v[1]. The answer is because x is a variable and v[1] is not a variabl…

Assignment and mutation are different things: [image] Assignment and mutation New to Julia The simplest version of this I can come up with is this: Assignment changes which object a name refers to: x = ex causes the name x to refer to the value resulting from the evalu…

Thanks a lot for the explanation. But what if someone defined another vector somewhere v = ['a', 1, 'b']. That will break my code without warning. This is something sort of spooky action at distance. Or is this incorrect?

That’s assignment rather than mutation. Assigning to v somewhere else has no effect on your v. Mutation is visible everywhere because everyone has a reference to the same object.

Sorry. I still do not understand. I understand the thing about mutation and assignment. I am trying to understand how the behavior can lead to spooky action for vectors. Here is the code in manual but I have made x a vector: x = [123, "hello"] # much later # maybe in a different file for i = 1:10…

The syntax x = and the syntax x[i] = do completely different and unrelated things: the former associates a variable with an object; the latter modifies an object. Changing code from doing one to the other and then asking why they behave differently is like putting a sheep in a chicken coop and then …

[image] StefanKarpinski: One is quite harmless and the other is not. So the code I posted above is harmless and the original one in the manual is harmful. Can you please explain why is that the case? Maybe it will be helpful to clear up what I am thinking incorrectly.

I really appreciate you taking time to explain it in such detail. Thank you very much. I understand the argument of mutability: 1. If I use mutable struct I do not need an global annotation to modify its fields. 2. Or for arrays the assignment calls the setindex! method and mutates the original arr…

I don’t see any spooky action at a distance in that code. First you assign x, then you mutate it, than you use it mutated. Here is where it is spooky: x = [1] f() = x[1] f() # returns 1 x[1] = 2 f() # returns 2 So, f uses a global array x and the behavior of f can be modified by changing the cont…

I was referring to it as spooky in the sense that the code did not do what it was expected to do since an element of global array x was mutated in between. But that might not be what spooky action at a distance refers to. I have started using functions and passing parameters in struct to avoid this…

Having the code split in self contained functions will certainly payoff in the long term for debugging. To inspect what is going on inside the function either you print values (rough, but simple, that’s what I do most of the time) or use a debugger. One remark about my comment above: the scoping ru…

Spooky action at distance not applicable to vectors?

General Usage

StefanKarpinski April 17, 2022, 2:53pm 2

Assignment and mutation are different things:

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Spooky action at distance not applicable to vectors?

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