The example is pretty self-explanatory, I want to make a function which changes a variable. But it doesn’t work, how do I make it to? a = 2 function change_a!(a) a = 3 nothing end change_a!(a) display(a); the output is 2. (I wish it was 3)

Perhaps it is worth mentinoning that: Scalars cannot be changed by default. They are immutable values. You can change, however, the value associated with the label a, and to do so you you better return the value from your function: function foo(a) a = a + 1 return a end and then call thi…

The keyword you’re looking for is global a, but this isn’t recommended (see: Avoid global variables ). Using global variables often leads to confusing behaviour, bugs, performance problems and a plethora of other nastiness. Instead, simply return the value and assign it to a variable. Note that cont…

Or maybe, this is what you are looking for: julia> a_ref=Ref(2) Base.RefValue{Int64}(2) julia> a_ref[] 2 julia> function self(ref_a) ref_a[]=4 end self (generic function with 1 method) julia> self(a_ref) 4 julia> a_ref[] 4

I understand, thanks. So this is appropriate code: a = [2] function change_a!(a) a[1] = 3 nothing end change_a!(a) display(a[1]); is it, however, appropriate (since it was inappropriate to make a global?

This is better, since in change_a! you’re modifying the contents of the argument, not the global variable directly. Note that global a would be used in change_a!, to tell the function to look in the containing scope for a instead of in the local scope. In both your original example and your second …

Thanks for the explanation. Also quite impressed by @oheil and whatever magic that is.

No magic at all: https://docs.julialang.org/en/v1/base/c/#Core.Ref It’s part of Julia’s interface calling to C. A good read I found about the topic: Working with and emulating References in Julia Using Ref, Array and Tuple types for reference like behavior Reading time: 10 m…

If you want to go the Ref or Array route, you’ll want to mark them as const (i.e., const a = Ref(2)) to somewhat avoid the performance cost of a maybe changing type (that’s the problem with globals). You will still have some overhead though because you have to dereference them - there’s no shortcut …

Make function which changes varriable

New to Julia

Henrique_Becker October 15, 2020, 1:58pm 10

Again late, but let shamelessly point out my post about how you should not see variables as boxes to complement what Leandro is saying.

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Make function which changes varriable

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