# Make a variable as a global variable within a function

**URL:** <https://discourse.julialang.org/t/make-a-variable-as-a-global-variable-within-a-function/63067>\
**Category:** New to Julia\
**Tags:** macros\
**Created:** [June 17, 2021, 6:29am UTC](https://discourse.julialang.org/t/make-a-variable-as-a-global-variable-within-a-function/63067 "2021-06-17T06:29:58Z")\
**Posts on this page:** 1\
**Showing post:** 12

<div class="post-metadata">

**Author:** ![iHany](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ihany/32/18151_2.png) [@iHany](https://discourse.julialang.org/u/iHany)\
**Post date:** [June 17, 2021, 2:10pm UTC](https://discourse.julialang.org/t/make-a-variable-as-a-global-variable-within-a-function/63067/12 "2021-06-17T14:10:01Z")

</div>

Hi you all,  
Probably the first question and example are too poor to understand my case.  
I wrote my situation and what I wanna do in detail below:

Briefly speaking, I wanna save specific variables annotated by a macro (namely, `@log`).  
More specifically, it is related to logging data in the use of [DifferentialEquations.jl](https://github.com/SciML/DifferentialEquations.jl) (abbrev.: DiffEq).

In DiffEq, I usually write a in-place DE (differential equation) like:

```julia
function sub_dynamics1!(dx, x, p, t)
    dx .= -1*x # an example dynamics
end
function sub_dynamics2!(dx, x, p, t)
    dx .= -2*x # an example dynamics
end

```

For nested systems, ODE sometimes calls subsystems’ ODE like:

```julia
function dynamics!(dx, x, p, t)
    sub_dynamics1!(dx.sub1, x.sub1, p.sub1, t)
    sub_dynamics2!(dx.sub2, x.sub2, p.sub2, t)
end

```

Note that the above “dot-access” (I mean, for example, `dx.sub1`) is possible via [ComponentArrays.jl](https://github.com/jonniedie/ComponentArrays.jl).

Now, I’d like to add a macro `@log` to indicate that the variable will be saved. For example, `sub_dyanmics1!` is changed as

```julia
function sub_dynamics1!(dx, x, p, t)
    @log x # will be saved
    dx .= -1*x
end

```

This idea is based on [SimulationLogs.jl](https://github.com/jonniedie/SimulationLogs.jl). However, [as discussed before](https://discourse.julialang.org/t/differentialequations-jl-saving-data-without-redundant-calculation-of-control-inputs/62559/6), SimulationLogs.jl saves data “after simulation”.  
This may be undesirable when the update of a simulation parameter (namely, `p`) occurs in stochastic sense. Also, I doubt that it works properly in the setting of thread-based parallel simulation.

So I wanna make (possibly some) macros, namely `@log` and `@LOG`, the former is for annotating saved variables and the latter is to transform `dynamics!` into a new function that contains a dictionary (which will contain variables annotated by `@log`).

I tested this functionality for some simple examples. The difficulty is, however, when a function (`dynamics!`) calls subsystems’ dynamics (`sub_dynamics1!` and `sub_dynamics2!`) and `@log` is added to `sub_dynamics1!` and `sub_dynamics2!`.

Here are my ideas:

1. (probably the best) Make a dictionary, which acts as a global variable only in `dynamics!` (that is, it is merely a local variable outside `dynamics!` and accessible in `sub_dynamics1!` and `sub_dynamics2!`).
2. (probably safe in terms of thread-based parallel simulation) Make some dictionaries, each of them acts as a global variable only in each thread, and add data to the thread-safe dictionaries.

Do you guys have solutions to my idea? Or, do you have a better idea?  
Feel free to leave comments 🙂

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