# How do I create an anonymous function from external variables?

**URL:** <https://discourse.julialang.org/t/how-do-i-create-an-anonymous-function-from-external-variables/24497>\
**Category:** New to Julia\
**Created:** [May 22, 2019, 11:39pm UTC](https://discourse.julialang.org/t/how-do-i-create-an-anonymous-function-from-external-variables/24497 "2019-05-22T23:39:02Z")\
**Posts on this page:** 9\
**Page:** 1

<div class="post-metadata">

**Author:** ![Elmer\_Cusipuma](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/elmer_cusipuma/32/11958_2.png) [@Elmer\_Cusipuma](https://discourse.julialang.org/u/Elmer_Cusipuma)\
**Post date:** [May 22, 2019, 11:39pm UTC](https://discourse.julialang.org/t/how-do-i-create-an-anonymous-function-from-external-variables/24497/1 "2019-05-22T23:39:02Z")

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I am trying create a anomymous function with external variables

```julia
idx = 3; #External Variable
f = x -> x[idx];

```

As an example

```julia
idx = 2 #change the value
x = [2,3,5]
f(x)
3

```

I would expect the output to be like

```julia
f = x -> x[3]
f(x)
5

```

Any suggestions for decoupling the external variable?

---

<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [May 22, 2019, 11:43pm UTC](https://discourse.julialang.org/t/how-do-i-create-an-anonymous-function-from-external-variables/24497/2 "2019-05-22T23:43:04Z")

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`f = x -> x[eval(:idx)];`

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<div class="post-metadata">

**Author:** ![Elmer\_Cusipuma](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/elmer_cusipuma/32/11958_2.png) [@Elmer\_Cusipuma](https://discourse.julialang.org/u/Elmer_Cusipuma)\
**Post date:** [May 22, 2019, 11:51pm UTC](https://discourse.julialang.org/t/how-do-i-create-an-anonymous-function-from-external-variables/24497/3 "2019-05-22T23:51:27Z")

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@jling Thank you so much for your help.

I get this error

```julia
idx = nothing
f(x)
ArgumentError: `nothing` should not be printed; use `show`, `repr`, or custom output instead.

```

---

<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [May 22, 2019, 11:52pm UTC](https://discourse.julialang.org/t/how-do-i-create-an-anonymous-function-from-external-variables/24497/4 "2019-05-22T23:52:21Z")

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what behavior do you expect to get, this is basically:

```julia
julia> x = [2,3,5]
3-element Array{Int64,1}:
 2
 3
 5

julia> x[nothing]

```

---

<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [May 22, 2019, 11:52pm UTC](https://discourse.julialang.org/t/how-do-i-create-an-anonymous-function-from-external-variables/24497/5 "2019-05-22T23:52:30Z")

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> [@jling](#):
>
> `f = x -> x[eval(:idx)];`

No that’s not going to make any difference.

It’s not super clear what you want. Your function is referencing two “external variables” (global variables) `x` and `idx`. Are you just saying that you want to make one of them refering to the global variable `x` while the other one doesn’t refer to the global variable `idx` anymore?

(edit: sorry, I somehow thought `x` is a global variable and not the argument, anyway, apart from the mentioning of `x` above, everything else are still correct so I’ll just leave it as is…)

In that case, the solution should come out on it’s own. You just need to make `idx` not be the global variable anymore, i.e. you need to make it a local variable. The way to introduce a local scope is `let` so you basically need

```julia
f = let idx=idx
   x -> x[idx]
end

```

Of course if you don’t need `idx` to be a variable, you can splice that into the code directly, like `@eval x->x[$idx]` but that’s really an overkill.

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<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [May 22, 2019, 11:53pm UTC](https://discourse.julialang.org/t/how-do-i-create-an-anonymous-function-from-external-variables/24497/6 "2019-05-22T23:53:23Z")

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what do you mean?

```julia
julia> f = x -> x[eval(:idx)];

julia> idx = 2
2

julia> x = [2,3,5]
3-element Array{Int64,1}:
 2
 3
 5

julia> f(x)
3

julia> idx = 3
3

julia> f(x)
5

```

and his function is **NOT** referring to 2 global variable because that’s an anonymous function

---

<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [May 22, 2019, 11:58pm UTC](https://discourse.julialang.org/t/how-do-i-create-an-anonymous-function-from-external-variables/24497/7 "2019-05-22T23:58:28Z")

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> [@jling](#):
>
> ```julia
> julia> f = x -> x[eval(:idx)];
> 
> julia> idx = 2
> 2
> 
> julia> x = [2,3,5]
> 3-element Array{Int64,1}:
> 2
> 3
> 5
> 
> julia> f(x)
> 3
> 
> julia> idx = 3
> 3
> 
> julia> f(x)
> 5
> 
> ```

I believe you are showing the exact result that was not expected.  
What I mean is that your function behaves identically with the original one.

```julia
julia> f = x -> x[idx]
#3 (generic function with 1 method)

julia> idx = 2
2

julia> x = [2,3,5]
3-element Array{Int64,1}:
 2
 3
 5

julia> f(x)
3

julia> idx = 3
3

julia> f(x)
5

```

> [@jling](#):
>
> and his function is **NOT** referring to 2 global variable because that’s an anonymous function

Yeah, see my edit above. However, it has nothing to do with the fact that it’s an anonymous function. It’s exactly the same for `f(x) = x[idx]`, which is not anonymous. I just missed the `x->` part, i.e. that `x` is the argument, which **IS** the reason why it’s only refering to 1 global variable and not 2.

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<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [May 22, 2019, 11:59pm UTC](https://discourse.julialang.org/t/how-do-i-create-an-anonymous-function-from-external-variables/24497/8 "2019-05-22T23:59:14Z")

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ah, my mistake, I thought he was trying to get that result. Sorry for the confusion.

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<div class="post-metadata">

**Author:** ![yuyichao](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/yuyichao/32/20_2.png) [@yuyichao](https://discourse.julialang.org/u/yuyichao)\
**Post date:** [May 23, 2019, 12:02am UTC](https://discourse.julialang.org/t/how-do-i-create-an-anonymous-function-from-external-variables/24497/9 "2019-05-23T00:02:08Z")

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If the question was to get that result for a function constructed in a local scope, then yes, what you had is a valid answer. It’s not a good answer though and should be discouraged like any unnecessary use of `eval`. The correct solution is to make sure you resolve your variable to the global one explicitly, i.e. by qualifying with the module it’s in, i.e. `x->x[<M>.idx]`. In this case, the `<M>` is the current module, i.e. `@ __MODULE__ ` so it should be `x->x[(@ __MODULE__ ).idx]` (too lazy to check if the `()` is required…edit: actually I’m pretty sure it is… anyway…)

edit2: of course using `global` also work. It’s just dirtier for one-liner, but you can do `x->(global idx; x[idx])`…