# Can you define variables inside a comprehension?
**URL:** https://discourse.julialang.org/t/can-you-define-variables-inside-a-comprehension/85642
**Category:** General Usage
**Tags:** scope
**Created:** [August 11, 2022, 10:03pm UTC](https://discourse.julialang.org/t/can-you-define-variables-inside-a-comprehension/85642 "2022-08-11T22:03:26Z")
**Posts on this page:** 4
**Page:** 1
### Author:  [@Albert\_Zevelev](https://discourse.julialang.org/u/Albert_Zevelev)
#### Post date: [August 11, 2022, 10:03pm UTC](https://discourse.julialang.org/t/can-you-define-variables-inside-a-comprehension/85642/1 "2022-08-11T22:03:26Z")
```julia
julia> M = [1 2; 3 4]
2×2 Matrix{Int64}:
1 2
3 4
julia> M = (M + M') ./ 2.0
2×2 Matrix{Float64}:
1.0 2.5
2.5 4.0
julia> M = [1 2; 3 4]
2×2 Matrix{Int64}:
1 2
3 4
julia> [x = (x + x') ./ 2.0 for x in [M] ]
1-element Vector{Matrix{Float64}}:
[1.0 2.5; 2.5 4.0]
julia> M
2×2 Matrix{Int64}:
1 2
3 4
```
Is there a way to redefine/mutate a variable inside a comprehension?
Or are all things supposed to be always be local inside a comprehension?
---
### Author:  [@thautwarm](https://discourse.julialang.org/u/thautwarm)
#### Post date: [August 12, 2022, 12:56am UTC](https://discourse.julialang.org/t/can-you-define-variables-inside-a-comprehension/85642/2 "2022-08-12T00:56:41Z")
> [@Albert\_Zevelev](#):
>
> Is there a way to redefine/mutate a variable inside a comprehension?
You can change an existing variable from outer scope as follows:
```julia
julia> function f(outer_var)
[
begin
outer_var = i
end
for i in 1:100 ]
return outer_var
end
f (generic function with 1 method)
julia> f(1)
100
```
---
### Author:  [@jar1](https://discourse.julialang.org/u/jar1)
#### Post date: [August 12, 2022, 1:01am UTC](https://discourse.julialang.org/t/can-you-define-variables-inside-a-comprehension/85642/3 "2022-08-12T01:01:53Z")
> [@Albert\_Zevelev](#):
>
> are all things supposed to be always be local inside a comprehension?
As a matter of style, I’d say it’s better that way.
---
### Author:  [@dawbarton](https://discourse.julialang.org/u/dawbarton)
#### Post date: [August 12, 2022, 10:27am UTC](https://discourse.julialang.org/t/can-you-define-variables-inside-a-comprehension/85642/4 "2022-08-12T10:27:55Z")
You can mutate a variable inside a comprehension but I wouldn’t recommend it as its very non-standard/confusing. If you really want to do it then
```julia
julia> M = [1.0 2; 3 4]
2×2 Matrix{Float64}:
1.0 2.0
3.0 4.0
julia> [x .= (x + x') ./ 2.0 for x in [M]]
1-element Vector{Matrix{Float64}}:
[1.0 2.5; 2.5 4.0]
julia> M
2×2 Matrix{Float64}:
1.0 2.5
2.5 4.0
```
(Note that you need to get the type of `M` right to mutate it like this.)
If you don’t want the resulting collection, a much cleaner way would be to declare a function and broadcast over the array.
```julia
julia> dosomething!(x) = (x .= (x + x') ./ 2.0)
dosomething! (generic function with 1 method)
julia> M = [1.0 2; 3 4]
2×2 Matrix{Float64}:
1.0 2.0
3.0 4.0
julia> dosomething!.([M])
1-element Vector{Matrix{Float64}}:
[1.0 2.5; 2.5 4.0]
julia> M
2×2 Matrix{Float64}:
1.0 2.5
2.5 4.0
```
or
```julia
julia> M = [1.0 2; 3 4]
2×2 Matrix{Float64}:
1.0 2.0
3.0 4.0
julia> foreach(x -> x .= (x + x') ./ 2.0, [M])
julia> M
2×2 Matrix{Float64}:
1.0 2.5
2.5 4.0
```