# I thought I understood "pass-by-sharing" in Julia until I found this

**URL:** <https://discourse.julialang.org/t/i-thought-i-understood-pass-by-sharing-in-julia-until-i-found-this/46897>\
**Category:** General Usage\
**Created:** [September 19, 2020, 5:59am UTC](https://discourse.julialang.org/t/i-thought-i-understood-pass-by-sharing-in-julia-until-i-found-this/46897 "2020-09-19T05:59:11Z")\
**Posts on this page:** 1\
**Showing post:** 19

<div class="post-metadata">

**Author:** ![Henrique\_Becker](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/henrique_becker/32/15443_2.png) [@Henrique\_Becker](https://discourse.julialang.org/u/Henrique_Becker)\
**Post date:** [September 19, 2020, 4:18pm UTC](https://discourse.julialang.org/t/i-thought-i-understood-pass-by-sharing-in-julia-until-i-found-this/46897/19 "2020-09-19T16:18:43Z")

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In the OP case, as other have already pointed out, I think the root of the problem lies in a very common misunderstanding about the scope of `for` loops.

> A `for` block does not define a single scope, but a succession of scopes one for each iteration.

The distinction between a single scope and a succession of scopes is often little relevant (I believe this is the reason for it being a common misunderstanding). However, together with closures it become relevant and crashes a mental model in which all iterations shared the same inner scope.

I think it can be illustrated by:

```julia
julia> function f()
           for i = 1:5
               if @isdefined x
                   println("x found: $x")
               else
                   println("x not found")
               end
               x = i
           end
           return
       end
f (generic function with 1 method)

julia> f()
x not found
x not found
x not found
x not found
x not found

```

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