# Is it defined behavior to modify \`iter\` while \`for\`-looping over it?

**URL:** <https://discourse.julialang.org/t/is-it-defined-behavior-to-modify-iter-while-for-looping-over-it/132477>\
**Category:** New to Julia\
**Tags:** question, for-loop\
**Created:** [September 18, 2025, 7:20am UTC](https://discourse.julialang.org/t/is-it-defined-behavior-to-modify-iter-while-for-looping-over-it/132477 "2025-09-18T07:20:42Z")\
**Posts on this page:** 1\
**Showing post:** 21

<div class="post-metadata">

**Author:** ![WalterMadelim](https://avatars.discourse-cdn.com/v4/letter/w/3e96dc/32.png) [@WalterMadelim](https://discourse.julialang.org/u/WalterMadelim)\
**Post date:** [September 18, 2025, 1:19pm UTC](https://discourse.julialang.org/t/is-it-defined-behavior-to-modify-iter-while-for-looping-over-it/132477/21 "2025-09-18T13:19:34Z")

</div>

> [@eldee](#):
>
> depending on how you define difference

If the local `i` used in the second pass associates to the same object to the `i` used in the first pass, then they are the same. (my understanding about julia is advanced again☺️)

```julia
julia> function test()
           v = Function[]
           for i = S(4)
               push!(v, () -> println("I'm ", i))
               map(x -> x(), v)
           end
       end
test (generic function with 1 method)

julia> test()
I'm [1]
I'm [1, 2]
I'm [1, 2]
I'm [1, 2, 3]
I'm [1, 2, 3]
I'm [1, 2, 3]
I'm [1, 2, 3, 4]
I'm [1, 2, 3, 4]
I'm [1, 2, 3, 4]
I'm [1, 2, 3, 4]

julia> function test()
           v = Function[]
           for i = 1:4
               push!(v, () -> println("I'm ", i))
               map(x -> x(), v)
           end
       end
test (generic function with 1 method)

julia> test()
I'm 1
I'm 1
I'm 2
I'm 1
I'm 2
I'm 3
I'm 1
I'm 2
I'm 3
I'm 4

```

---

_[View the full topic](https://discourse.julialang.org/t/is-it-defined-behavior-to-modify-iter-while-for-looping-over-it/132477)._