# Mutating a Set during iteration
**URL:** https://discourse.julialang.org/t/mutating-a-set-during-iteration/30558
**Category:** New to Julia
**Tags:** question
**Created:** [October 31, 2019, 7:55pm UTC](https://discourse.julialang.org/t/mutating-a-set-during-iteration/30558 "2019-10-31T19:55:05Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@Vasily\_Pisarev](https://discourse.julialang.org/u/Vasily_Pisarev)
#### Post date: [October 31, 2019, 7:55pm UTC](https://discourse.julialang.org/t/mutating-a-set-during-iteration/30558/1 "2019-10-31T19:55:05Z")
Is it safe to iterate over a set and mutate it in process?
Say, I have a `Set{Int64}` and want to replace all negative values in it by their absolute values. Is it OK to
```julia
for x in set_of_ints
if x < 0
pop!(set_of_ints, x)
push!(set_of_ints, -x)
end
end
```
?
---
### Author:  [@jw3126](https://discourse.julialang.org/u/jw3126)
#### Post date: [October 31, 2019, 8:25pm UTC](https://discourse.julialang.org/t/mutating-a-set-during-iteration/30558/2 "2019-10-31T20:25:39Z")
The answer is that it often works, but not always. Also it is bad practice to mutate a collection while iterating and should be avoided. You can generate counter examples like this:
```julia
using Test
for _ in 1:1000
set_of_ints = Set(rand(-1000:1000, 2000))
s2 = deepcopy(set_of_ints)
for x in set_of_ints
if x < 0
pop!(set_of_ints, x)
push!(set_of_ints, -x)
end
end
@test set_of_ints == Set(map(abs, collect(s2)))
end
```
---
### Author:  [@Vasily\_Pisarev](https://discourse.julialang.org/u/Vasily_Pisarev)
#### Post date: [October 31, 2019, 8:50pm UTC](https://discourse.julialang.org/t/mutating-a-set-during-iteration/30558/3 "2019-10-31T20:50:47Z")
Thanks, I suspected that this pattern working for me once was accidental. `replace!` seems to be the proper way to do that.