# Removing the elements of one array from another using setdiff
**URL:** https://discourse.julialang.org/t/removing-the-elements-of-one-array-from-another-using-setdiff/7977
**Category:** General Usage
**Created:** [December 25, 2017, 12:22am UTC](https://discourse.julialang.org/t/removing-the-elements-of-one-array-from-another-using-setdiff/7977 "2017-12-25T00:22:11Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@Qiyamah](https://discourse.julialang.org/u/Qiyamah)
#### Post date: [December 25, 2017, 12:22am UTC](https://discourse.julialang.org/t/removing-the-elements-of-one-array-from-another-using-setdiff/7977/1 "2017-12-25T00:22:11Z")
Hi fellows,
The following code works:
```nohighlight
a = [1,2,3,4,5]
b = [4,5,6]
setdiff(a,b)
3-element Array{Int64,1}:
1
2
3
```
However this one doesn’t:
```nohighlight
type X
a::Int
end
Base.broadcast(::typeof(==),x::X,y::X) = x.a == y.a
a = [X(1),X(2),X(4),X(5),X(6)]
b = [X(4),X(5),X(6)]
setdiff(a,b)
5-element Array{X,1}:
X(1)
X(2)
X(4)
X(5)
X(6)
```
What went wrong here?
---
### Author:  [@Non-Contradiction](https://discourse.julialang.org/u/Non-Contradiction)
#### Post date: [December 25, 2017, 1:27am UTC](https://discourse.julialang.org/t/removing-the-elements-of-one-array-from-another-using-setdiff/7977/2 "2017-12-25T01:27:30Z")
I’m on julia 0.6.2. I think this is what you might want to do in your code instead of the `broadcast` thing.
```julia
import Base.==
==(x::X,y::X) = x.a == y.a
```
This gives you `X(1) == X(1)`, not the `broadcast` function.
But this still cannot give you the `setdiff` result you want, at least on my julia v0.6.2. As the result is totally random on my julia 0.6.2. Sometime the result is a four element Array, sometime the result is a five element one.
I try to further dig into the issue. `setdiff(a, b)` leads to `in(a_elem, bset)` leads to `haskey(bset.dict, a_elem)`, where `bset = Set(b)`. And the following happens:
```julia
julia> haskey(Set(b).dict, Y(5))
false
julia> haskey(Set(b).dict, Y(5))
false
julia> haskey(Set(b).dict, Y(5))
true
```
So I think this problem is related to the dictionary in julia 0.6.2. I may have seen some issues similar to this before and the solution to it, and I will try to search for that.
---
### Author:  [@anon67531922](https://discourse.julialang.org/u/anon67531922)
#### Post date: [December 25, 2017, 1:29am UTC](https://discourse.julialang.org/t/removing-the-elements-of-one-array-from-another-using-setdiff/7977/3 "2017-12-25T01:29:38Z")
I suspect the reason is because `type` is mutable so even though the `a`s are the same, the objects aren’t equal. Try immutable.
```julia
julia> type mtest
a::Int
end
julia> immutable itest
a::Int
end
julia> m1 = mtest(1); m2 = mtest(1); m1 == m2
false
julia> i1 = itest(1); i2 = itest(1); i1 == i2
true
```
---
### Author:  [@Non-Contradiction](https://discourse.julialang.org/u/Non-Contradiction)
#### Post date: [December 25, 2017, 1:40am UTC](https://discourse.julialang.org/t/removing-the-elements-of-one-array-from-another-using-setdiff/7977/4 "2017-12-25T01:40:53Z")
`immutable` instead of `type` solves the problem on julia 0.6.2.
And the problem of `type` seems to relate to this:
[https://github.com/JuliaLang/julia/issues/12198](https://github.com/JuliaLang/julia/issues/12198)
---
### Author:  [@anon67531922](https://discourse.julialang.org/u/anon67531922)
#### Post date: [December 25, 2017, 2:05am UTC](https://discourse.julialang.org/t/removing-the-elements-of-one-array-from-another-using-setdiff/7977/5 "2017-12-25T02:05:56Z")
Yep. That seems to be the case.
```julia
julia> m1 = [mtest(i) for i in 1:5]; m2 = [mtest(i) for i in 4:5]; setdiff(m1,m2)
5-element Array{mtest,1}:
mtest(1)
mtest(2)
mtest(3)
mtest(4)
mtest(5)
julia> i1 = [itest(i) for i in 1:5]; m2 = [itest(i) for i in 4:5]; setdiff(i1,i2)
3-element Array{itest,1}:
itest(1)
itest(2)
itest(3)
```