# Is a Structure in a Collection

**URL:** <https://discourse.julialang.org/t/is-a-structure-in-a-collection/79957>\
**Category:** General Usage\
**Created:** [April 24, 2022, 9:07pm UTC](https://discourse.julialang.org/t/is-a-structure-in-a-collection/79957 "2022-04-24T21:07:19Z")\
**Posts on this page:** 8\
**Page:** 1

<div class="post-metadata">

**Author:** ![Ordens\_Ritter](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ordens_ritter/32/50363_2.png) [@Ordens\_Ritter](https://discourse.julialang.org/u/Ordens_Ritter)\
**Post date:** [April 24, 2022, 9:07pm UTC](https://discourse.julialang.org/t/is-a-structure-in-a-collection/79957/1 "2022-04-24T21:07:19Z")

</div>

I want to find whether a Structure of Type A is in included in A Collection. In the next step I wanna return this collection form a Collection of a Collection of Type A^^.

```julia
struct A
   a = Int
end

a = [A(1), A(2)]
in(a, A(1))

```

This throws the Error:

```julia
ERROR: MethodError: no method matching iterate(::A)
Closest candidates are:
  iterate(::Union{LinRange, StepRangeLen}) at ~/packages/julias/julia-1.7/share/julia/base/range.jl:826
  iterate(::Union{LinRange, StepRangeLen}, ::Integer) at ~/packages/julias/julia-1.7/share/julia/base/range.jl:826
  iterate(::T) where T<:Union{Base.KeySet{<:Any, <:Dict}, Base.ValueIterator{<:Dict}} at ~/packages/julias/julia-1.7/share/julia/base/dict.jl:695
  ...
Stacktrace:
 [1] in(x::Vector{A}, itr::A)
   @ Base ./operators.jl:1282
 [2] top-level scope
   @ REPL[6]:1

```

I assume I have to implement an iterate function?! How is this done? What needs to be returned?

---

<div class="post-metadata">

**Author:** ![oheil](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oheil/32/220745_2.png) [@oheil](https://discourse.julialang.org/u/oheil)\
**Post date:** [April 24, 2022, 9:14pm UTC](https://discourse.julialang.org/t/is-a-structure-in-a-collection/79957/2 "2022-04-24T21:14:51Z")

</div>

First things first:

```julia
julia> struct A
          a::Int
       end

julia> a = [A(1), A(2)]
2-element Vector{A}:
 A(1)
 A(2)

julia> findfirst(x->x==A(1),a)
1

julia> in(A(1),a)
true

```

---

<div class="post-metadata">

**Author:** ![oheil](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oheil/32/220745_2.png) [@oheil](https://discourse.julialang.org/u/oheil)\
**Post date:** [April 24, 2022, 9:19pm UTC](https://discourse.julialang.org/t/is-a-structure-in-a-collection/79957/3 "2022-04-24T21:19:19Z")

</div>

> [@Ordens\_Ritter](#):
>
> In the next step I wanna return this collection form a Collection of a Collection of Type A

Let us see what this is.

---

<div class="post-metadata">

**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [April 24, 2022, 10:21pm UTC](https://discourse.julialang.org/t/is-a-structure-in-a-collection/79957/4 "2022-04-24T22:21:36Z")

</div>

> [@Ordens\_Ritter](#):
>
> I want to find whether a Structure of Type A is in included in A Collection.

Seems that your question is answered, but this is another interpretation for that question:

```julia
julia> struct A a::Int end;

julia> struct B b::Int end;

julia> a = [B(1), A(2)]
2-element Vector{Any}:
 B(1)
 A(2)

julia> findfirst(x -> isa(x,A), a)
2

```

And then maybe the second question is:

```julia
julia> a = [A(1), B(1), A(2)]
3-element Vector{Any}:
 A(1)
 B(1)
 A(2)

julia> findall(x -> isa(x,A), a)
2-element Vector{Int64}:
 1
 3

julia> a2 = a[findall(x -> isa(x,A), a)]
2-element Vector{Any}:
 A(1)
 A(2)

```

---

<div class="post-metadata">

**Author:** ![Ordens\_Ritter](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ordens_ritter/32/50363_2.png) [@Ordens\_Ritter](https://discourse.julialang.org/u/Ordens_Ritter)\
**Post date:** [April 25, 2022, 11:59am UTC](https://discourse.julialang.org/t/is-a-structure-in-a-collection/79957/5 "2022-04-25T11:59:57Z")

</div>

Sorry, my desciption must have been not so clear. What I wanted to do is something similar to:

```julia

julia> struct Point{t<:Real}
           row:: t
           col:: t
       end

a = [Point(1, 1), Point(2, 2)]
b = [Point(1, 2), Point(2, 1)]
ab = [a, b]
ab[findfirst(x-> in(x, Point(1, 1), ab)]

```

From the error message I assume that the in function, applies an iteratertion which is not implemented for this Type “Point”.

---

<div class="post-metadata">

**Author:** ![oheil](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/oheil/32/220745_2.png) [@oheil](https://discourse.julialang.org/u/oheil)\
**Post date:** [April 25, 2022, 12:09pm UTC](https://discourse.julialang.org/t/is-a-structure-in-a-collection/79957/6 "2022-04-25T12:09:19Z")

</div>

for function `in` its:

```julia
in(item, collection)

```

you use it the other way round.

---

<div class="post-metadata">

**Author:** ![Ordens\_Ritter](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ordens_ritter/32/50363_2.png) [@Ordens\_Ritter](https://discourse.julialang.org/u/Ordens_Ritter)\
**Post date:** [April 25, 2022, 12:12pm UTC](https://discourse.julialang.org/t/is-a-structure-in-a-collection/79957/7 "2022-04-25T12:12:10Z")

</div>

thanks @oheil

---

<div class="post-metadata">

**Author:** ![lmiq](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lmiq/32/18314_2.png) [@lmiq](https://discourse.julialang.org/u/lmiq)\
**Post date:** [April 25, 2022, 12:18pm UTC](https://discourse.julialang.org/t/is-a-structure-in-a-collection/79957/8 "2022-04-25T12:18:55Z")

</div>

One step at the time: First, the `in` call is inverted, this works,

```julia
julia> a = [Point(1, 1), Point(2, 2)]
2-element Vector{Point{Int64}}:
 Point{Int64}(1, 1)
 Point{Int64}(2, 2)

julia> findfirst(x -> in(Point(1,1), a), a)
1

# and (more reasonably in this case):
julia> findfirst(x -> x==Point(1,1), a)
1

#or (I like this one)
julia> findfirst(isequal(Point(1,1)), a)
1

```

Now, you have a collection of collections. This command (with the dot: `findall.()`, will apply the `findall` operation to each element of the “outer” collection (this is the standard broadcast syntax of Julia):

```julia
julia> a = [Point(1, 1), Point(2, 2)];

julia> b = [Point(1, 2), Point(2, 1), Point(1,1)];

julia> ab = [a, b];

julia> findall.(isequal(Point(1,1)), ab)
2-element Vector{Vector{Int64}}:
 [1]
 [3]

```

Which is an array of vectors, containing the desired indexes of the elements at for each vector. It that is not what you want, you can use, for example:

```julia
julia> filter(isequal(Point(1,1)), reduce(vcat, ab))
2-element Vector{Point{Int64}}:
 Point{Int64}(1, 1)
 Point{Int64}(1, 1)

```

(where `reduce(vcat, ab)` flattens the `ab` array of arrays into a simple vector:

```julia
julia> reduce(vcat, ab)
5-element Vector{Point{Int64}}:
 Point{Int64}(1, 1)
 Point{Int64}(2, 2)
 Point{Int64}(1, 2)
 Point{Int64}(2, 1)
 Point{Int64}(1, 1)

```