findin does not work for strings?

[weijianzhg]

I am using Julia Version 0.4.0-dev+5149 (2015-06-01 18:58 UTC).
Suppose I have a list l

 julia> l = ["a", "a", "b", "c"]
 4-element Array{ASCIIString,1}:
 "a"
 "a"
 "b"
 "c"

then findfirst and findlast can find the correct index.

julia> findfirst(l, "a")
1
julia> findlast(l, "a")
2

But findin(l, "a") returns nothing

julia> findin(l, "a")
0-element Array{Int64,1}

Is this a bug or I misunderstand something?

Thanks,

Weijian

[KristofferC]

This is my interpretation from looking at the documentation:

findin(a, b)
Returns the indices of elements in collection a that appear in collection b

julia> findin(l, "a")
0-element Array{Int64,1}

julia> findin(l, ["a"])
2-element Array{Int64,1}:
 1
 2

[weijianzhg]

I see. Thanks @KristofferC , I think I misuse it. But it does return the correct result for integers.

julia> findin([1,2,3,3,4], 3)
2-element Array{Int64,1}:
 3
 4

[KristofferC]

Ok so this is the definition of findin

function findin(a, b)
    ind = Array(Int, 0)
    bset = Set(b)
    @inbounds for i = 1:length(a)
        a[i] in bset && push!(ind, i)
    end
    ind
end

The problem is that for a string b, the command Set(b) produces:

julia> Set("a")
Set(['a'])

julia> typeof(Set("a"))
Set{Char}

and thus later in the function this fails:

julia> "a" in Set("a")
false

On the other hand, these are true

julia> "a" in Set(["a"])
true

julia> 'a' in Set(["a"])
true

I am a bit surprised that Set(::ASCIIString) returns a set of Char. I guess since Set(Container{T}) should return a Set{T} and a string technically works as a container for Char it makes sense.

Maybe you should reopen this to confirm that this is what is expected.

[nalimilan]

String is indeed a collection of Chars, since that's what you get when indexing a string. If String wasn't a collection but a scalar, you would get an error.

If you want to look for strings in a string (as opposed to Charss in a string, as here), you can use search instead. See #10593 about attempts at making this more obivous.

[KristofferC]

@nalimilan: "If String wasn't a collection but a scalar, you would get an error."

This doesn't seem to be the case.

julia> findin([1,2,3,3,4], 3)
2-element Array{Int64,1}:
 3
 4

Because set of a scalar promotes it to a container with the scalar as the only item?

[nalimilan]

No, that's because numbers are considered as collections at the moment:

julia> 2[1]
2

I find it a bit confusing, for people have advocated that it was often useful.

[KristofferC]

Oh, I see. Thanks.

[weijianzhg]

Thanks @nalimilan and @KristofferC for clarifying this.