Change indexin sentinel to nothing

base/array.jl

@@ -2164,7 +2164,7 @@ indmin(a) = findmin(a)[2]
 
 Return an array containing the highest index in `b` for
 each value in `a` that is a member of `b`. The output
-array contains 0 wherever `a` is not a member of `b`.
+array contains `nothing` wherever `a` is not a member of `b`.
 
 # Examples
 ```jldoctest
@@ -2173,24 +2173,27 @@ julia> a = ['a', 'b', 'c', 'b', 'd', 'a']
 julia> b = ['a', 'b', 'c']
 
 julia> indexin(a, b)
-6-element Array{Int64,1}:
+6-element Array{Union{Nothing, Int64},1}:
  1
  2
  3
  2
- 0
+  nothing
  1
 
 julia> indexin(b, a)
-3-element Array{Int64,1}:
+3-element Array{Union{Nothing, Int64},1}:
  6
  4
  3
 ```
 """
 function indexin(a, b::AbstractArray)
-    bdict = Dict(zip(b, 1:length(b)))
-    map(i -> get(bdict, i, 0), a)
+    indexes = keys(b)
+    bdict = Dict(zip(b, indexes))
+    return Union{eltype(indexes), Nothing}[
+        get(bdict, i, nothing) for i in a
+    ]
 end
 
 function _findin(a, b)

test/arrayops.jl

@@ -1394,13 +1394,13 @@ end
 @test i7197() == (2,2)
 
 # PR #8622 and general indexin tests
-@test indexin([1,3,5,7], [5,4,3]) == [0,3,1,0]
-@test indexin([1 3; 5 7], [5 4; 3 2]) == [0 2; 1 0]
-@test indexin((2 * x + 1 for x in 0:3), [5,4,3,5,6]) == [0,3,4,0]
-@test indexin(6, [1,3,6,6,2]) == 4
+@test indexin([1,3,5,7], [5,4,3]) == [nothing,3,1,nothing]
+@test indexin([1 3; 5 7], [5 4; 3 2]) == [nothing CartesianIndex(2, 1); CartesianIndex(1, 1) nothing]
+@test indexin((2 * x + 1 for x in 0:3), [5,4,3,5,6]) == [nothing,3,4,nothing]
+@test indexin(6, [1,3,6,6,2]) == fill(4, ())
 @test indexin([6], [1,3,6,6,2]) == [4]
-@test indexin(3, 2:5) == 2
-@test indexin(3.0, 2:5) == 2
+@test indexin([3], 2:5) == [2]
+@test indexin([3.0], 2:5) == [2]
 
 #6828 - size of specific dimensions
 let a = Array{Float64}(uninitialized, 10)