Add indexin (#515)

README.md

@@ -360,6 +360,10 @@ Currently, the `@compat` macro supports the following syntaxes:
 
 * `indmin` and `indmax` are now `argmin` and `argmax`, respectively ([#25654]).
 
+* `Compat.indexin` accepts any iterable as first argument, returns `nothing` (rather than `0`)
+   for entries with no match and gives the index of the first (rather than the last) match 
+   ([#25662], [#25998]).
+
 * `isabstract` and `isleaftype` are now `isabstracttype` and `isconcretetype`, respectively
   ([#23666], [#25496]).
 
@@ -576,6 +580,7 @@ includes this fix. Find the minimum version from there.
 [#25646]: https://github.com/JuliaLang/julia/issues/25646
 [#25647]: https://github.com/JuliaLang/julia/issues/25647
 [#25654]: https://github.com/JuliaLang/julia/issues/25654
+[#25662]: https://github.com/JuliaLang/julia/issues/25662
 [#25705]: https://github.com/JuliaLang/julia/issues/25705
 [#25706]: https://github.com/JuliaLang/julia/issues/25706
 [#25738]: https://github.com/JuliaLang/julia/issues/25738
@@ -585,6 +590,7 @@ includes this fix. Find the minimum version from there.
 [#25896]: https://github.com/JuliaLang/julia/issues/25896
 [#25959]: https://github.com/JuliaLang/julia/issues/25959
 [#25990]: https://github.com/JuliaLang/julia/issues/25990
+[#25998]: https://github.com/JuliaLang/julia/issues/25998
 [#26069]: https://github.com/JuliaLang/julia/issues/26069
 [#26089]: https://github.com/JuliaLang/julia/issues/26089
 [#26149]: https://github.com/JuliaLang/julia/issues/26149

src/Compat.jl

@@ -1664,6 +1664,21 @@ end
         Base.mv(src, dst; remove_destination = force)
 end
 
+if VERSION < v"0.7.0-DEV.3972"
+    function indexin(a, b::AbstractArray)
+        inds = keys(b)
+        bdict = Dict{eltype(b),eltype(inds)}()
+        for (val, ind) in zip(b, inds)
+            get!(bdict, val, ind)
+        end
+        return Union{eltype(inds), Nothing}[
+             get(bdict, i, nothing) for i in a
+         ]
+    end
+else
+    const indexin = Base.indexin
+end
+
 include("deprecated.jl")
 
 end # module Compat

test/runtests.jl

@@ -1488,4 +1488,7 @@ mktempdir(@__DIR__) do dir
     @test readdir(dir) == ["dest.jl"]
 end
 
+# 0.7.0-DEV.3972
+@test Compat.indexin([1, 2], [1, 0, 1]) == [1, nothing]
+
 nothing