NEWS, tests and some fixes

NEWS.md

@@ -39,6 +39,8 @@ New library functions
 
 * Two argument methods `findmax(f, domain)`, `argmax(f, domain)` and the corresponding `min` versions ([#27613]).
 * `isunordered(x)` returns true if `x` is value that is normally unordered, such as `NaN` or `missing`.
+* New `keepat!(vector, inds)` function which is the inplace equivalent of `vector[inds]`
+  for a list `inds` of integers ([#36229]).
 * New macro `Base.@invokelatest f(args...; kwargs...)` provides a convenient way to call `Base.invokelatest(f, args...; kwargs...)` ([#37971])
 * New macro `Base.@invoke f(arg1::T1, arg2::T2; kwargs...)` provides an easier syntax to call `invoke(f, Tuple{T1,T2}; kwargs...)` ([#38438])
 * Two arguments method `lock(f, lck)` now accepts a `Channel` as the second argument. ([#39312])

base/abstractarray.jl

@@ -2560,7 +2560,7 @@ Remove the items at all the indices which are not given by `inds`,
 and return the modified `a`.
 Items which are kept are shifted to fill the resulting gaps.
 
-`inds` can be either an iterator or a collection of sorted integer indices.
+`inds` must be an iterator of sorted and unique integer indices.
 See also [`deleteat!`](@ref).
 
 !!! compat "Julia 1.7"
@@ -2576,13 +2576,13 @@ julia> keepat!([6, 5, 4, 3, 2, 1], 1:2:5)
 ```
 """
 function keepat!(a::AbstractVector, inds)
-    isempty(inds) && return a
     local prev
     i = firstindex(a)
     for k in inds
-        @isdefined(prev) && (prev <= k || throw(ArgumentError("indices must be sorted")))
+        @isdefined(prev) && (prev < k || throw(ArgumentError("indices must be sorted")))
+        ak = a[k] # must happen even when i==k for bounds checking
         if i != k
-            a[i] = a[k]
+            @inbounds a[i] = ak # k > i, so a[i] is inbounds
         end
         prev = k
         i = nextind(a, i)

test/abstractarray.jl

@@ -1298,3 +1298,24 @@ end
     @test @inferred(reduce(vcat, x_vecs)) == [5.0, 1.0, 2.0, 3.0]
     @test @inferred(reduce(vcat, ([10.0], [20.0], Bool[]))) == [10.0, 20.0]
 end
+
+@testset "keepat!" begin
+    a = [1:6;]
+    @test a === keepat!(a, 1:5)
+    @test a == 1:5
+    @test keepat!(a, [2, 4]) == [2, 4]
+    @test isempty(keepat!(a, []))
+
+    a = [1:6;]
+    @test_throws BoundsError keepat!(a, 1:10) # make sure this is not a no-op
+    @test_throws BoundsError keepat!(a, 2:10)
+    @test_throws ArgumentError keepat!(a, [2, 4, 3])
+
+    b = BitVector([1, 1, 1, 0, 0])
+    @test b === keepat!(b, 1:5)
+    @test b == [1, 1, 1, 0, 0]
+    @test keepat!(b, 2:4) == [1, 1, 0]
+    @test_throws BoundsError keepat!(a, -1:10)
+    @test_throws ArgumentError keepat!(a, [2, 1])
+    @test isempty(keepat!(a, []))
+end