add keepat! (the opposite of deleteat!)

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

@@ -2549,3 +2549,46 @@ function rest(a::AbstractArray{T}, state...) where {T}
     sizehint!(v, length(a))
     return foldl(push!, Iterators.rest(a, state...), init=v)
 end
+
+
+## keepat! ##
+
+"""
+    keepat!(a::AbstractVector, inds)
+
+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` must be an iterator of sorted and unique integer indices.
+See also [`deleteat!`](@ref).
+
+!!! compat "Julia 1.7"
+    This function is available as of Julia 1.7.
+
+# Examples
+```jldoctest
+julia> keepat!([6, 5, 4, 3, 2, 1], 1:2:5)
+3-element Array{Int64,1}:
+ 6
+ 4
+ 2
+```
+"""
+function keepat!(a::AbstractVector, inds)
+    local prev
+    i = firstindex(a)
+    for k in inds
+        if @isdefined(prev)
+            prev < k || throw(ArgumentError("indices must be unique and sorted"))
+        end
+        ak = a[k] # must happen even when i==k for bounds checking
+        if i != k
+            @inbounds a[i] = ak # k > i, so a[i] is inbounds
+        end
+        prev = k
+        i = nextind(a, i)
+    end
+    deleteat!(a, i:lastindex(a))
+    return a
+end

base/exports.jl

@@ -503,6 +503,7 @@ export
     count,
     delete!,
     deleteat!,
+    keepat!,
     eltype,
     empty!,
     empty,

doc/src/base/collections.md

@@ -273,6 +273,7 @@ Base.pushfirst!
 Base.popfirst!
 Base.insert!
 Base.deleteat!
+Base.keepat!
 Base.splice!
 Base.resize!
 Base.append!

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