add docs, compat, move to abstractarray.jl

base/abstractarray.jl

@@ -2549,3 +2549,44 @@ 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` can be either an iterator or a collection of sorted 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)
+    isempty(inds) && return a
+    local prev
+    i = firstindex(a)
+    for k in inds
+        @isdefined(prev) && (prev <= k || throw(ArgumentError("indices must be sorted")))
+        if i != k
+            a[i] = a[k]
+        end
+        prev = k
+        i = nextind(a, i)
+    end
+    deleteat!(a, i:lastindex(a))
+    return a
+end

base/array.jl

@@ -1367,40 +1367,6 @@ function insert!(a::Array{T,1}, i::Integer, item) where T
     return a
 end
 
-"""
-    keepat!(a::Vector, 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` can be either an iterator or a collection of sorted and unique integer indices.
-See also [`deleteat!`](@ref).
-
-# 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)
-    isempty(inds) && return a
-    local prev
-    i = firstindex(a)
-    for k in inds
-        @isdefined(prev) && (prev <= k || throw(ArgumentError("indices must be sorted")))
-        if i != k
-            a[i] = a[k]
-        end
-        prev = k
-        i = nextind(a, i)
-    end
-    deleteat!(a, i:lastindex(a))
-    return a
-end
-
 """
     deleteat!(a::Vector, i::Integer)
 

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!