Add {first,last}(::AbstractVector, ::Integer) methods

Author: giordano

base/abstractarray.jl

@@ -343,6 +343,27 @@ function first(itr)
     x[1]
 end
 
+"""
+    first(v::AbstractVector, n::Integer)
+
+Get the first n elements of the vector, without going out-of-bounds.
+
+# Examples
+```jldoctest
+julia> first(["foo", "bar", "qux"], 2)
+2-element Array{String,1}:
+ "foo"
+ "bar"
+
+julia> first(1:6, 10)
+1:6
+
+julia> first(Bool[], 1)
+0-element Array{Bool,1}
+```
+"""
+first(v::AbstractVector, n::Integer) = @inbounds v[firstindex(v):min(firstindex(v) - 1 + n, end)]
+
 """
     last(coll)
 
@@ -361,6 +382,27 @@ julia> last([1; 2; 3; 4])
 """
 last(a) = a[end]
 
+"""
+    last(v::AbstractVector, n::Integer)
+
+Get the last n elements of the vector, without going out-of-bounds.
+
+# Examples
+```jldoctest
+julia> last(["foo", "bar", "qux"], 2)
+2-element Array{String,1}:
+ "bar"
+ "qux"
+
+julia> last(1:6, 10)
+1:6
+
+julia> first(Float64[], 1)
+0-element Array{Float64,1}
+```
+"""
+last(v::AbstractArray, n::Integer) = @inbounds v[max(firstindex(v), end + 1 - n):end]
+
 """
     strides(A)
 

test/abstractarray.jl

@@ -999,3 +999,15 @@ end
 @testset "vcat with mixed elements" begin
     @test vcat(Nothing[], [missing], [1.0], [Int8(1)]) isa Vector{Union{Missing, Nothing, Float64}}
 end
+
+@testset "first/last n elements of vector" begin
+    v = [1, 13, 42]
+    @test first(v, -2) == []
+    @test first(v, 2) == v[1:2]
+    @test first(v, 100) == v
+    @test first(v, 1) != v[1]
+    @test last(v, -2) == []
+    @test last(v, 2) == v[end-1:end]
+    @test last(v, 100) == v
+    @test last(v, 1) != v[end]
+end