Make median non-mutating on arrays.

Due to a change in the behaviour of `mapslices` (JuliaLang#16260), `median(X,k)` would mutate the underlying array. Fixes JuliaLang#17153.

NEWS.md

@@ -84,6 +84,8 @@ Breaking changes
     If a reshaped copy is needed, use `copy(reshape(a))` or `copy!` to a new array of
     the desired shape ([#4211]).
 
+  * `mapslices` will always pass a view, so passing mutating functions will mutate the underlying array ([#16260])
+
   * Local variables and arguments are represented in lowered code as numbered `Slot`
     objects instead of as symbols ([#15609]).
 

base/statistics.jl

@@ -490,9 +490,10 @@ function median!{T}(v::AbstractVector{T})
     end
 end
 median!{T}(v::AbstractArray{T}) = median!(vec(v))
-
 median{T}(v::AbstractArray{T}) = median!(copy!(Array(T, length(v)), v))
-median{T}(v::AbstractArray{T}, region) = mapslices(median!, v, region)
+
+median!{T}(v::AbstractArray{T}, region) = mapslices(median!, v, region)
+median{T}(v::AbstractArray{T}, region) = median!(copy(v), region)
 
 # for now, use the R/S definition of quantile; may want variants later
 # see ?quantile in R -- this is type 7

test/statistics.jl

@@ -32,8 +32,10 @@ end
 @test median([1.,-1.,Inf,-Inf]) == 0.0
 @test isnan(median([-Inf,Inf]))
 
-@test all(median([2 3 1 -1; 7 4 5 -4], 2) .== [1.5, 4.5])
-@test all(median([2 3 1 -1; 7 4 5 -4], 1) .== [4.5 3.5 3.0 -2.5])
+X = [2 3 1 -1; 7 4 5 -4]
+@test all(median(X, 2) .== [1.5, 4.5])
+@test all(median(X, 1) .== [4.5 3.5 3.0 -2.5])
+@test X == [2 3 1 -1; 7 4 5 -4] # issue #17153
 
 @test_throws ArgumentError median([])
 @test isnan(median([NaN]))
@@ -44,6 +46,7 @@ end
 @test median!([1 2 3 4]) == 2.5
 @test median!([1 2; 3 4]) == 2.5
 
+
 @test invoke(median, (AbstractVector,), 1:10) == median(1:10) == 5.5
 
 # mean