Add view kwarg to first and last (#2951)

NEWS.md

@@ -94,6 +94,8 @@
   ([#2459](https://github.com/JuliaData/DataFrames.jl/pull/2459))
 * `sort!` now supports general `AbstractDataFrame`
   ([#2946](https://github.com/JuliaData/DataFrames.jl/pull/2946))
+* `filter` now supports `view` keyword argument
+  ([#2951](https://github.com/JuliaData/DataFrames.jl/pull/2951))
 
 ## Bug fixes
 

src/abstractdataframe/abstractdataframe.jl

@@ -486,11 +486,15 @@ Get the first row of `df` as a `DataFrameRow`.
 Base.first(df::AbstractDataFrame) = df[1, :]
 
 """
-    first(df::AbstractDataFrame, n::Integer)
+    first(df::AbstractDataFrame, n::Integer; view::Bool=false)
 
 Get a data frame with the `n` first rows of `df`.
+
+If `view=false` a freshly allocated `DataFrame` is returned.
+If `view=true` then a `SubDataFrame` view into `df` is returned.
 """
-Base.first(df::AbstractDataFrame, n::Integer) = df[1:min(n, nrow(df)), :]
+@inline Base.first(df::AbstractDataFrame, n::Integer; view::Bool=false) = 
+    view ? Base.view(df, 1:min(n ,nrow(df)), :) : df[1:min(n, nrow(df)), :]
 
 """
     last(df::AbstractDataFrame)
@@ -500,11 +504,15 @@ Get the last row of `df` as a `DataFrameRow`.
 Base.last(df::AbstractDataFrame) = df[nrow(df), :]
 
 """
-    last(df::AbstractDataFrame, n::Integer)
+    last(df::AbstractDataFrame, n::Integer; view::Bool=false)
 
 Get a data frame with the `n` last rows of `df`.
+
+If `view=false` a freshly allocated `DataFrame` is returned.
+If `view=true` then a `SubDataFrame` view into `df` is returned.
 """
-Base.last(df::AbstractDataFrame, n::Integer) = df[max(1, nrow(df)-n+1):nrow(df), :]
+@inline Base.last(df::AbstractDataFrame, n::Integer; view::Bool=false) = 
+    view ? Base.view(df, max(1, nrow(df)-n+1):nrow(df), :) : df[max(1, nrow(df)-n+1):nrow(df), :]
 
 
 """

test/dataframe.jl

@@ -1105,6 +1105,19 @@ end
     @test last(df, 6) == DataFrame(A=5:10)
     @test last(df, 1) == DataFrame(A=10)
 
+    @inferred first(df, 6)
+    @inferred last(df, 6)
+    @inferred first(df)
+    @inferred last(df)
+
+    @test first(df, 6, view=true) == DataFrame(A=1:6)
+    @test last(df, 6, view=true) == DataFrame(A=5:10)
+
+    @test first(df, 6, view=true) isa SubDataFrame
+    @test first(df, 6, view=false) isa DataFrame
+    @test last(df, 6, view=true) isa SubDataFrame
+    @test last(df, 6, view=false) isa DataFrame
+
     @test_throws ArgumentError only(df)
     @test_throws ArgumentError only(DataFrame())
     df = DataFrame(a=1, b=2)