fix first and last for negative row count

NEWS.md

@@ -16,6 +16,9 @@
 
 ## Bug fixes
 
+* Correctly throw an error if negative number of rows is passed
+  to `first` or `last`
+  ([#3402](https://github.com/JuliaData/DataFrames.jl/pull/3402))
 * Always use the default thread pool for multithreaded operations,
   instead of using the interactive thread pool when Julia was started
   with `-tM,N` with N > 0

src/abstractdataframe/abstractdataframe.jl

@@ -558,14 +558,19 @@ Base.first(df::AbstractDataFrame) = df[1, :]
     first(df::AbstractDataFrame, n::Integer; view::Bool=false)
 
 Get a data frame with the `n` first rows of `df`.
+If `n` is greater than number of rows of `df` get all rows.
+Error if `n` is negative.
 
 If `view=false` a freshly allocated `DataFrame` is returned.
 If `view=true` then a `SubDataFrame` view into `df` is returned.
 
 $METADATA_FIXED
 """
-@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)), :]
+@inline function Base.first(df::AbstractDataFrame, n::Integer; view::Bool=false)
+    n < 0 && throw(ArgumentError("Number of elements must be nonnegative"))
+    r = min(n, nrow(df))
+    return view ? Base.view(df, 1:r, :) : df[1:r, :]
+end
 
 """
     last(df::AbstractDataFrame)
@@ -580,14 +585,19 @@ Base.last(df::AbstractDataFrame) = df[nrow(df), :]
     last(df::AbstractDataFrame, n::Integer; view::Bool=false)
 
 Get a data frame with the `n` last rows of `df`.
+If `n` is greater than number of rows of `df` get all rows.
+Error if `n` is negative.
 
 If `view=false` a freshly allocated `DataFrame` is returned.
 If `view=true` then a `SubDataFrame` view into `df` is returned.
 
 $METADATA_FIXED
 """
-@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), :]
+@inline function Base.last(df::AbstractDataFrame, n::Integer; view::Bool=false)
+    n < 0 && throw(ArgumentError("Number of elements must be nonnegative"))
+    r = max(1, nrow(df) - n + 1)
+    return view ? Base.view(df, r:nrow(df), :) : df[r:nrow(df), :]
+end
 
 """
     describe(df::AbstractDataFrame; cols=:)

test/dataframe.jl

@@ -1180,10 +1180,16 @@ end
     @test_throws BoundsError first(DataFrame(x=[]))
     @test_throws BoundsError last(DataFrame(x=[]))
 
-    @test first(df, 6) == DataFrame(A=1:6)
-    @test first(df, 1) == DataFrame(A=1)
-    @test last(df, 6) == DataFrame(A=5:10)
-    @test last(df, 1) == DataFrame(A=10)
+    for v in (true, false)
+        @test first(df, 6, view=v) == DataFrame(A=1:6)
+        @test first(df, 1, view=v) == DataFrame(A=1)
+        @test first(df, 0, view=v) == DataFrame()
+        @test_throws ArgumentError first(df, -1, view=v)
+        @test last(df, 6, view=v) == DataFrame(A=5:10)
+        @test last(df, 1, view=v) == DataFrame(A=10)
+        @test last(df, 0, view=v) == DataFrame()
+        @test_throws ArgumentError last(df, -1, view=v)
+    end
 
     @inferred first(df, 6)
     @inferred last(df, 6)