Allow permutaion of names in rename!

src/abstractdataframe/abstractdataframe.jl

@@ -150,21 +150,21 @@ rename(f::Function, df::AbstractDataFrame)
 
 * `::AbstractDataFrame` : the updated result
 
-New names are processed sequentially. A new name must not already exist in the `DataFrame`
-at the moment an attempt to rename a column is performed.
+Each name is changed at most once. Permutation of names is allowed.
 
 **Examples**
 
 ```julia
 df = DataFrame(i = 1:10, x = rand(10), y = rand(["a", "b", "c"], 10))
 rename(df, :i => :A, :x => :X)
+rename(df, :x => :y, :y => :x)
 rename(df, [:i => :A, :x => :X])
 rename(df, Dict(:i => :A, :x => :X))
 rename(x -> Symbol(uppercase(string(x))), df)
 rename(df) do x
     Symbol(uppercase(string(x)))
 end
-rename!(df, Dict(:i =>: A, :x => :X))
+rename!(df, Dict(:i => :A, :x => :X))
 ```
 
 """

src/other/index.jl

@@ -46,14 +46,41 @@ function names!(x::Index, nms::Vector{Symbol}; makeunique::Bool=false)
 end
 
 function rename!(x::Index, nms)
+    xbackup = copy(x)
+    processedfrom = Set{Symbol}()
+    processedto = Set{Symbol}()
+    toholder = Dict{Symbol,Int}()
     for (from, to) in nms
+        if from ∈ processedfrom
+            merge!(empty!(x.lookup), xbackup.lookup)
+            x.names .= xbackup.names
+            throw(ArgumentError("Tried renaming $from multiple times."))
+        end
+        if to ∈ processedto
+            merge!(empty!(x.lookup), xbackup.lookup)
+            x.names .= xbackup.names
+            throw(ArgumentError("Tried renaming to $to multiple times."))
+        end
+        push!(processedfrom, from)
+        push!(processedto, to)
         from == to && continue # No change, nothing to do
+        if !haskey(xbackup, from)
+            merge!(empty!(x.lookup), xbackup.lookup)
+            x.names .= xbackup.names
+            throw(ArgumentError("Tried renaming $from to $to, when $from does not exist in the Index."))
+        end
         if haskey(x, to)
-            error("Tried renaming $from to $to, when $to already exists in the Index.")
+            toholder[to] = x.lookup[to]
         end
-        x.lookup[to] = col = pop!(x.lookup, from)
+        col = haskey(toholder, from) ? pop!(toholder, from) : pop!(x.lookup, from)
+        x.lookup[to] = col
         x.names[col] = to
     end
+    if !isempty(toholder)
+        merge!(empty!(x.lookup), xbackup.lookup)
+        x.names .= xbackup.names
+        throw(ArgumentError("Tried renaming to $(first(keys(toholder))), when it already exists in the Index."))
+    end
     return x
 end
 

test/dataframe.jl

@@ -1127,6 +1127,33 @@ end
     @test names(df) == [:A_4, :B_4]
     @test rename!(x->Symbol(lowercase(string(x))), df) === df
     @test names(df) == [:a_4, :b_4]
+
+    df = DataFrame(A = 1:3, B = 'A':'C', C = [:x, :y, :z])
+    @test rename!(df, :A => :B, :B => :A) === df
+    @test names(df) == [:B, :A, :C]
+    @test rename!(df, :A => :B, :B => :A, :C => :D) === df
+    @test names(df) == [:A, :B, :D]
+    @test rename!(df, :A => :B, :B => :C, :D => :A) === df
+    @test names(df) == [:B, :C, :A]
+    @test rename!(df, :A => :C, :B => :A, :C => :B) === df
+    @test names(df) == [:A, :B, :C]
+    @test rename!(df, :A => :A, :B => :B, :C => :C) === df
+    @test names(df) == [:A, :B, :C]
+
+    @test_throws ArgumentError rename!(df, :X => :Y)
+    @test names(df) == [:A, :B, :C]
+    @test_throws ArgumentError rename!(df, :A => :X, :X => :Y)
+    @test names(df) == [:A, :B, :C]
+    @test_throws ArgumentError rename!(df, :A => :B)
+    @test names(df) == [:A, :B, :C]
+    @test_throws ArgumentError rename!(df, :A => :X, :A => :X)
+    @test names(df) == [:A, :B, :C]
+    @test_throws ArgumentError rename!(df, :A => :X, :B => :X)
+    @test names(df) == [:A, :B, :C]
+    @test_throws ArgumentError rename!(df, :A => :B, :B => :A, :C => :B)
+    @test names(df) == [:A, :B, :C]
+    @test_throws ArgumentError rename!(df, :A => :B, :B => :A, :A => :X)
+    @test names(df) == [:A, :B, :C]
 end
 
 @testset "size" begin