Fix wrong makeunique suggestion in stack error message (#3526)

Author: anurag-mds

src/abstractdataframe/reshape.jl

@@ -155,8 +155,22 @@ function stack(df::AbstractDataFrame,
     end
     N = length(ints_measure_vars)
     cnames = _names(df)[ints_id_vars]
+
+    if variable_name_s in cnames
+        throw(ArgumentError("Column name :$variable_name_s already exists in the " *
+                            "data frame. Pass `variable_name` keyword argument to " *
+                            "use a different name for the variable column."))
+    end
+
+    if value_name_s in cnames
+        throw(ArgumentError("Column name :$value_name_s already exists in the " * 
+                            "data frame. Pass `value_name` keyword argument to " * 
+                            "use a different name for the value column."))
+    end
+
     push!(cnames, variable_name_s)
     push!(cnames, value_name_s)
+
     if variable_eltype === Symbol
         catnms = PooledArray(_names(df)[ints_measure_vars])
     elseif variable_eltype === String
@@ -187,6 +201,18 @@ function _stackview(df::AbstractDataFrame, measure_vars::AbstractVector{Int},
                     value_name::Symbol, variable_eltype::Type)
     N = length(measure_vars)
     cnames = _names(df)[ints_id_vars]
+
+    if variable_name in cnames
+        throw(ArgumentError("Column name :$variable_name already exists in the " *
+                            "data frame. Pass `variable_name` keyword argument to " *
+                            "use a different name for the variable column."))
+    end
+    if value_name in cnames
+        throw(ArgumentError("Column name :$value_name already exists in the " *
+                            "data frame. Pass `value_name` keyword argument to " *
+                            "use a different name for the value column."))
+    end
+
     push!(cnames, variable_name)
     push!(cnames, value_name)
     if variable_eltype === Symbol

test/reshape.jl

@@ -1006,4 +1006,18 @@ end
     DataFrame(x=1:3, a=[11, missing, missing], b=[missing, 12, missing], missing=[missing, missing, 13])
 end
 
+
+@testset "better stack errors" begin
+    df = DataFrame(foo=["a","a","b"], variable=["x","x","x"], v1=[1,1.5,2], v2=[10,11.5,13.5])
+    @test_throws ArgumentError stack(df)
+    @test_throws ArgumentError stack(df, [:v1, :v2], variable_name=:foo)
+    @test_throws ArgumentError stack(df, [:v1, :v2], value_name=:foo)
+    df2 = DataFrame(foo=["a","b"], value=["x","y"], v1=[1,2])
+    @test_throws ArgumentError stack(df2, [:v1], value_name=:foo)
+    # view=true path
+    @test_throws ArgumentError stack(df, [:v1, :v2], view=true)
+    @test_throws ArgumentError stack(df, [:v1, :v2], variable_name=:foo, view=true)
+end
+
+
 end # module