Add DropConstant transform (#252)

* Add 'DropConstant' transform

* Apply suggestions

Author: eliascarv

docs/src/transforms.md

@@ -92,6 +92,12 @@ DropExtrema
 DropUnits
 ```
 
+## DropConstant
+
+```@docs
+DropConstant
+```
+
 ## AbsoluteUnits
 
 ```@docs

src/TableTransforms.jl

@@ -54,6 +54,7 @@ export
   Satisfies,
   Only,
   Except,
+  DropConstant,
   Rename,
   StdNames,
   StdFeats,

src/transforms/satisfies.jl

@@ -66,3 +66,10 @@ Except(DST.Categorical)
 ```
 """
 Except(S::Type{<:SciType}) = Satisfies(x -> !(elscitype(x) <: S))
+
+"""
+    DropConstant()
+
+Drops the constant columns using the `allequal` function.
+"""
+DropConstant() = Satisfies(!allequal)

test/transforms/satisfies.jl

@@ -71,3 +71,19 @@ end
   tₒ = revert(T, n, c)
   @test t == tₒ
 end
+
+@testset "DropConstant" begin
+  a = [4, 6, 7, 8, 1, 2]
+  b = fill(5, 6)
+  c = [1.9, 7.4, 8.6, 8.9, 2.4, 7.7]
+  d = fill(5.5, 6)
+  t = Table(; a, b, c, d)
+
+  T = DropConstant()
+  n, c = apply(T, t)
+  @test Tables.schema(n).names == (:a, :c)
+  @test Tables.getcolumn(n, :a) == t.a
+  @test Tables.getcolumn(n, :c) == t.c
+  tₒ = revert(T, n, c)
+  @test t == tₒ
+end