Provide formatter for labeling categories in cut function

src/extras.jl

@@ -42,8 +42,16 @@ function fill_refs!(refs::AbstractArray, X::AbstractArray{>: Missing},
 end
 
 """
+    default_formatter(from, to, i; closed=false)
+
+Provide the default label format for the `cut` function.
+"""
+default_formatter(from, to, i; closed=false) = string("[", from, ", ", to, closed ? "]" : ")")
+
+@doc raw"""
     cut(x::AbstractArray, breaks::AbstractVector;
-        extend::Bool=false, labels::AbstractVector=[], allow_missing::Bool=false)
+        labels::Union{AbstractVector{<:AbstractString},Function}, 
+        extend::Bool=false, allow_missing::Bool=false)
 
 Cut a numeric array into intervals and return an ordered `CategoricalArray` indicating
 the interval into which each entry falls. Intervals are of the form `[lower, upper)`,
@@ -56,14 +64,76 @@ also accept them.
 * `extend::Bool=false`: when `false`, an error is raised if some values in `x` fall
   outside of the breaks; when `true`, breaks are automatically added to include all
   values in `x`, and the upper bound is included in the last interval.
-* `labels::AbstractVector=[]`: a vector of strings giving the names to use for the
-  intervals; if empty, default labels are used.
+* `labels::Union{AbstractVector,Function}: a vector of strings giving the names to use for
+  the intervals; or a function `f(from, to, i; extend)` that generates the labels from the
+  left and right interval boundaries and the group index. Defaults to 
+  `string("[", from, ", ", to, extend ? "]" : ")")`, e.g. `"[1, 5)"`.
 * `allow_missing::Bool=true`: when `true`, values outside of breaks result in missing values.
   only supported when `x` accepts missing values.
+
+# Examples
+```jldoctest
+julia> using CategoricalArrays
+
+julia> x = collect(-1:0.5:1);
+
+julia> cut(x, [0, 1], extend=true)
+5-element CategoricalArray{String,1,UInt32}:
+ "[-1.0, 0.0)"
+ "[-1.0, 0.0)"
+ "[0.0, 1.0]" 
+ "[0.0, 1.0]" 
+ "[0.0, 1.0]" 
+
+julia> cut(x, 2)
+5-element CategoricalArray{String,1,UInt32}:
+ "[-1.0, 0.0)"
+ "[-1.0, 0.0)"
+ "[0.0, 1.0]" 
+ "[0.0, 1.0]" 
+ "[0.0, 1.0]" 
+
+julia> cut(x, 2, labels=["A", "B"])
+5-element CategoricalArray{String,1,UInt32}:
+ "A"
+ "A"
+ "B"
+ "B"
+ "B"
+
+julia> fmt(from, to, i; closed) = "grp $i ($from//$to)"
+fmt (generic function with 1 method)
+
+julia> cut(x, 3, labels=fmt)
+5-element CategoricalArray{String,1,UInt32}:
+ "grp 1 (-1.0//-0.333333)"    
+ "grp 1 (-1.0//-0.333333)"    
+ "grp 2 (-0.333333//0.333333)"
+ "grp 3 (0.333333//1.0)"      
+ "grp 3 (0.333333//1.0)"      
+
+julia> using StatsBase: ecdf
+
+julia> percentile(x) = round(Int,100*parse(Float64,x))
+percentile (generic function with 1 method)
+
+julia> fmt2(from, to, i; closed) = "P$(percentile(from))P$(percentile(to))"
+fmt2 (generic function with 1 method)
+
+julia> cut(ecdf(x)(x), 3, labels=fmt2)
+5-element CategoricalArray{String,1,UInt32}:
+ "P20P47" 
+ "P20P47" 
+ "P47P73" 
+ "P73P100"
+ "P73P100"
+```
 """
 function cut(x::AbstractArray{T, N}, breaks::AbstractVector;
-             extend::Bool=false, labels::AbstractVector{U}=String[],
-             allow_missing::Bool=false) where {T, N, U<:AbstractString}
+             extend::Bool=false,
+             labels::Union{AbstractVector{<:AbstractString},Function}=default_formatter,
+             allow_missing::Bool=false) where {T, N}
+
     if !issorted(breaks)
         breaks = sort(breaks)
     end
@@ -92,7 +162,7 @@ function cut(x::AbstractArray{T, N}, breaks::AbstractVector;
     end
 
     n = length(breaks)
-    if isempty(labels)
+    if labels isa Function
         @static if VERSION >= v"0.7.0-DEV.4524"
             from = map(x -> sprint(show, x, context=:compact=>true), breaks[1:n-1])
             to = map(x -> sprint(show, x, context=:compact=>true), breaks[2:n])
@@ -102,13 +172,9 @@ function cut(x::AbstractArray{T, N}, breaks::AbstractVector;
         end
         levs = Vector{String}(undef, n-1)
         for i in 1:n-2
-            levs[i] = string("[", from[i], ", ", to[i], ")")
-        end
-        if extend
-            levs[end] = string("[", from[end], ", ", to[end], "]")
-        else
-            levs[end] = string("[", from[end], ", ", to[end], ")")
+            levs[i] = labels(from[i], to[i], i, closed=false)
         end
+        levs[end] = labels(from[end], to[end], n-1, closed=extend)
     else
         length(labels) == n-1 || throw(ArgumentError("labels must be of length $(n-1), but got length $(length(labels))"))
         # Levels must have element type String for type stability of the result
@@ -122,11 +188,11 @@ end
 
 """
     cut(x::AbstractArray, ngroups::Integer;
-        labels::AbstractVector=String[])
+        labels::Union{AbstractVector{<:AbstractString},Function})
 
 Cut a numeric array into `ngroups` quantiles, determined using
 [`quantile`](@ref).
 """
 cut(x::AbstractArray, ngroups::Integer;
-    labels::AbstractVector{U}=String[]) where {U<:AbstractString} =
+    labels::Union{AbstractVector{<:AbstractString},Function}=default_formatter) =
     cut(x, Statistics.quantile(x, (1:ngroups-1)/ngroups); extend=true, labels=labels)

test/15_extras.jl

@@ -108,4 +108,13 @@ end
     @test levels(x) == ["[2.0, 3.5)", "[3.5, 5.0]"]
 end
 
+@testset "formatter function" begin
+  my_formatter(from, to, i; extend) = "$i: $from -- $to"
+
+  x = collect(0.15:0.20:0.95)
+  p = [0, 0.4, 0.8, 1.0]
+
+  @test cut(x, p, labels=my_formatter2) == ["1: 0.0 -- 0.4", "1: 0.0 -- 0.4", "2: 0.4 -- 0.8", "2: 0.4 -- 0.8", "3: 0.8 -- 1.0"]
+end
+
 end