Provide formatter for labeling categories in cut function

src/extras.jl

@@ -41,6 +41,13 @@ function fill_refs!(refs::AbstractArray, X::AbstractArray{>: Missing},
     end
 end
 
+"""
+    _default_formatter_
+
+Provide the default label format for the `cut` function.
+"""
+_default_formatter_(from, to, i; extend=false) = string("[", from, ", ", to, extend ? "]" : ")")
+
 """
     cut(x::AbstractArray, breaks::AbstractVector;
         extend::Bool=false, labels::AbstractVector=[], allow_missing::Bool=false)
@@ -56,14 +63,16 @@ 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}=_default_formatter_`: a vector of strings giving the names to use for the
+  intervals; or a function `f(from,to,i;extend=false)` 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.
 """
 function cut(x::AbstractArray{T, N}, breaks::AbstractVector;
-             extend::Bool=false, labels::AbstractVector{U}=String[],
+             extend::Bool=false, labels=_default_formatter_,
              allow_missing::Bool=false) where {T, N, U<:AbstractString}
+    (labels isa AbstractVector) || (labels isa Function) || throw(ArgumentError("labels must be a formatter function or an AbstractVector"))
+
     if !issorted(breaks)
         breaks = sort(breaks)
     end
@@ -92,7 +101,7 @@ 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])
@@ -102,12 +111,12 @@ 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], ")")
+            levs[i] = labels(from[i], to[i], i)
         end
         if extend
-            levs[end] = string("[", from[end], ", ", to[end], "]")
+            levs[end] = labels(from[end], to[end], n-1, extend=extend)
         else
-            levs[end] = string("[", from[end], ", ", to[end], ")")
+            levs[end] = labels(from[end], to[end], n-1)
         end
     else
         length(labels) == n-1 || throw(ArgumentError("labels must be of length $(n-1), but got length $(length(labels))"))
@@ -128,5 +137,8 @@ Cut a numeric array into `ngroups` quantiles, determined using
 [`quantile`](@ref).
 """
 cut(x::AbstractArray, ngroups::Integer;
-    labels::AbstractVector{U}=String[]) where {U<:AbstractString} =
+    labels=_default_formatter_) =
     cut(x, Statistics.quantile(x, (1:ngroups-1)/ngroups); extend=true, labels=labels)
+
+
+

test/15_extras.jl

@@ -108,4 +108,20 @@ end
     @test levels(x) == ["[2.0, 3.5)", "[3.5, 5.0]"]
 end
 
+@testset "formatter function" begin
+  my_formatter1(from,to,i;extend=false) = "group $i"
+  my_formatter2(from,to,i;extend=false) = "$i: $from -- $to"
+  function my_formatter3(from,to,i;extend=true)
+    percentile(x) = Int(round(100 * parse.(Float64,x),digits=0))
+    string("P",percentile(from),"P",percentile(to))
+  end
+
+  x = collect(0.15:0.20:0.95)
+  p = [0, 0.4, 0.8, 1.0]
+
+  @test cut(x, p, labels=my_formatter1) == ["group 1", "group 1", "group 2", "group 2", "group 3"]
+  @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"]
+  @test cut(x, p, labels=my_formatter3) == ["P0P40"  , "P0P40"  , "P40P80" , "P40P80" , "P80P100"]
+end
+
 end