Fix bug in quantile for case of 1 element vector (JuliaStats#42)

* Fix bug in quantile for case of 1 element vector

* add tests

* make the implementation type-stable

* Remove )

Co-authored-by: Milan Bouchet-Valat <nalimilan@club.fr>

src/Statistics.jl

@@ -969,14 +969,22 @@ end
     require_one_based_indexing(v)
 
     n = length(v)
+
+    @assert n > 0 # this case should never happen here
+
     m = alpha + p * (one(alpha) - alpha - beta)
     aleph = n*p + oftype(p, m)
     j = clamp(trunc(Int, aleph), 1, n-1)
     γ = clamp(aleph - j, 0, 1)
 
-    a = v[j]
-    b = v[j + 1]
-
+    if n == 1
+        a = v[1]
+        b = v[1]
+    else
+        a = v[j]
+        b = v[j + 1]
+    end
+
     if isfinite(a) && isfinite(b)
         return a + γ*(b-a)
     else

test/runtests.jl

@@ -557,6 +557,7 @@ end
     @test_throws ArgumentError quantile([1, missing], 0.5)
     @test_throws ArgumentError quantile([1, NaN], 0.5)
     @test quantile(skipmissing([1, missing, 2]), 0.5) === 1.5
+    @test quantile([1], 0.5) === 1.0
 
     # make sure that type inference works correctly in normal cases
     for T in [Int, BigInt, Float64, Float16, BigFloat, Rational{Int}, Rational{BigInt}]