fix: propagate error from Quartile for inputs too small for quartiles

Quartile computed the median of empty slices for inputs with fewer than
three elements and discarded the resulting error, returning NaN quartiles
with a nil error. The silent NaN then leaked into InterQuartileRange,
Midhinge and Trimean. Propagate the error instead, mirroring the existing
EmptyInputErr handling.

quartile.go

@@ -32,10 +32,22 @@ func Quartile(input Float64Data) (Quartiles, error) {
 		c2 = c1 + 1
 	}
 
-	// Find the Medians with the cutoff points
-	Q1, _ := Median(copy[:c1])
-	Q2, _ := Median(copy)
-	Q3, _ := Median(copy[c2:])
+	// Find the Medians with the cutoff points.
+	// For il < 3 the lower and upper halves are empty, so Median
+	// returns ErrEmptyInput; propagate it instead of silently
+	// returning NaN quartiles.
+	Q1, err := Median(copy[:c1])
+	if err != nil {
+		return Quartiles{}, err
+	}
+	Q2, err := Median(copy)
+	if err != nil {
+		return Quartiles{}, err
+	}
+	Q3, err := Median(copy[c2:])
+	if err != nil {
+		return Quartiles{}, err
+	}
 
 	return Quartiles{Q1, Q2, Q3}, nil
 
@@ -46,7 +58,10 @@ func InterQuartileRange(input Float64Data) (float64, error) {
 	if input.Len() == 0 {
 		return math.NaN(), EmptyInputErr
 	}
-	qs, _ := Quartile(input)
+	qs, err := Quartile(input)
+	if err != nil {
+		return math.NaN(), err
+	}
 	iqr := qs.Q3 - qs.Q1
 	return iqr, nil
 }
@@ -56,7 +71,10 @@ func Midhinge(input Float64Data) (float64, error) {
 	if input.Len() == 0 {
 		return math.NaN(), EmptyInputErr
 	}
-	qs, _ := Quartile(input)
+	qs, err := Quartile(input)
+	if err != nil {
+		return math.NaN(), err
+	}
 	mh := (qs.Q1 + qs.Q3) / 2
 	return mh, nil
 }
@@ -68,7 +86,10 @@ func Trimean(input Float64Data) (float64, error) {
 	}
 
 	c := sortedCopy(input)
-	q, _ := Quartile(c)
+	q, err := Quartile(c)
+	if err != nil {
+		return math.NaN(), err
+	}
 
 	return (q.Q1 + (q.Q2 * 2) + q.Q3) / 4, nil
 }

quartile_test.go

@@ -41,6 +41,19 @@ func TestQuartile(t *testing.T) {
 	}
 }
 
+func TestQuartileSingleElement(t *testing.T) {
+	// A single element has no lower or upper half, so it cannot have
+	// quartiles. Quartile must return an error instead of silently
+	// producing NaN values with a nil error.
+	quartiles, err := stats.Quartile([]float64{1})
+	if err == nil {
+		t.Errorf("Single element slice should have returned an error")
+	}
+	if quartiles != (stats.Quartiles{}) {
+		t.Errorf("Errored result should be the zero Quartiles value, got %+v", quartiles)
+	}
+}
+
 func TestInterQuartileRange(t *testing.T) {
 	s1 := []float64{102, 104, 105, 107, 108, 109, 110, 112, 115, 116, 118}
 	iqr, _ := stats.InterQuartileRange(s1)
@@ -53,6 +66,11 @@ func TestInterQuartileRange(t *testing.T) {
 	if err == nil {
 		t.Errorf("Empty slice should have returned an error")
 	}
+
+	_, err = stats.InterQuartileRange([]float64{1})
+	if err == nil {
+		t.Errorf("Single element slice should have returned an error")
+	}
 }
 
 func TestMidhinge(t *testing.T) {
@@ -67,6 +85,11 @@ func TestMidhinge(t *testing.T) {
 	if err == nil {
 		t.Errorf("Empty slice should have returned an error")
 	}
+
+	_, err = stats.Midhinge([]float64{1})
+	if err == nil {
+		t.Errorf("Single element slice should have returned an error")
+	}
 }
 
 func TestTrimean(t *testing.T) {
@@ -81,4 +104,9 @@ func TestTrimean(t *testing.T) {
 	if err == nil {
 		t.Errorf("Empty slice should have returned an error")
 	}
+
+	_, err = stats.Trimean([]float64{1})
+	if err == nil {
+		t.Errorf("Single element slice should have returned an error")
+	}
 }