Added cumprod function. Fixed error in CumSum

sliceops.go

@@ -98,7 +98,24 @@ func CumSum(receiver, s []float64) []float64 {
 	}
 	receiver[0] = s[0]
 	for i := 1; i < len(s); i++ {
-		receiver[i] += receiver[i-1] + s[i]
+		receiver[i] = receiver[i-1] + s[i]
+	}
+	return receiver
+}
+
+// Finds the cumulative product of the first i elements in 
+// s and puts them in place into the ith element of the
+// receiver. If the receiver is nil a new slice is created
+func CumProd(receiver, s []float64) []float64 {
+	if receiver == nil {
+		receiver = make([]float64, len(s))
+	}
+	if len(s) == 0 {
+		return receiver[:0]
+	}
+	receiver[0] = s[0]
+	for i := 1; i < len(s); i++ {
+		receiver[i] = receiver[i-1] + s[i]
 	}
 	return receiver
 }

sliceops_test.go

@@ -108,6 +108,10 @@ func TestCumSum(t *testing.T) {
 	val := CumSum(nil, s)
 	truth := []float64{3, 7, 8, 15, 20}
 	if !Eq(val, truth, 1E-15) {
-		t.Errorf("Wrong cumsum returned. Expected %v, returned %v", truth, val)
+		t.Errorf("Wrong cumsum returned with nil receiver. Expected %v, returned %v", truth, val)
+	}
+	val = CumSum(val, s)
+	if !Eq(val, truth, 1E-15) {
+		t.Errorf("Wrong cumsum returned with non-nil receiver. Expected %v, returned %v", truth, val)
 	}
 }