doc: add example for Product and ProductBy

Author: samber

math.go

@@ -88,7 +88,6 @@ func SumBy[T any, R constraints.Float | constraints.Integer | constraints.Comple
 // Product gets the product of the values in a collection. If collection is empty 0 is returned.
 // Play: https://go.dev/play/p/2_kjM_smtAH
 func Product[T constraints.Float | constraints.Integer | constraints.Complex](collection []T) T {
-
 	if collection == nil {
 		return 0
 	}
@@ -107,7 +106,6 @@ func Product[T constraints.Float | constraints.Integer | constraints.Complex](co
 // ProductBy summarizes the values in a collection using the given return value from the iteration function. If collection is empty 0 is returned.
 // Play: https://go.dev/play/p/wadzrWr9Aer
 func ProductBy[T any, R constraints.Float | constraints.Integer | constraints.Complex](collection []T, iteratee func(item T) R) R {
-
 	if collection == nil {
 		return 0
 	}

math_example_test.go

@@ -67,12 +67,33 @@ func ExampleSumBy() {
 	// Output: 6
 }
 
+func ExampleProduct() {
+	list := []int{1, 2, 3, 4, 5}
+
+	result := Product(list)
+
+	fmt.Printf("%v", result)
+	// Output: 120
+}
+
+func ExampleProductBy() {
+	list := []string{"foo", "bar"}
+
+	result := ProductBy(list, func(item string) int {
+		return len(item)
+	})
+
+	fmt.Printf("%v", result)
+	// Output: 9
+}
+
 func ExampleMean() {
 	list := []int{1, 2, 3, 4, 5}
 
 	result := Mean(list)
 
 	fmt.Printf("%v", result)
+	// Output: 3
 }
 
 func ExampleMeanBy() {
@@ -83,4 +104,5 @@ func ExampleMeanBy() {
 	})
 
 	fmt.Printf("%v", result)
+	// Output: 3
 }