sort: new example: Sorting slices with sort.SliceStable

ExampleSliceStable echoes the sort.Slice example, to demonstrate sorting
on two fields together preserving order between sorts.

Change-Id: I8afc20c0203991bfd57260431eda73913c165355
Reviewed-on: https://go-review.googlesource.com/37196Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/sort/example_test.go

@@ -41,3 +41,31 @@ func ExampleSlice() {
 	// Output: By name: [{Alice 55} {Bob 75} {Gopher 7} {Vera 24}]
 	// By age: [{Gopher 7} {Vera 24} {Alice 55} {Bob 75}]
 }
+
+func ExampleSliceStable() {
+
+	people := []struct {
+		Name string
+		Age  int
+	}{
+		{"Alice", 25},
+		{"Elizabeth", 75},
+		{"Alice", 75},
+		{"Bob", 75},
+		{"Alice", 75},
+		{"Bob", 25},
+		{"Colin", 25},
+		{"Elizabeth", 25},
+	}
+
+	// Sort by name, preserving original order
+	sort.SliceStable(people, func(i, j int) bool { return people[i].Name < people[j].Name })
+	fmt.Println("By name:", people)
+
+	// Sort by age preserving name order
+	sort.SliceStable(people, func(i, j int) bool { return people[i].Age < people[j].Age })
+	fmt.Println("By age,name:", people)
+
+	// Output: By name: [{Alice 25} {Alice 75} {Alice 75} {Bob 75} {Bob 25} {Colin 25} {Elizabeth 75} {Elizabeth 25}]
+	// By age,name: [{Alice 25} {Bob 25} {Colin 25} {Elizabeth 25} {Alice 75} {Alice 75} {Bob 75} {Elizabeth 75}]
+}