通常你在應用中定義了一個結構體,那么你也可能需要這個結構體的(指針)對象集合,比如:
type Any interface{}
type Car struct {
Model string
Manufacturer string
BuildYear int
// ...
}
type Cars []*Car
在定義所需功能時我們可以利用函數可以作為(其它函數的)參數的事實來使用高階函數,例如:
1)定義一個通用的 Process()
函數,它接收一個作用于每一輛 car 的 f 函數作參數:
// Process all cars with the given function f:
func (cs Cars) Process(f func(car *Car)) {
for _, c := range cs {
f(c)
}
}
2)在上面的基礎上,實現一個查找函數來獲取子集合,并在 Process()
中傳入一個閉包執(zhí)行(這樣就可以訪問局部切片 cars
):
// Find all cars matching a given criteria.
func (cs Cars) FindAll(f func(car *Car) bool) Cars {
cars := make([]*Car, 0)
cs.Process(func(c *Car) {
if f(c) {
cars = append(cars, c)
}
})
return cars
}
3)實現 Map 功能,產出除 car 對象以外的東西:
// Process cars and create new data.
func (cs Cars) Map(f func(car *Car) Any) []Any {
result := make([]Any, 0)
ix := 0
cs.Process(func(c *Car) {
result[ix] = f(c)
ix++
})
return result
}
現在我們可以定義下面這樣的具體查詢:
allNewBMWs := allCars.FindAll(func(car *Car) bool {
return (car.Manufacturer == "BMW") && (car.BuildYear > 2010)
})
4)我們也可以根據入參返回不同的函數。也許我們想根據不同的廠商添加汽車到不同的集合,但是這可能會是多變的。所以我們可以定義一個函數來產生特定的添加函數和 map 集:
func MakeSortedAppender(manufacturers[]string)(func(car*Car),map[string]Cars) {
// Prepare maps of sorted cars.
sortedCars := make(map[string]Cars)
for _, m := range manufacturers {
sortedCars[m] = make([]*Car, 0)
}
sortedCars["Default"] = make([]*Car, 0)
// Prepare appender function:
appender := func(c *Car) {
if _, ok := sortedCars[c.Manufacturer]; ok {
sortedCars[c.Manufacturer] = append(sortedCars[c.Manufacturer], c)
} else {
sortedCars["Default"] = append(sortedCars["Default"], c)
}
}
return appender, sortedCars
}
現在我們可以用它把汽車分類為獨立的集合,像這樣:
manufacturers := []string{"Ford", "Aston Martin", "Land Rover", "BMW", "Jaguar"}
sortedAppender, sortedCars := MakeSortedAppender(manufacturers)
allUnsortedCars.Process(sortedAppender)
BMWCount := len(sortedCars["BMW"])
我們讓這些代碼在下面的程序 cars.go 中執(zhí)行:
示例 11.18 cars.go:
// cars.go
package main
import (
"fmt"
)
type Any interface{}
type Car struct {
Model string
Manufacturer string
BuildYear int
// ...
}
type Cars []*Car
func main() {
// make some cars:
ford := &Car{"Fiesta", "Ford", 2008}
bmw := &Car{"XL 450", "BMW", 2011}
merc := &Car{"D600", "Mercedes", 2009}
bmw2 := &Car{"X 800", "BMW", 2008}
// query:
allCars := Cars([]*Car{ford, bmw, merc, bmw2})
allNewBMWs := allCars.FindAll(func(car *Car) bool {
return (car.Manufacturer == "BMW") && (car.BuildYear > 2010)
})
fmt.Println("AllCars: ", allCars)
fmt.Println("New BMWs: ", allNewBMWs)
//
manufacturers := []string{"Ford", "Aston Martin", "Land Rover", "BMW", "Jaguar"}
sortedAppender, sortedCars := MakeSortedAppender(manufacturers)
allCars.Process(sortedAppender)
fmt.Println("Map sortedCars: ", sortedCars)
BMWCount := len(sortedCars["BMW"])
fmt.Println("We have ", BMWCount, " BMWs")
}
// Process all cars with the given function f:
func (cs Cars) Process(f func(car *Car)) {
for _, c := range cs {
f(c)
}
}
// Find all cars matching a given criteria.
func (cs Cars) FindAll(f func(car *Car) bool) Cars {
cars := make([]*Car, 0)
cs.Process(func(c *Car) {
if f(c) {
cars = append(cars, c)
}
})
return cars
}
// Process cars and create new data.
func (cs Cars) Map(f func(car *Car) Any) []Any {
result := make([]Any, len(cs))
ix := 0
cs.Process(func(c *Car) {
result[ix] = f(c)
ix++
})
return result
}
func MakeSortedAppender(manufacturers []string) (func(car *Car), map[string]Cars) {
// Prepare maps of sorted cars.
sortedCars := make(map[string]Cars)
for _, m := range manufacturers {
sortedCars[m] = make([]*Car, 0)
}
sortedCars["Default"] = make([]*Car, 0)
// Prepare appender function:
appender := func(c *Car) {
if _, ok := sortedCars[c.Manufacturer]; ok {
sortedCars[c.Manufacturer] = append(sortedCars[c.Manufacturer], c)
} else {
sortedCars["Default"] = append(sortedCars["Default"], c)
}
}
return appender, sortedCars
}
輸出:
AllCars: [0xf8400038a0 0xf840003bd0 0xf840003ba0 0xf840003b70]
New BMWs: [0xf840003bd0]
Map sortedCars: map[Default:[0xf840003ba0] Jaguar:[] Land Rover:[] BMW:[0xf840003bd0 0xf840003b70] Aston Martin:[] Ford:[0xf8400038a0]]
We have 2 BMWs