function

function 的結構如下，使用 func 宣告且需要有名稱、輸入參數、回傳結果跟 function 主體

func name(parameter-list) (result-list) {
	body
}

輸入參數如果是同型別的話可以簡化成 add(x, y int)．輸入參數沒用到的話可以用 _ 省略．
回傳結果可以給一個變數，這樣在 function 裡就可以不用最後再 return 一個值．

package main

import (
	"crypto/rand"
	"fmt"
	"math/big"
	mathRand "math/rand"
	"time"
)

func main() {
	fmt.Println(add(1, 2))         // 3
	fmt.Println(randomNum(100, 3)) // 66
	fmt.Println(mathRandom(100))   // 2813524261773535146

}
func add(x int, y int) int {
	return x + y
}
func randomNum(seed, _ int) (ret *big.Int) {
	ret, _ = rand.Int(rand.Reader, big.NewInt(int64(seed)))
	return
}
func mathRandom(seed int) (ret int) {
	mathRand.Seed(int64(time.Now().UnixNano()))
	ret = mathRand.Int()
	return
}

把匿名函式傳給變數的呼叫方式

sayHello := func(name string) {
	fmt.Println("Hello " + name)
}
sayHello("Daniel") // Hello Daniel

把函式當作參數傳遞

package main

import (
	"crypto/rand"
	"fmt"
	"math/big"
	"strconv"
)

func main() {
	sayHello := func(name string) string {
		return "Hello " + name
	}
	fmt.Println(randomNum(100, 1, sayHello)) // Hello Daniel,your number is 75
}

func randomNum(seed, _ int, sayHello func(string) string) (ret string) {
	number, _ := rand.Int(rand.Reader, big.NewInt(int64(seed)))
	ret = sayHello("Daniel") + ",your number is " + strconv.FormatInt(number.Int64(), 10)
	return
}

像 strings.Map 是把 funtcion 當參數傳遞，第 2 個參數字串會根據第 1 個參數的方法來做轉換

package main

import (
	"fmt"
	"strings"
)

func main() {
	trans := func(r rune) rune {
		if r == 'A' || r == 97 {
			return '*'
		}
		return r
	}
	retstr := strings.Map(trans, "ABCDEabcde")
	fmt.Println(retstr) // *BCDE*bcde
}

function 回傳多個結果

package main

import (
	"fmt"
)

func main() {
	sum, message := add(5, 10)
	fmt.Println(sum)     // 15
	fmt.Println(message) // High
}

func add(x, y int) (retSum int, message string) {
	sum := x + y
	retSum = sum
	if sum > 10 {
		message = "High"
	} else {
		message = "Low"
	}
	return
}

可變的輸入參數

package main

import (
	"fmt"
)

func main() {
	fmt.Println(sum())        // 0
	fmt.Println(sum(1))       // 1
	fmt.Println(sum(1, 2))    // 3
	fmt.Println(sum(1, 2, 3)) // 6
}

func sum(nums ...int) int {
	sum := 0
	for _, num := range nums {
		sum += num
	}
	return sum
}

匿名函式的使用方式

package main

import "fmt"

func main() {
	sum := 10
	func(num int) { sum += num }(3) // 有參數呼叫
	func() { sum -= 5 }() // 無參數呼叫
	func() { sum *= 2 }()
	fmt.Println(sum) // 16
}