slice 做function的參數

package main

import "fmt"

func modify(foo []string) {
	foo[1] = "c"
	fmt.Println("modify foo", foo)
}

func main() {
	foo := []string{"a", "b"}
	fmt.Println("before foo: ", foo)
	modify(foo)
	fmt.Println("after foo: ", foo)
}

// before foo: [a b]
// modify foo [a c]
// after foo: [a c]
因為是call by reference
所以會影響foo的值

嘗試用append 改變slice

package main

import "fmt"

func addVal(foo []string) {
	foo = append(foo, "c")
	fmt.Println("modify foo", foo)
}

func main() {
	foo := []string{"a", "b"}
	fmt.Println("before foo: ", foo)
	addVal(foo)
	fmt.Println("after foo: ", foo)
}

// before foo: [a b]
// modify foo [a b c]
// after foo: [a b]
因為append回傳新的value
所以addVal function裡的foo是得到append回傳的value
但外面的foo是不會污染的

但如果要用append 改變外頭foo的值呢

可以如下

package main

import "fmt"

func addVal(foo *[]string) {
	*foo = append(*foo, "c")
	fmt.Println("modify foo", foo)
}

func main() {
	foo := []string{"a", "b"}
	fmt.Println("before foo: ", foo)
	addVal(&foo)
	fmt.Println("after foo: ", foo)
}

會得到結果為
// before foo: [a b]
// modify foo &[a b c]
// after foo: [a b c]

但不建議用此方法，維護程式碼會很複雜，很亂

建議寫法如下

package main

import "fmt"

func addVal(foo []string) []string {
	foo = append(foo, "c")
	fmt.Println("modify foo", foo)
	return foo
}

func main() {
	foo := []string{"a", "b"}
	fmt.Println("before foo: ", foo)
	foo = addVal(foo)
	fmt.Println("after foo: ", foo)
}

// before foo: [a b]
// modify foo [a b c]
// after foo: [a b c]
這樣閱讀性提高

another observe

package main

import "fmt"

func main() {
	foo := []string{"a", "b"}
	bar := foo[:1]
	fmt.Println("bar: ", bar) // [a]
	s1 := append(bar, "c")
	fmt.Println("foo: ", foo) // [a c]
	fmt.Println("s1: ", s1)   // [a c]
	s2 := append(bar, "d")
	fmt.Println("foo: ", foo) // [a d]
	fmt.Println("s2: ", s2)   // [a d]
	s3 := append(bar, "e", "f")
	fmt.Println("foo: ", foo) // [a d] 因為超過自己原本是兩個值的空間 =>foo := []string{"a", "b"}
	fmt.Println("s3: ", s3)   // [a e f]
}