深入理解Go语言中的多返回值处理：用户定义函数与内置机制的差异

func divide(a, b int) (int, error) {
    if b == 0 {
        return 0, fmt.Errorf("division by zero")
    }
    return a / b, nil
}

package main

import "fmt"

func f2() (k, v string) {
    return "Hello", "World"
}

func main() {
    // 错误示例：多返回值函数在单值上下文中
    // k := f2() // 编译错误: multiple-value f2() in single-value context

    // 正确示例1：接收所有返回值
    k, v := f2()
    fmt.Printf("f2() returns: k=%s, v=%s\n", k, v)

    // 正确示例2：使用下划线忽略不需要的返回值
    greeting, _ := f2()
    fmt.Printf("f2() greeting: %s\n", greeting)

    // 正确示例3：完全忽略所有返回值 (如果函数没有副作用，通常不推荐)
    f2()
}

package main

import "fmt"

func main() {
    m := map[string]int{"One": 1, "Two": 2}

    // 单值形式：获取值
    v1 := m["One"]
    fmt.Printf("m[\"One\"] (single-value): %d\n", v1) // Output: 1

    v_nonexistent := m["Three"]
    fmt.Printf("m[\"Three\"] (single-value, nonexistent): %d\n", v_nonexistent) // Output: 0 (int的零值)

    // 双值形式：获取值并检查键是否存在
    v2, ok2 := m["Two"]
    if ok2 {
        fmt.Printf("m[\"Two\"] (double-value): %d, key exists: %t\n", v2, ok2) // Output: 2, key exists: true
    }

    v3, ok3 := m["Three"]
    if !ok3 {
        fmt.Printf("m[\"Three\"] (double-value): %d, key exists: %t\n", v3, ok3) // Output: 0, key exists: false
    }
}

package main

import "fmt"

func main() {
    m := map[string]int{"One": 1, "Two": 2, "Three": 3}

    fmt.Println("Iterating map with single-value range (keys only):")
    for k := range m {
        fmt.Printf("Key: %s\n", k)
    }

    fmt.Println("\nIterating map with double-value range (keys and values):")
    for k, v := range m {
        fmt.Printf("Key: %s, Value: %d\n", k, v)
    }
}

package main

import "fmt"

func main() {
    var i interface{} = "Hello Go"

    // 单值形式：如果断言失败会panic
    s1 := i.(string)
    fmt.Printf("Type assertion (single-value, success): %s\n", s1)

    // var j interface{} = 123
    // s_panic := j.(string) // This would panic: interface conversion: interface {} is int, not string

    // 双值形式：安全地进行类型断言
    s2, ok2 := i.(string)
    if ok2 {
        fmt.Printf("Type assertion (double-value, success): %s, ok: %t\n", s2, ok2)
    }

    n, ok3 := i.(int)
    if !ok3 {
        fmt.Printf("Type assertion (double-value, failure): %d, ok: %t\n", n, ok3) // Output: 0, ok: false
    }
}