Go函数学习

package main

import (
    "fmt"
    "reflect"
    "runtime"
    "math"
)

//函数
//函数可以返回两个值。返回多个值时可以起名字
//返回值类型写在最后面
//函数可以作为参数
//没有默认参数，可选参数

func eval( a, b int , op string) ( int , error) {
    switch op {
    case "+":
        return a + b , nil
    case "-":
        return a - b , nil
    case "*":
        return a * b , nil
    case "/":
        q, _ := div(a , b)  //下划线表示不想用那个返回值
        return q , nil
    default:
        return 0 , fmt.Errorf("unsupported operation: %s" , op )
    }
}

func div( a, b int ) ( int , int)  {
    return a / b, a % b
}

func apply( op func( int , int ) int , a, b int ) int {  //参数名写在前，类型写在后
    p := reflect.ValueOf(op).Pointer()
    opName := runtime.FuncForPC(p).Name()
    fmt.Printf("calling function %s with agrs" + "(%d,%d)  ",opName,a,b)
    return op(a , b)
}

func pow( a, b int) int {
    return int (math.Pow( float64(a),float64(b)))
}

func sum( numbers ...int) int {   //...传可变参数列表
    s := 0
    for i := range numbers {
        s += numbers[i]
    }
    return  s
}
func main() {
    if result , err := eval( 3,4,"x"); err != nil {
        fmt.Println("Error",err)   //Error unsupported operation: x
    }else {
        fmt.Println(result)
    }
    fmt.Println(eval(3, 4, "*"))  // 12 <nil>
    fmt.Println(eval(3, 4, "/"))  // 0  <nil>
    fmt.Println( div(13 , 3))         // 4 1
    fmt.Println(apply( pow,3,4))   //calling function main.pow with agrs(3,4)  81

    fmt.Println(apply(
        func(a int, b int) int {
            return int( math.Pow(
                float64(a), float64(b) ))
        },3,4,
    ))   //calling function main.main.func1 with agrs(3,4)  81

    fmt.Println(sum(1,2,3,4,5))  //15
}