package main

import (
	"fmt"
	"io/ioutil"
)

func eval (a,b int , op string)int{
	var result int
	switch op {
	case "+":
		result = a + b
	case "-":
		result = a - b
	case "*":
		result = a*b
	case "/":
		result = a / b
	default:
		print("unsupported operator:"+ op)
	}
	return result
}

func main(){
	const filename = "abc.txt"
	//contents,err := ioutil.ReadFile(filename)
	//if err != nil{
	//	fmt.Println(err)
	//}else {
	//	fmt.Printf("%s\n",contents)
	//}
	if contents,err := ioutil.ReadFile(filename);err != nil{
		fmt.Println(err)
	}else {
		fmt.Printf("%s\n",contents)
	}

	fmt.Println(eval(3,3,"o"))
}

运行结果展示的顺序有点奇怪：
abcde
unsupported operator:o12345
hello
again
0
为什么 unsupported operator:o 被插在了中间？？？而不是在最后？
还有 panic 与 println 有何区别？