package main

import (
   "io/ioutil"
   "fmt"
)

func grade(score int) string{
   g:=""
   switch {//可以省略掉switch中的判断
   case score<0 || score>100:
      panic(fmt.Sprintf(
         "Wrong score:%d",score))
   case score <60:
      g="f"
   case score <80:
      g="C"
   case score <90:
      g="B"
   case score <=100:
      g="A"
   }
   return g
}

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:
      panic("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)
   //}
   //简便写法,if的条件里可以赋值，条件里赋值的变量作用域就在这个if语句里
   if contents,err:= ioutil.ReadFile(filename);err !=nil{
      fmt.Println(err)
   }else {
      fmt.Printf("%s\n",contents)
   }

   fmt.Printf("%s\n",grade(99))
   fmt.Println(eval(1,2,"*"))
}

package main

import (

	"fmt"

	"strconv"

	"os"

	"bufio"

)

func convertToBin(n int) string{

	result :=""

	for ;n > 0 ; n /= 2 {

		lsb:=n % 2

		result = strconv.Itoa(lsb) + result

	}

	return result

}

func printFile(filename string){

	file,err := os.Open(filename)

	if err !=nil{

		panic(err)

	}

	scanner := bufio.NewScanner(file)

	for  scanner.Scan() {

		fmt.Println(scanner.Text())

	}

}

func main() {

	fmt.Println(

		convertToBin(5),

		convertToBin(128),

	)

	printFile("abc.txt")

}