go build 通过文件名后缀实现不同平台的条件编译操作

package main
import (
 "io/ioutil"
 "fmt"
)
// switch会自动break，除非使用fallthrough；可以swich多条件；panic()报错让程序终止
func grade(score int) string {
 g := ""
 switch {
 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"
 //default:
 // panic(fmt.Sprintf("Wrong score: %d", score))
 }
 return g
}
func readFile(filename string) {
 contents, err := ioutil.ReadFile(filename)
 if err != nil {
  fmt.Println(err)
 } else {
  fmt.Printf("%s\n", contents)
 }
 // if的条件里可以赋值，其赋值的变量作用域就在if的block中
 if contents, err := ioutil.ReadFile(filename); err != nil {
  fmt.Println(err)
 } else {
  fmt.Printf("%s\n", contents)
 }
}
func main() {
 const filename = "abc.txt"
 readFile(filename)
 fmt.Println()
 fmt.Println(
  grade(0),
  grade(56),
  grade(82),
  grade(100),
  //grade(101), // 中断报错
  )
}

abcde
12345
hello
world
abcde
12345
hello
world
F F B A

package main
import (
 "fmt"
 "strconv"
 "os"
 "bufio"
)
// 十进制转二进制，for中省略初始条件
func convertToBin(n int) string {
 result := ""
 for ; n>0; n /= 2 {
  lsb := n % 2
  result = strconv.Itoa(lsb) + result
 }
 return result
}
// 一行行读文件，for中省略初始条件，相当于while
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())
 }
}
// 死循环，不要while了
func forever() {
 for {
  fmt.Println("abc")
 }
}
func main() {
 fmt.Println(
  convertToBin(5), // 101
  convertToBin(13), // 1101
  convertToBin(72387885),
  convertToBin(0),
  )
 printFile("abc.txt")
 //forever()
}

101 1101 100010100001000110100101101 
abcde
12345
hello
world