golang 基本语法

一、变量定义

package main

import (
	"fmt"
	"math"
	"math/cmplx"
)

var (
	aa = 3
	ss = "kkk"
	bb = true
)

func variableZeroValue() {
	var a int
	var s string
	fmt.Printf("%d %q\n", a, s)
}

func variableInitialValue() {
	var a, b int = 3, 4
	var s string = "abc"
	fmt.Println(a, b, s)
}

func variableTypeDeduction() {
	var a, b, c, s = 3, 4, true, "def"
	fmt.Println(a, b, c, s)
}

func variableShorter() {
	a, b, c, s := 3, 4, true, "def"
	b = 5
	fmt.Println(a, b, c, s)
}

func euler() {
	fmt.Printf("%.3f\n",
		cmplx.Exp(1i*math.Pi)+1)
}

func triangle() {
	var a, b int = 3, 4
	fmt.Println(calcTriangle(a, b))
}

func calcTriangle(a, b int) int {
	var c int
	c = int(math.Sqrt(float64(a*a + b*b)))
	return c
}

func consts() {
	const (
		filename = "abc.txt"
		a, b     = 3, 4
	)
	var c int
	c = int(math.Sqrt(a*a + b*b))
	fmt.Println(filename, c)
}

func enums() {
	const (
		cpp = iota
		_
		python
		golang
		javascript
	)

	const (
		b = 1 << (10 * iota)
		kb
		mb
		gb
		tb
		pb
	)

	fmt.Println(cpp, javascript, python, golang)
	fmt.Println(b, kb, mb, gb, tb, pb)
}

func main() {
	fmt.Println("Hello world")
	variableZeroValue()
	variableInitialValue()
	variableTypeDeduction()
	variableShorter()
	fmt.Println(aa, ss, bb)

	euler()
	triangle()
	consts()
	enums()
}

二、switch、if

package main

import (
	"fmt"
	"io/ioutil"
)

func grade(score int) string {//switch不需要break，且可以加入条件判断
	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"
	}
	return g
}

func main() {
	// If "abc.txt" is not found,
	// please check what current directory is,
	// and change filename accordingly.
	const filename = "abc.txt"
	if contents, err := ioutil.ReadFile(filename); err != nil {//if不需要括号，且可以写两个语句
		fmt.Println(err)
	} else {
		fmt.Printf("%s\n", contents)
	}

	fmt.Println(
		grade(0),
		grade(59),
		grade(60),
		grade(82),
		grade(99),
		grade(100),
		// Uncomment to see it panics.
		// grade(-3),
	)
}

三、for

package main

import (
	"bufio"
	"fmt"
	"io"
	"os"
	"strconv"
	"strings"
)

func convertToBin(n int) string {
	result := ""
	for ; n > 0; n /= 2 {//一个判断，一个赋值
		lsb := n % 2
		result = strconv.Itoa(lsb) + result
	}
	return result
}

func printFile(filename string) {
	file, err := os.Open(filename)
	if err != nil {
		panic(err)
	}

	printFileContents(file)
}

func printFileContents(reader io.Reader) {
	scanner := bufio.NewScanner(reader)

	for scanner.Scan() {//一个判断，循环
		fmt.Println(scanner.Text())
	}
}

func forever() {
	for {//死循环
		fmt.Println("abc")
	}
}

func main() {
	fmt.Println("convertToBin results:")
	fmt.Println(
		convertToBin(5),  // 101
		convertToBin(13), // 1101
		convertToBin(72387885),
		convertToBin(0),
	)

	fmt.Println("abc.txt contents:")
	printFile("basic/branch/abc.txt")

	fmt.Println("printing a string:")
	s := `abc"d"
	kkkk
	123

	p`
	printFileContents(strings.NewReader(s))

	// Uncomment to see it runs forever
	// forever()
}

四、函数

package main

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

func eval(a, b int, op string) (int, error) {//返回两个值
	switch op {
	case "+":
		return a + b, nil
	case "-":
		return a - b, nil
	case "*":
		return a * b, nil
	case "/":
		q, _ := div(a, b)//第二值不使用，所以用_代替
		return q, nil
	default:
		return 0, fmt.Errorf(
			"unsupported operation: %s", op)
	}
}

func div(a, b int) (q, r int) {//定义了返回两个值的名字，调用函数时编译器可自动推断出来
	return a / b, a % b
}

func apply(op func(int, int) int, a, b int) int {//函数作为参数
	p := reflect.ValueOf(op).Pointer()
	opName := runtime.FuncForPC(p).Name()
	fmt.Printf("Calling function %s with args "+
		"(%d, %d)\n", opName, a, b)

	return op(a, b)
}

func sum(numbers ...int) int {
	s := 0
	for i := range numbers {
		s += numbers[i]
	}
	return s
}

func swap(a, b int) (int, int) {
	return b, a
}

func main() {
	fmt.Println("Error handling")
	if result, err := eval(3, 4, "x"); err != nil {//if语法
		fmt.Println("Error:", err)
	} else {
		fmt.Println(result)
	}
	q, r := div(13, 3)
	fmt.Printf("13 div 3 is %d mod %d\n", q, r)

	fmt.Println("pow(3, 4) is:", apply(//匿名函数
		func(a int, b int) int {
			return int(math.Pow(
				float64(a), float64(b)))
		}, 3, 4))

	fmt.Println("1+2+...+5 =", sum(1, 2, 3, 4, 5))

	a, b := 3, 4
	a, b = swap(a, b)//值传递
	fmt.Println("a, b after swap is:", a, b)
}