Golang Learning Road 2-Basic Understanding (Part 1)

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Preface

To learn a language, first understand the basic grammar of Golang, such as variable definition, data type, etc.

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1. Definition of variables and constants

1.Variables

A variable is a name given to a memory location to store a value of a specific type.
Common definitions:

// 方式一
name := xxx
// 方式二
var (
name = x
age = 18
)

2.Constant

Constants and variables are just the opposite. Once a constant is declared, its value cannot be changed, such as reassignment.
Generally available when the usage scenario is immutable data.

// 方式一
const name = x
// 方式二
const (
	name = ""
	age = 18
)

Complete code:

package main

import "fmt"

func main() {
    
    
	// 1、变量定义：var
	name := "ppl"

	var name1 = "ppl"

	var name2 string
	name2 = "ppl"

	// 定义多个变量
	a, b := "a", "b"

	// 常量定义：const
	const const1 = 3.14

	const (
		const2 = 3.14
		const3 = 0.618
	)

	fmt.Println(name, name1, name2, a, b)
	fmt.Println(const1, const2, const3)
}

2.Data type

1 Boolean type: true or false
2 Numeric type: int and floating point type float32, float64
3 String type: string string
4 Derived type: Map type, etc.

func main() {
    
    
	// 1	布尔型： true 或 false
	bool1 := true
	// 2	数字类型：整型 int 和浮点型 float32、float64（没有自动类型提升或转换）
	number := 10
	// 3	字符串类型：字符串string
	name := "ppl"
	// 4	派生类型：Map 类型等等
	Map := make(map[string]string)
	Map ["map"] = "yes"

	fmt.Println(bool1, number, name, Map)
}

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2. Self-increasing and self-decreasing

Syntax must be on a single line

package main

import "fmt"

func main() {
    
    
	age := 0
	age++

	//fmt.Println(age++)  不允许，必须单独一行 ++
	fmt.Println(age)

	age--
	fmt.Println(age)
}

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3. Pointer

What are pointers?
A pointer variable points to the memory address of a value, the internal garbage collection mechanism (gc).

1. Use pointers & and *

• Variable pointer storage address: &name
• Access value using pointer: *ptr

// 声明实际变量
name := "ppl"
// 声明指针变量且存储地址=&name
ptr := &name
fmt.Println("name:", name)
fmt.Println("name ptr:", ptr)
fmt.Println("name ptr使用指针访问值:", *ptr)

2. Null pointer

Null pointer: nil, a null value such as Python’s None, mysql’s null

// 空指针：nil,既空值如Python的None,mysql的null
var ptrNil *string
// 指针做判断 != 与 ==
if ptrNil == nil {
    
    
	fmt.Println("ptrNil:", ptrNil)
}

3. Pointer complete code

package main

import "fmt"

func main() {
    
    
	// 声明实际变量
	name := "ppl"
	// 声明指针变量且存储地址=&name
	ptr := &name
	fmt.Println("name:", name)
	fmt.Println("name ptr:", ptr)
	fmt.Println("name ptr使用指针访问值:", *ptr)

	// 使用new关键字定义
	name1 := new(string)
	*name1 = "ppl"
	fmt.Println(name1)
	fmt.Println(*name1)
	fmt.Println("strPtr()：", strPtr(*name1))

	// 空指针：nil,既空值如Python的None,mysql的null
	var ptrNil *string
	// 指针做判断 != 与 ==
	if ptrNil == nil {
    
    
		fmt.Println("ptrNil:", ptrNil)
	}
}

func strPtr(str string) *string {
    
    
	// 返回 string 的指针
	return &str
}

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4. Grammar is not supported

1. Self-increasing and self-decreasing--i and ++i

Correct writing: i++ or i–

2. Does not support memory address (pointer) addition and subtraction

3. The ternary operator is not supported

java：a > b ? a : b
python：a if a > b else b

4. In conditional judgment, only bool false is logically false.

The numbers 0 and nil are not supported as conditional judgment syntax. Only the Boolean type false is supported, unlike Python.

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5. String string

The bytes of strings in the go language use UTF-8 encoding to identify Unicode text.
If you need to replace and other operations, you need to combine the strings package. The following are some basic usages:

1. Native output

Use rhetorical question marks: ``

2.len gets the string length

len method: get character length len(xx)

3. Splicing & formatting output

str1 + str2
above detailed code as follows:

package main

import (
	"fmt"
)

func main() {
    
    
	str := "ppl\nPPL"
	// 1.原生输出，使用反问号：``
	str1 := `Use "go help <command>" for more information about a command.
		Additional help topics:
        buildconstraint build constraints
        buildmode       build modes

Use "go help <topic>" for more information about that topic.
`
	fmt.Println(str)
	fmt.Println(str1)

	// 2.len 方法:获取字符长度
	lenStr := len(str)
	fmt.Println("len(str)=", lenStr)

	// 3.拼接&格式化输出
	joinStr := str + "+++join+++join"
	fmt.Println(joinStr)
	fmt.Printf("joinStr=%s", joinStr)
}

4. String slicing

Left closed right open (including the left, not including the right)

// 4.字符串切片
str11 := "HelloWord"
fmt.Println(str11[:5])	// 打印 Hello (切片0-4的字符)
fmt.Println(str11[5:])	// 打印 Word	 (切片5-最后的字符)
fmt.Println(str11[2:5])	// 打印 第2-第5个字符

End