Write and run the first go language program

Preface

stay Go Introduction to project directory structure In this article , We have a preliminary understanding of Go Project structure , So let's start writing the first Go Language program —— Output at console “Hello World!”.

Write the first Go Language program

1	package main // Statement main package 
2	
3	import "fmt" // Import fmt package , Required when printing strings 
4	
5	func main(){
     // Statement main The main function 
6		fmt.Println("Hello world!") // Print Hello world!
7	}

Let's introduce the meaning of these lines of code in detail .

package（ Create a package ）

Go Language “ package ” As a management unit , Every Go The source file must first declare the package it belongs to , So we'll see everyone Go The beginning of the source file is a package Statement , The format is as follows ：

package name

among package Is the keyword that declares the package name ,name For the name of the package .
Go Language packages and folders are one-to-one correspondence , It has the following characteristics ：

• Peer files in a directory belong to the same package .
• The package name can be different from its directory name .
• main Bag is Go Entry package of language program , One Go Language programs must have and only have one main package . If a program does not main package , Then there will be an error when compiling , Unable to generate executable .

import（ Import package ）

After the package declaration , yes import sentence , Used to import the package that the program depends on , The imported package name uses double quotation marks "" Surround , The format is as follows ：

import "name"

among import Is the keyword of the import package ,name Is the name of the imported package .

Code No. 3 Row imported fmt package , This line of code will tell Go compiler , We need to use fmt Functions or variables in the package ,fmt Bag is Go The language standard library provides us with , Used to format the contents of input and output （ Be similar to C In language stdio.h The header file ）, Similar to that os package 、io Bag, etc .

There is another point to be noted , The imported package cannot contain packages not used in the code , otherwise Go The compiler will report compilation errors , for example imported and not used: “xxx”,“xxx” Represents the package name .

You can also use a import Keyword import multiple packages , In this case, you need to use parentheses ( ) Surround the name of the bag , And each package name takes one line , That is, write it as follows ：

import(
    "name1"
    "name2"
)

main function

Code number 5 Line creates a main function , It is Go The entry function of language program , That is, the first function to run after the program starts .main Functions can only be declared in main In bag , Cannot be declared in other packages , also , One main There must also be and only one... In the package main function .

main Function is a kind of custom function , stay Go In language , All functions are keyed func At the beginning , The definition format is as follows ：

func  Function name  ( parameter list ) ( Return value list ){
    
     The body of the function 
}

The format description is as follows ：

• Function name ： By letter 、 Numbers 、 Draw the line down _ form , among , The first letter of a function name cannot be a number , also , In the same package , Function name cannot be duplicate .
• parameter list ： A parameter consists of parameter variables and parameter types , for example func foo( a int, b string ).
• Return value list ： It can be a list of return value types , It can also be a combination of variable name and type like parameter list , When the function has a return value , Must be used in the function body return Statement returns .
• The body of the function ： Code snippets that can be called repeatedly .
  Be careful ：Go The left brace of a language function { Must be on the same line as the function name , Otherwise, an error will be reported .

Print Hello World

Code number 6 That's ok fmt.Println(“Hello World!”) in ,Println yes fmt A function in a package , It is used to format the output data , Like strings 、 Integers 、 Decimals, etc , Be similar to C In language printf function . Here we use Println Function to print a string , That is to say ( ) It uses "" The part of the package .
Be careful ,Println The function will wrap automatically after printing ,ln yes line Abbreviation .
Order number . yes Go A kind of language operator , This means calling fmt In bag Println function .
in addition , Code fmt.Println(“Hello World!”) Ending , No need to use ; As a Terminator ,Go The compiler will automatically add , Of course , Add here ; It's OK, too .

compile & function

Go Language is a compiled static language （ and C The language is the same ）, So it's running Go Before language program , First compile it into binary executable file .
Can pass Go Language provides go build perhaps go run Command to Go Language program to compile ：

• go build command Can be Go The language program code is compiled into binary executable files , But we need to run the binary manually ;
• go run command It's more convenient , It will run directly after compilation Go Language program , A temporary file will be generated during compilation , But the executable file will not be generated , This feature is very suitable for debugging programs .
  It's already written Go Language program , We use go run Run the command ：
  
  Come here , I believe you will know how to create a simple Go Language program , Let's try it ！