awesome-cheatsheets/languages/golang.md

# Golang

## Summary

-   Introduction
    -   [Hello World](#hello-world)
    -   [Go CLI Commands](#go-cli-commands)
    -   [Go Modules](#go-modules)
-   Basic
    -   [Basic Types](#basic-types)
    -   [Variables](#variables)
    -   [Operators](#operators)
    -   [Conditional Statements](#conditional-statements)
    -   [Loops](#loops)
    -   [Arrays](#arrays)
    -   [Functions](#functions)
-   Advanced
    -   [Structs](#structs)
    -   [Maps](#maps)
    -   [Pointers](#pointers)
    -   [Methods and Interfaces](#methods-and-interfaces)
    -   [Errors](#errors)
    -   [Testing](#testing)
-   Standard Libs
    -   [Package fmt](#package-fmt)

## Hello World

```go
package main

import "fmt"

func main() {
    fmt.Println("Hello Gophers!")
}
```

[Return to Summary](#summary)

<hr/>

## Go CLI Commands

```bash
# Compile & Run code
$ go run [file.go]

# Compile
$ go build [file.go]
# Running compiled file
$ ./hello

# Test packages
$ go test [folder]

# Install packages/modules
$ go install [package]

# List installed packages/modules
$ go list

# Update packages/modules
$ go fix

# Format package sources
$ go fmt

# See package documentation
$ go doc [package]

# Add dependencies and install
$ go get [module]

# See Go environment variables
$ go env

# See version
$ go version
```

[Return to Summary](#summary)

<hr/>

## Go Modules

-   Go projects are called **modules**
-   Each module has multiple **packages**
-   Each package should has a scoped functionality. Packages talk to each other to compose the code
-   A module needs at least one package, the **main**
-   The package main needs a entry function called **main**

```bash
# Create Module
$ go mod init [name]
```

Tip: By convention, modules names has the follow structure:

domain.com/user/module/package

Example: github.com/spf13/cobra

<hr/>

[Return to Summary](#summary)

## Basic Types

|    Type    |               Set of Values                |                    Values                     |
| :--------: | :----------------------------------------: | :-------------------------------------------: |
|    bool    |                  boolean                   |                  true/false                   |
|   string   |            array of characters             |             needs to be inside ""             |
|    int     |                  integers                  |             32 or 64 bit integer              |
|    int8    |               8-bit integers               |                 [ -128, 128 ]                 |
|   int16    |              16-bit integers               |               [ -32768, 32767]                |
|   int32    |              32-bit integers               |          [ -2147483648, 2147483647]           |
|   int64    |              64-bit integers               | [ -9223372036854775808, 9223372036854775807 ] |
|   uint8    |          8-bit unsigned integers           |                  [ 0, 255 ]                   |
|   uint16   |          16-bit unsigned integers          |                 [ 0, 65535 ]                  |
|   uint32   |          32-bit unsigned integers          |               [ 0, 4294967295 ]               |
|   uint64   |          64-bit unsigned integers          |          [ 0, 18446744073709551615 ]          |
|  float32   |                32-bit float                |                                               |
|  float64   |                64-bit float                |                                               |
| complex64  | 32-bit float with real and imaginary parts |                                               |
| complex128 | 64-bit float with real and imaginary parts |                                               |
|    byte    |                sets of bits                |                alias for uint8                |
|    rune    |             Unicode characters             |                alias for int32                |

[Return to Summary](#summary)

<hr/>

## Variables

```go
// Declaration
var int value

// Initialization
value = 10

// Declaration + Initialization + Type inference
var isActive = true

// Short declaration (only inside functions)
text := "Hello"

// Multi declaration
var i, j, k = 1, 2, 3

// Variable not initialized = Zero values
// Numeric: 0
// Boolean: false
// String: ""
// Special value: nil (same as null)

var number int // 0
var text string // false
var boolean bool // ""

// Type conversions
// T(v) converts v to type T

i := 1.234 // float
int(i) // 1

// Constants
const pi = 3.1415
```

<hr/>

## Operators

[Return to Summary](#summary)

Arithmetic Operators
| Symbol | Operation | Valid Types |
|:---------:|:-------------:|:-------------:|
| `+` | Sum | integers, floats, complex values, strings |
| `-` | Difference | integers, floats, complex values |
| `*` | Product | integers, floats, complex values |
| `/` | Quotient | integers, floats, complex values |
| `%` | Remainder | integers |
| `&` | Bitwise AND | integers |
| `|` | Bitwise OR | integers |
| `^` | Bitwise XOR | integers |
| `&^` | Bit clear (AND NOT) | integers |
| `<<` | Left shift | integer << unsigned integer |
| `>>` | Right shift | integer >> unsigned integer |

Comparison Operators
| Symbol | Operation |
|:---------:|:-------------:|
| `==` | Equal |
| `!=` | Not equal |
| `<` | Less |
| `<=` | Less or equal |
| `>` | Greater |
| `>=` | Greater or equal |

Logical Operators
| Symbol | Operation |
|:---------:|:-------------:|
| `&&` | Conditional AND |
| `||` | Conditional OR |
| `!` | NOT |

[Return to Summary](#summary)

<hr/>

## Conditional Statements

```go
// If / Else
i := 1

if i > 0 {
    // Condition is True! i is greater than zero
} else {
    // Condition is False! i is lower or equal to zero
}

// Else if
i := 1

if i > 0 {
    // Condition is True! i is greater than zero
} else if i > 0 && i < 2 {
    // Condition is True! i greater than zero and lower than two
} else if i > 1 && i < 4 {
    // Condition is True! i greater than one and lower than four
} else {
    // None of the above conditions is True, so it falls here
}

// If with short statements
i := 2.567

if j := int(i); j == 2 {
    // Condition is True! j, the integer value of i, is equal to two
} else {
    // Condition is False! j, the integer value of i, is not equal to two
}

// Switch
text := 'hey'

switch text {
    case 'hey':
        // 'Hello!'
    case 'bye':
        // 'Byee'
    default:
        // 'Ok'
}

// Switch without condition
value := 5

switch {
    case value < 2:
        // 'Hello!'
    case value >= 2 && value < 6:
        // 'Byee'
    default:
        // 'Ok'
}
```

[Return to Summary](#summary)

<hr/>

## Loops

```go
// Golang only has the for loop
for i := 0; i < 10; i++ {
    // i
}

// The first and third parameters are ommitable
// For as a while
i := 0;

for i < 10 {
    i++
}

// Forever loop
for {

}
```

[Return to Summary](#summary)

<hr/>

## Arrays

```go
// Declaration with specified size
var array [3]string
array[0] = "Hello"
array[1] = "Golang"
array[2] = "World"

// Declaration and Initialization
values := [5]int{1, 2, 3, 4, 5}

// Slices: A subarray that acts as a reference of an array
// Determining min and max
values[1:3] // {2, 3, 4}

// Determining only max will use min = 0
values[:2] // {1, 2, 3}

// Determining only min will use max = last element
values[3:] // {3, 4}

// Length: number of elements that a slice contains
len(values) // 5

// Capacity: number of elements that a slice can contain
values = values[:1]
len(values) // 2
cap(values) // 5

// Slice literal
slice := []bool{true, true, false}

// make function: create a slice with length and capacity
slice := make([]int, 5, 6) // make(type, len, cap)

// Append new element to slice
slice := []int{ 1, 2 }
slice = append(slice, 3)
slice // { 1, 2, 3 }
slice = append(slice, 3, 2, 1)
slice // { 1, 2, 3, 3, 2, 1 }

// For range: iterate over a slice
slice := string["W", "o", "w"]

for i, value := range slice {
    i // 0, then 1, then 2
    value // "W", then "o", then "w"
}

// Skip index or value

for i := range slice {
    i // 0, then 1, then 2
}

for _, value := range slice {
   value // "W", then "o", then "w"
}
```

[Return to Summary](#summary)

<hr/>

## Functions

```go
// Functions acts as a scoped block of code
func sayHello() {
    // Hello World!
}
sayHello() // Hello World!

// Functions can take zero or more parameters, as so return zero or more parameters
func sum(x int, y int) int {
    return x + y
}
sum(3, 7) // 10

// Returned values can be named and be used inside the function
func doubleAndTriple(x int) (double, triple int) {
    double = x * 2
    triple = x * 3
    return
}
d, t := doubleAndTriple(5)
// d = 10
// t = 15

// Skipping one of the returned values
_, t := doubleAndTriple(3)
// t = 9

// Functions can defer commands. Defered commands are
// runned in a stack order after the execution and
// returning of a function
var aux = 0

func switchValuesAndDouble(x, y int) {
    aux = x
    defer aux = 0 // cleaning variable to post use
    x = y * 2
    y = aux * 2
}

a, b = 2, 5
switchValuesAndDouble(2, 5)

// a = 10
// b = 4
// aux = 0

// Functions can be handled as values
func calc(fn func(int, int) int) int {
    return fn(2, 6)
}

func sum(x, y int) int {
    return x + y
}

func mult(x, y int) int {
    return x * y
}

calc(sum) // 8
calc(mult) // 12

// Function closures: a function that returns a function
// that remembers the original context
func calc() func(int) int {
    value := 0
    return func(x int) int {
        value += x
        return value
    }
}

calculator := calc()
calculator(3) // 3
calculator(45) // 48
calculator(12) // 60
```

[Return to Summary](#summary)

<hr/>

## Structs

Structs are a way to arrange data in specific formats.

```go
// Declaring a struct
type Person struct {
    Name string
    Age int
}

// Initializing
person := Person{"John", 34}
person.Name // "John"
person.Age // 34

person2 := Person{Age: 20}
person2.Name // ""
person2.Age // 20

person3 := Person{}
person3.Name // ""
person3.Age // 0
```

[Return to Summary](#summary)

<hr/>

## Maps

Maps are data structures that holds values assigneds to a key.

```go
// Declaring a map
var cities map[string]string

// Initializing
cities = make(map[string]string)
cities // nil

// Insert
cities["NY"] = "EUA"

// Retrieve
newYork = cities["NY"]
newYork // "EUA"

// Delete
delete(cities, "NY")

// Check if a key is setted
value, ok := cities["NY"]
ok // false
value // ""
```

[Return to Summary](#summary)

<hr/>

## Pointers

Pointers are a direct reference to a memory address that some variable or value is being stored.

```go
// Pointers has *T type
var value int
var pointer *int

// Point to a variable memory address with &
value = 3
pointer = &value

pointer // 3
pointer = 20
pointer // 20
pointer += 5
pointer // 25

// Pointers to structs can access the attributes
type Struct struct {
    X int
}

s := Struct{3}
pointer := &s

s.X // 3
```

Obs: Unlike C, Go doesn't have pointer arithmetics.

[Return to Summary](#summary)

<hr/>

## Methods and Interfaces

Go doesn't have classes. But you can implement methods, interfaces and almost everything contained in OOP, but in what gophers call "Go Way"

```go
type Dog struct {
    Name string
}

func (dog *Dog) bark() string {
    return dog.Name + " is barking!"
}

dog := Dog{"Rex"}
dog.bark() // Rex is barking!
```

Interfaces are implicitly implemented. You don't need to inform that your struct are correctly implementing a interface if it already has all methods with the same name of the interface.
All structs implement the `interface{}` interface. This empty interface means the same as `any`.

```go
// Car implements Vehicle interface
type Vehicle interface {
    Accelerate()
}

type Car struct {

}

func (car *Car) Accelerate() {
    return "Car is moving on ground"
}
```

[Return to Summary](#summary)

<hr/>

## Errors

Go doesn't support `throw`, `try`, `catch` and other common error handling structures. Here, we use `error` package to build possible errors as a returning parameter in functions

```go
import "errors"

// Function that contain a logic that can cause a possible exception flow 
func firstLetter(text string) (string, error) {
    if len(text) < 1 {
        return nil, errors.New("Parameter text is empty")
    }
    return string(text[0]), nil
}

a, errorA := firstLetter("Wow")
a // "W"
errorA // nil

b, errorB := firstLetter("")
b // nil
errorB // Error("Parameter text is empty")
```

[Return to Summary](#summary)

<hr/>

## Testing

Go has a built-in library to unit testing. In a separate file you insert tests for functionalities of a file and run `go test package` to run all tests of the actual package or `go test path` to run a specific test file.

```go
// main.go
func Sum(x, y int) int {
    return x + y
}

// main_test.go
import ( 
    "testing"
    "reflect"
)

func TestSum(t *testing.T) {
    x, y := 2, 4
    expected := 2 + 4

    if !reflect.DeepEqual(sum(x, y), expected) {
        t.Fatalf("Function Sum not working as expected")
    }
}
```

[Return to Summary](#summary)

<hr/>

## Package `fmt`

```go
import "fmt"

fmt.Print("Hello World") // Print in console
fmt.Println("Hello World") // Print and add a new line in end
fmt.Printf("%s is %d years old", "John", 32) // Print with formatting
fmt.Errorf("User %d not found", 123) // Print a formatted error
```

[Return to Summary](#summary)

<hr/>