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🐹Go Cheatsheet v1.21+

Complete Reference for Seasoned Developers

πŸ”§Variables & Types 

// Variable declaration

var name string = "gopher"
var name = "gopher" // Type inferred
name := "gopher" // Short declaration (only in functions)

// Constants
const Pi = 3.14
const (
StatusOK = 200
StatusCreated = 201
)

// iota (auto-incrementing)
const (
Read = 1 << iota // 1
Write // 2
Execute // 4
)

// Multiple assignment
i, j := 0, 1

πŸ“¦Data Structures 

// Arrays (fixed size)

var arr [5]int // [0,0,0,0,0]
arr := [3]int{1, 2, 3} // [1,2,3]
arr := [...]int{1, 2, 3, 4, 5} // Size inferred

// Slices (dynamic size)
slice := []int{1, 2, 3}
slice := make([]int, 3) // [0,0,0]
slice := make([]int, 3, 5) // len=3, cap=5
slice = append(slice, 4, 5) // [0,0,0,4,5]
copy(dest, src) // Copy elements

// Slice operations
slice := arr[1:4] // Elements 1-3
slice := arr[:3] // First 3 elements
slice := arr[2:] // From element 2
len(slice) // Length
cap(slice) // Capacity

// Maps
m := make(map[string]int)
m := map[string]int{"foo": 1, "bar": 2}
m["key"] = value // Set value
value, exists := m["key"] // Check exists
delete(m, "key") // Remove key

πŸ—οΈStructs & Pointers 

// Struct definition

type Person struct {
Name string
Age int
}

// Creating structs
p := Person{Name: "Bob", Age: 20}
p := Person{"Bob", 20} // Same order as defined
p := new(Person) // Returns a pointer

// Pointers
x := 10
p := &x // Point to x
fmt.Println(*p) // Dereference
*p = 20 // Change value

// Struct embedding (composition)
type Employee struct {
Person // Embedded struct
Salary int
}
emp.Name = "Bob" // Access embedded field

πŸ”€String Operations 

// String basics

s := "Hello, δΈ–η•Œ"
len(s) // byte length, not char count
s[0] // byte at position (not rune)

// Rune handling (Unicode characters)
for i, r := range "Hello, δΈ–η•Œ" {
fmt.Printf("%d: %c\n", i, r)
}

// String manipulation
s := strings.Join([]string{"a", "b"}, ",")
parts := strings.Split("a,b,c", ",")
strings.Contains("seafood", "foo") // true
strings.HasPrefix("prefix", "pre") // true
strings.ToUpper("Hello") // "HELLO"
strings.TrimSpace(" Hello ") // "Hello"
strings.Replace("hello", "l", "L", 1) // "heLlo"
strings.Replace("hello", "l", "L", -1) // "heLLo"

// String conversions
i, err := strconv.Atoi("42") // string to int
s := strconv.Itoa(42) // int to string
b := []byte("Hello") // string to bytes
s := string([]byte{72, 101, 108, 108, 111}) // bytes to string

πŸ”€Control Flow 

// If statement

if x > 0 {
// code
} else if x < 0 {
// code
} else {
// code
}

// If with short statement
if err := doSomething(); err != nil {
// handle error
}

// Switch statement
switch os := runtime.GOOS; os {
case "darwin":
// code
case "linux":
// code
default:
// code
}

// Switch with no expression (clean if-else)
switch {
case condition1:
// code
case condition2:
// code
}

// For loop
for i := 0; i < 10; i++ {
// code
}

// For as while
for condition {
// code
}

// Infinite loop
for {
// code
if done { break }
if skip { continue }
}

// Range - iterate over slice, map, string, channel
for i, v := range slice {
// i is index, v is value
}
for k, v := range map {
// k is key, v is value
}

βš™οΈFunctions 

// Basic function

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

// Multiple return values
func divMod(a, b int) (int, int) {
return a / b, a % b
}

// Named return values
func split(sum int) (x, y int) {
x = sum \* 4 / 9
y = sum - x
return // Returns x, y
}

// Variadic functions
func sum(nums ...int) int {
total := 0
for \_, num := range nums {
total += num
}
return total
}
sum(1, 2, 3)
nums := []int{1, 2, 3}
sum(nums...) // Spread operator

// Function as value
f := func(x int) int {
return x \* x
}
fmt.Println(f(5)) // 25

// Closures
func adder() func(int) int {
sum := 0
return func(x int) int {
sum += x
return sum
}
}

⏰Defer, Panic & Recover 

// Defer (executes after surrounding function returns)

defer file.Close() // Common use case
defer mu.Unlock()

// Multiple defers (executed in LIFO order)
defer fmt.Println("1")
defer fmt.Println("2") // "2" prints before "1"

// Panic (similar to throwing exception)
panic("something went wrong")

// Recover (catch panic)
defer func() {
if r := recover(); r != nil {
fmt.Println("Recovered from", r)
}
}()

🎭Methods & Interfaces 

// Method definition (on struct)

type Rectangle struct {
width, height float64
}

// Value receiver (gets copy)
func (r Rectangle) Area() float64 {
return r.width \* r.height
}

// Pointer receiver (can modify receiver)
func (r _Rectangle) Scale(factor float64) {
r.width _= factor
r.height \*= factor
}

// Interface definition
type Shape interface {
Area() float64
}

// Implicit implementation (no "implements" keyword)
func CalculateArea(s Shape) float64 {
return s.Area()
}

// Empty interface (accepts any value)
func PrintAny(v interface{}) {
fmt.Println(v)
}

// Type assertion
val, ok := v.(string) // Check if v is string
if !ok {
// handle not a string
}

// Type switch
switch v := v.(type) {
case int:
fmt.Println("int:", v)
case string:
fmt.Println("string:", v)
default:
fmt.Println("unknown type")
}

πŸš€Concurrency 

// Goroutines (lightweight threads)

go func() {
// code runs concurrently
}()

// Channels (communicate between goroutines)
ch := make(chan int) // Unbuffered channel
ch := make(chan int, 10) // Buffered channel

// Send/receive on channel
ch <- 42 // Send value
val := <-ch // Receive value
val, ok := <-ch // Check if closed

// Select statement (multiplex channels)
select {
case v := <-ch1:
// handle value from ch1
case v := <-ch2:
// handle value from ch2
case ch3 <- x:
// sent x on ch3
default:
// run if no channels ready
}

// Channel directions
func receive(ch <-chan int) {} // Receive-only
func send(ch chan<- int) {} // Send-only

// Close channel (no more sends allowed)
close(ch)

// WaitGroup (wait for goroutines)
var wg sync.WaitGroup
wg.Add(1)
go func() {
defer wg.Done()
// code
}()
wg.Wait()

❌Error Handling 

// Error interface

type error interface {
Error() string
}

// Returning errors
if err != nil {
return nil, err
}

// Create errors
errors.New("error message")
fmt.Errorf("error: %v", value)

// Custom error
type MyError struct {
Code int
Msg string
}

func (e \*MyError) Error() string {
return fmt.Sprintf("code %d: %s", e.Code, e.Msg)
}

// Handle errors with if err != nil
if err != nil {
log.Fatal(err)
}

// Multiple error checks pattern
func doStuff() error {
err := step1()
if err != nil {
return fmt.Errorf("step1 failed: %w", err)
}

    err = step2()
    if err != nil {
        return fmt.Errorf("step2 failed: %w", err)
    }
    return nil

}

πŸ“¦Packages & Modules 

// Package declaration

package main

// Imports
import (
"fmt"
"strings"
)

// Import with alias
import (
"encoding/json"
str "strings"
)

// Blank import (init() runs only)
import \_ "image/png"

// Exported names (capitalized)
func ExportedFunc() {} // Accessible outside
func privateFunc() {} // Package-private

// Package initialization
var DBConn = initDB()
func init() {
// Run before main()
}

// Create module
// go mod init github.com/user/module

// Add dependency
// go get github.com/pkg/errors

πŸ“File Operations 

// Reading files

data, err := os.ReadFile("file.txt")

// Using bufio for line reading
file, err := os.Open("file.txt")
if err != nil {
return err
}
defer file.Close()

scanner := bufio.NewScanner(file)
for scanner.Scan() {
line := scanner.Text()
// process line
}

// Writing files
err := os.WriteFile("file.txt", data, 0644)

// File options (os.O_CREATE|os.O_WRONLY|os.O_APPEND)
file, err := os.OpenFile(
"file.txt",
os.O_WRONLY|os.O_CREATE,
0644,
)
if err != nil {
return err
}
defer file.Close()

πŸ§ͺTesting 

// In file ending with _test.go

package mypackage

import "testing"

// Test function (run with go test)
func TestAdd(t \*testing.T) {
got := Add(2, 3)
want := 5
if got != want {
t.Errorf("Add(2, 3) = %d; want %d", got, want)
}
}

// Table-driven tests
func TestMultiply(t \*testing.T) {
tests := []struct {
x, y, want int
}{
{2, 3, 6},
{-1, 5, -5},
{0, 10, 0},
}
for \_, tt := range tests {
got := Multiply(tt.x, tt.y)
if got != tt.want {
t.Errorf("Multiply(%d, %d) = %d; want %d",
tt.x, tt.y, got, tt.want)
}
}
}

// Benchmarks
func BenchmarkFib(b \*testing.B) {
for i := 0; i < b.N; i++ {
Fibonacci(10)
}
}

πŸ“šStandard Library 

// Time operations

now := time.Now()
future := now.Add(time.Hour _ 24)
duration := future.Sub(now)
time.Sleep(time.Millisecond _ 100)
formatted := now.Format("2006-01-02 15:04:05")

// JSON handling
type Person struct {
Name string `json:"name"`
Age int `json:"age,omitempty"`
}

// Marshal (Go struct to JSON)
data, err := json.Marshal(person)

// Unmarshal (JSON to Go struct)
err := json.Unmarshal(data, &person)

// HTTP client
resp, err := http.Get("https://example.com")
if err != nil {
return err
}
defer resp.Body.Close()
body, err := io.ReadAll(resp.Body)

// HTTP server
http.HandleFunc("/", func(w http.ResponseWriter, r \*http.Request) {
fmt.Fprintf(w, "Hello, %q", r.URL.Path)
})
http.ListenAndServe(":8080", nil)

πŸ”—Quick Links: golang.org β€’ pkg.go.dev β€’ Go Tour β€’ Effective Go

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