Go Standard Library

Standard Library Categories

import (
    "fmt"
    "io"
    "bufio"
)

import (
    "strings"
    "strconv"
    "regexp"
)

import (
    "net/http"
    "net/url"
    "encoding/json"
)

import (
    "time"
    "math"
    "math/rand"
)

🔹 Essential I/O Packages

Most commonly used packages for input and output:

🔸 fmt Package - Formatted I/O:

package main

import "fmt"

func main() {
    name := "Alice"
    age := 30
    
    // Print functions
    fmt.Print("Hello ")
    fmt.Println("World!")
    
    // Formatted printing
    fmt.Printf("Name: %s, Age: %d\n", name, age)
    
    // Sprint functions (return strings)
    message := fmt.Sprintf("User: %s (%d years old)", name, age)
    fmt.Println(message)
}

🔹 String Manipulation

Working with strings using the strings package:

package main

import (
    "fmt"
    "strings"
    "strconv"
)

func main() {
    text := "  Hello, Go World!  "
    
    // String operations
    fmt.Println("Original:", text)
    fmt.Println("Trimmed:", strings.TrimSpace(text))
    fmt.Println("Uppercase:", strings.ToUpper(text))
    fmt.Println("Lowercase:", strings.ToLower(text))
    fmt.Println("Contains 'Go':", strings.Contains(text, "Go"))
    fmt.Println("Replace:", strings.Replace(text, "World", "Universe", 1))
    
    // String conversion
    number := 42
    str := strconv.Itoa(number)
    fmt.Println("Number to string:", str)
    
    back, _ := strconv.Atoi(str)
    fmt.Println("String to number:", back)
}

🔹 Time and Date

Working with time using the time package:

package main

import (
    "fmt"
    "time"
)

func main() {
    // Current time
    now := time.Now()
    fmt.Println("Current time:", now)
    
    // Formatted time
    fmt.Println("Formatted:", now.Format("2006-01-02 15:04:05"))
    fmt.Println("Date only:", now.Format("January 2, 2006"))
    fmt.Println("Time only:", now.Format("3:04 PM"))
    
    // Time calculations
    tomorrow := now.Add(24 * time.Hour)
    fmt.Println("Tomorrow:", tomorrow.Format("2006-01-02"))
    
    // Duration
    duration := time.Since(now)
    fmt.Println("Duration since now:", duration)
}

🔹 Math Operations

Mathematical functions using the math package:

package main

import (
    "fmt"
    "math"
    "math/rand"
    "time"
)

func main() {
    // Basic math operations
    fmt.Println("Square root of 16:", math.Sqrt(16))
    fmt.Println("Power 2^3:", math.Pow(2, 3))
    fmt.Println("Absolute value of -5:", math.Abs(-5))
    fmt.Println("Ceiling of 4.2:", math.Ceil(4.2))
    fmt.Println("Floor of 4.8:", math.Floor(4.8))
    
    // Constants
    fmt.Println("Pi:", math.Pi)
    fmt.Println("E:", math.E)
    
    // Random numbers
    rand.Seed(time.Now().UnixNano())
    fmt.Println("Random int (0-99):", rand.Intn(100))
    fmt.Println("Random float:", rand.Float64())
}

🔹 JSON Handling

Working with JSON data using encoding/json:

package main

import (
    "encoding/json"
    "fmt"
)

type Person struct {
    Name string `json:"name"`
    Age  int    `json:"age"`
    City string `json:"city"`
}

func main() {
    // Create a person
    person := Person{
        Name: "John Doe",
        Age:  30,
        City: "New York",
    }
    
    // Convert to JSON
    jsonData, err := json.Marshal(person)
    if err != nil {
        fmt.Println("Error:", err)
        return
    }
    fmt.Println("JSON:", string(jsonData))
    
    // Parse JSON
    jsonString := `{"name":"Jane Smith","age":25,"city":"London"}`
    var newPerson Person
    err = json.Unmarshal([]byte(jsonString), &newPerson)
    if err != nil {
        fmt.Println("Error:", err)
        return
    }
    fmt.Printf("Parsed: %+v\n", newPerson)
}

🔹 Common Standard Library Packages

Quick reference of essential standard library packages: