Go: Hello World III

Functions

package main

import (
    "fmt"
)


// add adds a to b
func add(a int, b int) int {
    return a + b
}

// divmod return quotient and reminder
func divmod(a int, b int) (int, int) {
   return a / b, a % b
}

func main() {
	val := add(1, 2)
    fmt.Println(val)
    // 3
    
    div, mod := divmod(7, 2)
    fmt.Printf("div = %d, mode = %d", div, mod)
    // div = 3, mode = 1
}

• params passing

  • Go pass integer to a func: it passese by value, which means it will copy it then pass into func. 不會改變原本的變數
  • Go pass slices or map: passing by reference. func 裡面運作的就是原本的 object!!

func doubleAt(values []int, i int) {
    values[i] *=2
}

func double(n int) {
     n *= 2
}

func main() {
    values := []int{1, 2, 3, 4}
    doubleAt(values, 2)
    fmt.Println(values)
    // [1 2 6 4]

    val := 10
    double(10)
    fmt.Println(val)
    // 10
}

• 用 pointer 可以解決傳 integer 的 copy by value 狀況

  •     func doublePtr(n *int) {
          *n *=2
      }
    

  •    doublePtr(&val)
       fmt.Println(val)
       // 20
       fmt.Println(&val)
       // 0xc0000100c8
    

   func doubleAt(values []int, i int) {
       values[i] *=2
   }
   
   func double(n int) {
        n *= 2
   }
   
   func doublePtr(n *int) {
       *n *=2
   }
   
   func main() {
       values := []int{1, 2, 3, 4}
       doubleAt(values, 2)
       fmt.Println(values)
       // [1 2 6 4]
   
       val := 10
       double(10)
       fmt.Println(val)
       // 10
       
       doublePtr(&val)
       fmt.Println(val)
       // 20
       fmt.Println(&val)
       // 0xc0000100c8
   }

• Error return

  • go func can return more than one value
  • nil

package main

import (
    "fmt"
    "math"
)

func sqrt(n float64) (float64, error) {
    if n < 0 {
        return 0.0, fmt.Errorf("sqrt of negative value (%f)", n)
    }

    return math.Sqrt(n), nil
}


func main() {

    s1, err := sqrt(2.0)
    if err != nil {
        fmt.Printf("ERROR: %s\n", err)
    } else {
        fmt.Println(s1)
    }
    // 1.4142135623730951

    s2, err2 := sqrt(-2.0)
    if err2 != nil {
        fmt.Printf("ERROR: %s\n", err2)
    } else {
        fmt.Println(s2)
    }
    // ERROR: sqrt of negative value (-2.000000)
}

• defer

  • go has its garbage collector
  • defer: make sure resources is closed / free

func cleanup(name string) {
    fmt.Printf("Cleaning up %s\n", name)
}

func worker() {
    defer cleanup("A")
    defer cleanup("B")
    fmt.Println("worker")
}



func main() {
   worker()
   // worker
   // Cleaning up B
   // Cleaning up A
}

Challenge: Functions

• Write a function that gets a URL and returns the value of Content-Type response HTTP header

• The function should return an error if it can’t perform a GET request

• func contentType(url string) (string, error)

• Use net/http Get function to make HTTP call
  • resp, err := http.Get(“https:/golang.org”)
• Use res.Header.Get to get the value of a header
  • resp.Header.Get(“Content-Length”)

• Maker sure the response body is closed

  • resp.Body.Close()

我的解

package main

import (
	"fmt"
	"net/http"
)


func contentType(url string) (string, error) {
   resp, err := http.Get(url)
   defer resp.Body.Close()
   if err != nil {
	   return "error", fmt.Errorf("Error: %s\n", err)
   } else {
	   result := resp.Header.Get("Content-Type")
	   fmt.Printf("type: %T\n", result)
	   return result, nil
   }

}


func main() {

	sr, err  := contentType("https://golang.org")
	fmt.Println("content-type: ", sr ,"\n", "Error:", err)
}

// type: string
// content-type:  text/html; charset=utf-8
// Error: <nil>

老師的解

package main

import (
	"fmt"
	"net/http"
)


func contentType(url string) (string, error) {
   resp, err := http.Get(url)

   if err != nil {
	   return "", err
   }
   
   defer resp.Body.Close() // Make sure we close the body

   ctype := resp.Header.Get("Content-Type")
   if ctype == "" { // Return error if Content-Type header not found
      return "", fmt.Errorf("can't find Content-Type header")
   }

   return ctype, nil

}


func main() {

	ctype, err  := contentType("https://linkedin.com")
    if err != nil {
	    fmt.Printf("ERRPR: %s\n", err)
	} else {
        fmt.Println(ctype)
	}
}
// text/html; charset=utf-8