03_Functions.md

Functions

Let's learn about Go's functions

Types in function

• You must specify the type of function's arguments and return values

  • By telling the compiler what type it is, you can compile quickly!

    • Similar to Java! Very different from Python...

  • ex)

    package main
    
    import "fmt"
    
    func multiply(a int, b int) int {
     return a * b
    }
    
    func main() {
     fmt.Println(multiply(2, 4))
    }

• If arguments have the same type, you only need to specify once

  • ex)

    func multiply(a, b int) int {
     return a * b
    }

• Functions that need return must specify return type!

Multiple Return Values

• Go supports multiple return values

  • A characteristic of Go that's different from many programming languages!

  • ex)

    package main
    
    import (
     "fmt"
     "strings"
    )
    
    func lenAndUpper(name string) (int, string) {
     return len(name), strings.ToUpper(name)
    }
    
    func main() {
     totalLength, upperName := lenAndUpper("chloe")
     fmt.Println(totalLength, upperName)
    }

• However, you cannot receive fewer values than the number of returns!

  • ex) Example that will cause an error

     totalLength := lenAndUpper("chloe")

    • Since lenAndUpper returns two values, it produces the following error

      go101/03_functions on  master [!?] via 🐹 v1.15.6 
      ➜ go run main.go
      # command-line-arguments
      ./main.go:18:14: assignment mismatch: 1 variable but lenAndUpper returns 2 values

• In this case, you can put _ to ignore the value

  • ex)

     totalLength, _ := lenAndUpper("chloe")

    • This way underscore (_) can be used as ignored value!
      • The compiler literally ignores ignored values

Variadic functions

• You can create variadic functions using ...

  • The method is to put ... before the type! That's it!

  • ex)

    func repeatMe(words ...string) {
     fmt.Println(words)
    }

Naked returns

• When specifying the return type of a function, pre-declare the values to return and make an empty return

  • Since it's initialized when declared, inside the function you're updating that variable!

    • You can return without specifying the return value

  • ex)

    package main
    
    import (
     "fmt"
     "strings"
    )
    
    func lenAndLower(name string) (length int, lowercase string) {
     length = len(name)
     lowercase = strings.ToLower(name)
     return
    }
    
    func main() {
     length, name := lenAndLower("CHLOE")
     fmt.Println(length, name)
    }

• If you want to clearly specify what value is returned, it's better not to use this

defer

• Code that executes after the function ends

  • You can set something to operate additionally through defer when the function terminates

    • If it's a function with a return value, defer executes after return

  • ex)

    func lenAndLower(name string) (length int, lowercase string) {
      // defer
     defer fmt.Println("Done!")
      
     length = len(name)
     lowercase = strings.ToLower(name)
     return
    }

• You can use defer to close or delete images or files after function termination, and send API requests!