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flag.String and flag.Int

Learn to define flags with a type.

flag.String and flag.Int

Functions such as flag.String, flag.Int and flag.Duration return a pointer; the value is filled after Parse and read with *name. Alternatively the ...Var forms bind to an existing variable:

ad := flag.String("ad", "dünya", "selamlanacak kişi")

var sure time.Duration
flag.DurationVar(&sure, "sure", 5*time.Second, "zaman aşımı")
flag.Parse()
fmt.Println("Merhaba", *ad, sure)

Duration options accept readable values such as -sure=1m30s.

Your own option type

Any type implementing the flag.Value interface (String() and Set(string) error) can be an option. For example, a repeatable option:

type liste []string

func (l *liste) String() string     { return strings.Join(*l, ",") }
func (l *liste) Set(v string) error { *l = append(*l, v); return nil }

var etiketler liste
flag.Var(&etiketler, "etiket", "tekrarlanabilir")
// -etiket=go -etiket=web → [go web]

Practice Repeatable option

Write the liste type above. Register it in a FlagSet with fs.Var(&etiketler, "etiket", "..."), parse []string{"-etiket=go", "-etiket=web"} and print etiketler.

Starter code

main.go
package main

import (
    "flag"
    "fmt"
    "strings"
)

func main() {
    // buraya yaz
}

Sample solution

main.go
package main

import (
    "flag"
    "fmt"
    "strings"
)

type liste []string

func (l *liste) String() string     { return strings.Join(*l, ",") }
func (l *liste) Set(v string) error { *l = append(*l, v); return nil }

func main() {
    var etiketler liste
    fs := flag.NewFlagSet("app", flag.ContinueOnError)
    fs.Var(&etiketler, "etiket", "tekrarlanabilir")
    if err := fs.Parse([]string{"-etiket=go", "-etiket=web"}); err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(etiketler)
}

Expected output

[go web]

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 If <code>*ad</code> is read before <code>Parse</code> after <code>ad := flag.String(...)</code>, what do you get?

  • nil
  • A panic
  • The default value Correct answer
  • The value the user gave

Explanation: The pointer is filled with the default at definition; command-line values are written only when Parse is called.

2 Read the value

Question 
ad := flag.String("ad", "dünya", "kişi")
flag.Parse()
fmt.Println("Merhaba", ____ad)
Answer 
ad := flag.String("ad", "dünya", "kişi")
flag.Parse()
fmt.Println("Merhaba", *ad)

Explanation: flag.String returns a pointer; the value is reached with *ad.

3 Function and return

Term Meaning
flag.String *string
flag.Int *int
flag.DurationVar Binds an existing variable
flag.Var Custom flag.Value

All lessons in Intermediate

1 Goroutines

  1. What is a goroutine?
  2. The go keyword
  3. Goroutine and function
  4. The main goroutine
  5. Number of goroutines
  6. Summary - Goroutines

2 Channels

  1. What is a channel?
  2. Channel with make
  3. Sending data
  4. Receiving data
  5. Buffered channel
  6. Summary - Channels

3 Select

  1. What is select?
  2. Multiple channels
  3. default and non-blocking
  4. Select with timeout
  5. Closing a channel
  6. Summary - Select

4 sync package

  1. sync.Mutex
  2. sync.RWMutex
  3. sync.WaitGroup
  4. sync.Once
  5. sync.Pool
  6. Summary - sync package

5 Project: Concurrency

  1. Worker pool skeleton
  2. Job queue
  3. Collecting results
  4. Error handling

6 Context

  1. context.Background
  2. WithCancel
  3. WithTimeout
  4. WithValue
  5. WithDeadline
  6. Summary - Context

7 io package

  1. io.Reader
  2. io.Writer
  3. os.Stdin and os.Stdout
  4. bufio.Scanner
  5. io.Copy
  6. Summary - io package

8 Testing

  1. Test function
  2. t.Error and t.Fatal
  3. Table-driven tests
  4. Test helpers
  5. Example tests
  6. Summary - Testing

9 Benchmark

  1. Benchmark function
  2. b.ResetTimer
  3. Memory benchmark
  4. Sub-benchmark
  5. Comparing benchmarks
  6. Summary - Benchmark

10 fmt package

  1. fmt.Printf
  2. Format verbs
  3. fmt.Sprintf
  4. fmt.Errorf
  5. fmt.Fprintln
  6. Summary - fmt package

11 encoding/json

  1. The encoding/json package
  2. Struct tags
  3. Marshal and Unmarshal
  4. Encoder and Decoder
  5. JSON and map
  6. Summary - encoding/json

12 net/http

  1. Client with net/http
  2. http.Get
  3. http.Handler
  4. http.HandleFunc
  5. JSON API
  6. Summary - net/http

13 flag package

  1. The flag package
  2. flag.String and flag.Int
  3. flag.Parse
  4. Subcommand
  5. os.Args
  6. Summary - flag package

14 Intermediate final

  1. Challenge 1 - Concurrency
  2. Challenge 2 - HTTP service
  3. Challenge 3 - Test and benchmark
  4. Challenge 4 - CLI tool