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Marshal and Unmarshal

Learn to produce JSON with Marshal and parse with Unmarshal.

Unmarshal

json.Unmarshal(data, &target) decodes JSON bytes into a Go value. The target must be a pointer; otherwise the package can't fill it.

var u Urun
err := json.Unmarshal([]byte(`{"ad":"Kalem","fiyat":12.5}`), &u)
if err != nil {
    return err
}
fmt.Println(u.Ad, u.Fiyat)

A type mismatch (e.g. text where a number is expected) returns *json.UnmarshalTypeError; broken JSON returns *json.SyntaxError.

Telling a missing field apart

"adet":0 and not sending the count at all look the same. To tell them apart, use a pointer field:

type Guncelle struct {
    Adet *int `json:"adet"`
}
// {"adet":0}  → Adet != nil, *Adet == 0
// {}          → Adet == nil (not sent)

This distinction matters in PATCH-style APIs.

Practice Decode JSON

Define type Siparis struct { ID int `json:"id"`; Urunler []string `json:"urunler"` }. Decode the text {"id":7,"urunler":["kalem","defter"]} and print ID and the number of products.

Starter code

main.go
package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    // buraya yaz
}

Sample solution

main.go
package main

import (
    "encoding/json"
    "fmt"
)

type Siparis struct {
    ID      int      `json:"id"`
    Urunler []string `json:"urunler"`
}

func main() {
    veri := []byte(`{"id":7,"urunler":["kalem","defter"]}`)
    var s Siparis
    if err := json.Unmarshal(veri, &s); err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(s.ID, len(s.Urunler))
}

Expected output

7 2

Questions and answers

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

1 After Unmarshal, what happens to a struct field missing from the JSON?

  • It is deleted
  • It keeps its previous value (usually the zero value) Correct answer
  • An error is returned
  • It becomes nil

Explanation: Unmarshal only touches fields present in the JSON; that lets you decode into a struct with defaults and change only the given fields.

2 Pass a pointer

Question 
var s Sepet
err := json.Unmarshal(veri, ____)
Answer 
var s Sepet
err := json.Unmarshal(veri, &s)

Explanation: Unmarshal needs a pointer to be able to fill the target.

3 Error type

Term Meaning
Bozuk JSON *json.SyntaxError
Wrong type *json.UnmarshalTypeError
Not a pointer *json.InvalidUnmarshalError
Missing field No error, value kept

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