10 — Maps & JSON

Two things Go gives you built in: a hash-table type (map) and struct-driven JSON. Every example is live — edit it and press Run.

Maps and struct-tag JSON

A map is Go's built-in hash table, map[K]V. Reading a missing key returns the value's zero value — so use the comma-ok form (v, ok := m[k]) to tell "absent" from "present but zero". For JSON, you declare a struct with field tags: only exported (capitalised) fields are serialised, the tag sets the JSON key, and omitempty drops a zero-valued field. Marshal and Unmarshal convert both ways; unknown and missing fields are handled gracefully.

package main

import (
    "encoding/json"
    "fmt"
)

type User struct {
    Name  string `json:"name"`
    Age   int    `json:"age"`
    Email string `json:"email,omitempty"` // dropped when empty
}

func main() {
    m := map[string]int{"x": 0}
    v, ok := m["x"]
    fmt.Println(v, ok)         // 0 true  — present but zero
    v, ok = m["missing"]
    fmt.Println(v, ok)         // 0 false — absent

    b, _ := json.Marshal(User{Name: "andy", Age: 40, Email: "a@example.com"})
    fmt.Println(string(b))
    b, _ = json.Marshal(User{Name: "a", Age: 1})
    fmt.Println(string(b))     // email omitted

    var u User
    json.Unmarshal([]byte(`{"name":"cara","age":22,"extra":true}`), &u)
    fmt.Printf("%s %d %q\n", u.Name, u.Age, u.Email) // extra ignored, email zero
}

Output:

0 true
0 false
{"name":"andy","age":40,"email":"a@example.com"}
{"name":"a","age":1}
cara 22 ""

Unmarshal takes the address (&u) so it can fill the struct in. One trap: the zero value of a map is nil, and writing to a nil map panics — initialise with map[K]V{} or make first. Ranging a map visits entries in randomised order.

Recap

Next: 11 — defer, panic & recover.