Maps and structs¶

• Maps
  • Collections of value types that are acessed via keys
  • Created via literals or via make function
  • Members accessed via [key] syntax
    • myMap["key"] = "value"
  • Check for presence with "value, ok" form of result
  • Multiple assignments refer to same underlying data
• Structs
  • Collections of disparate data types that describe a single concept
  • Keyed by named fields
  • Normally created as types, but anonymous structs are allowed
  • Structs are value types
  • No inheritance, but can use composition via embedding
  • Tags can be added to struct fields to describe field

package main

import (
        "fmt"
    "reflect"
)

type Doctor struct {
    number int
    actorName string
    companions []string
}

func main() {

    // Maps
        statePopulations := map[string]int{
                "California":   39250017,
                "Texas":        27862596,
                "Florida":      20612439,
                "New York":     19745289,
                "Pennsylvania": 12802503,
                "Illinois":     12801539,
                "Ohio":         11614373,
        }
        fmt.Println(statePopulations)

    // You can use arrays as keys
    m := map[[3]int]string{}
    fmt.Println(m)

    // make function
    stpop := make(map[string]int)
    stpop = map[string]int{"California": 39250017}
    fmt.Println(stpop)

    // 

    fmt.Println(statePopulations["California"])
    statePopulations["Georgia"] = 10310371
    fmt.Println(statePopulations)
    delete(statePopulations, "Georgia")
    fmt.Println(statePopulations)
    fmt.Println(statePopulations["Georgia"]) // 0
    pop, ok := statePopulations["Georgia"]
    fmt.Println(pop, ok) // 0, false

    // underlying data is passed by references

    sp := statePopulations
    delete(sp, "Ohio")
    fmt.Println(sp)
    fmt.Println(statePopulations)

    // structs

    aDoctor := Doctor {
        number: 3,
        actorName:  "Jon Pertwee",
        companions: []string {
            "Liz Shaw",
            "Jo Grant",
            "Sarah Jane Smith",
        },
    }

    fmt.Println(aDoctor)
    fmt.Println(aDoctor.companions)
    fmt.Println(aDoctor.companions[1])

    // you can also initialize the struct without
    // providing field names (positional structs), but later on
    // if there are any changes in the code, this might break.
    // not recommended.

    bDoctor := Doctor{3, "Jon Pertwee", []string{"1", "2"}}
    fmt.Println(bDoctor)

    // anonymous structs
    doc := struct{name string}{name: "John Pertwee"}
    fmt.Println(doc)

    // unlike maps, we create copies
    anotherDoctor := doc
    anotherDoctor.name = "Tom Baker"
    fmt.Println(doc)
    fmt.Println(anotherDoctor)

    // If we like to to reference, we use a pointer
    yetAnotherDoc := &doc
    yetAnotherDoc.name = "Thomas Baker"
    fmt.Println(doc)
    fmt.Println(yetAnotherDoc)

    // Composition, similar concept to inheritance

    type Animal struct {
        Name string
        Origin string
    }

    type Bird struct {
        Animal
        SpeedKPH    float32
        CanFly      bool
    }

    b := Bird{}
    b.Name = "Emu"
    b.Origin = "Australia"
    b.SpeedKPH = 48
    b.CanFly = false

    fmt.Println(b) // {{Emu Australia} 48 false}
    fmt.Println(b.Name) // Emu

    c := Bird {
        Animal:     Animal{Name: "Emu", Origin: "Australia"},
        SpeedKPH: 48,
        CanFly: false,
    }
    fmt.Println(c)

    // tags, using reflect package

    type AnimalWithTags struct {
        Name        string     `required max:"100"`
        Origin      string
    }

    t := reflect.TypeOf(AnimalWithTags{})
    field, _ := t.FieldByName("Name")
    fmt.Println(field.Tag) // required max:"100"

}

map[California:39250017 Florida:20612439 Illinois:12801539 New York:19745289 Ohio:11614373 Pennsylvania:12802503 Texas:27862596]
map[]
map[California:39250017]
39250017
map[California:39250017 Florida:20612439 Georgia:10310371 Illinois:12801539 New York:19745289 Ohio:11614373 Pennsylvania:12802503 Texas:27862596]
map[California:39250017 Florida:20612439 Illinois:12801539 New York:19745289 Ohio:11614373 Pennsylvania:12802503 Texas:27862596]
0
0 false
map[California:39250017 Florida:20612439 Illinois:12801539 New York:19745289 Pennsylvania:12802503 Texas:27862596]
map[California:39250017 Florida:20612439 Illinois:12801539 New York:19745289 Pennsylvania:12802503 Texas:27862596]
{3 Jon Pertwee [Liz Shaw Jo Grant Sarah Jane Smith]}
[Liz Shaw Jo Grant Sarah Jane Smith]
Jo Grant
{3 Jon Pertwee [1 2]}
{John Pertwee}
{John Pertwee}
{Tom Baker}
{Thomas Baker}
&{Thomas Baker}
{{Emu Australia} 48 false}
Emu
{{Emu Australia} 48 false}
required max:"100"