Field access

Learn to access struct fields with the dot operator.

Field access

Fields are read and written with a dot. If the struct is in a slice, an element's field is changed through the index:

kitaplar := []Kitap{{"Dune", "Herbert", 412}, {"Vakıf", "Asimov", 255}}
kitaplar[1].Sayfa = 300
for _, k := range kitaplar {
    fmt.Println(k.Title, k.Sayfa)
}

The k in a for _, k := range loop is a copy; k.Sayfa = ... doesn't change the slice.

Struct tags

A tag can be written in backticks after a field; libraries read these tags. The most common are JSON field names:

type Kitap struct {
    Title string `json:"title"`
    Sayfa  int    `json:"sayfa,omitempty"`
}

Tags don't change how Go code behaves; only packages that read them through reflection (encoding/json, database libraries) use them.

Exercise Bulk update

Define the Kitap struct (Title string, Sayfa int). Increase every book's pages by 10 through the index in a two-book slice and print the total page count (starting values: 412 and 255).

Show example solution

package main

import "fmt"

type Kitap struct {
    Title string
    Sayfa  int
}

func main() {
    kitaplar := []Kitap{{"Dune", 412}, {"Vakıf", 255}}
    for i := range kitaplar {
        kitaplar[i].Sayfa += 10
    }
    toplam := 0
    for _, k := range kitaplar {
        toplam += k.Sayfa
    }
    fmt.Println(toplam)
}

Test yourself

Which correctly increases every book's page count by 10?
- kitaplar.Sayfa += 10
- It isn't possible
- for i := range kitaplar { kitaplar[i].Sayfa += 10 }
- for _, k := range kitaplar { k.Sayfa += 10 }
Show answer

for i := range kitaplar { kitaplar[i].Sayfa += 10 } — The change must go through the index; the value range gives is a copy of the element.

Change by index

for i := range kitaplar {
    kitaplar____.Sayfa += 10
}

Show answer

for i := range kitaplar {
    kitaplar[i].Sayfa += 10
}

Expression and effect

k.Title

Reads the field
kitaplar[1].Sayfa = 300

Changes the element in the slice
for _, k := range

Gives a copy
`json:"title"`

A tag

Next lesson: Struct embedding

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