Adding to cart

Add products to the cart with append.

Adding to the cart

The cart is a slice of products. Adding is done with append. If the same product is added a second time, its quantity should increase instead of adding a new row; so the cart is searched first:

type Sepet struct{ Urunler []Urun }

func (s *Sepet) Ekle(u Urun) {
    for i := range s.Urunler {
        if s.Urunler[i].Ad == u.Ad {
            s.Urunler[i].Adet += u.Adet
            return
        }
    }
    s.Urunler = append(s.Urunler, u)
}

Changing with for i := range

Using for _, u := range s.Urunler while increasing the quantity doesn't work: u is a copy and the change doesn't reach the slice. That's why we iterate by index and change it through s.Urunler[i].Adet. This is one of the most common mistakes for Go beginners.

Exercise The Ekle method

Write Urun, Sepet and the Ekle method. Add Kalem (3), Defter (1) and Kalem again (2) to the cart. Print the number of products and the pen's quantity.

Show example solution

package main

import "fmt"

type Urun struct {
    Ad    string
    Fiyat float64
    Adet  int
}

type Sepet struct{ Urunler []Urun }

func (s *Sepet) Ekle(u Urun) {
    for i := range s.Urunler {
        if s.Urunler[i].Ad == u.Ad {
            s.Urunler[i].Adet += u.Adet
            return
        }
    }
    s.Urunler = append(s.Urunler, u)
}

func main() {
    var s Sepet
    s.Ekle(Urun{"Kalem", 12.5, 3})
    s.Ekle(Urun{"Defter", 20, 1})
    s.Ekle(Urun{"Kalem", 12.5, 2})
    fmt.Println(len(s.Urunler), s.Urunler[0].Adet)
}

Test yourself

Why does <code>Ekle</code> use a pointer receiver (<code>*Sepet</code>)?
- Because it is shorter
- Because slices require pointers
- It isn't required
- To change the cart itself; a value receiver would work on a copy
Show answer

To change the cart itself; a value receiver would work on a copy — append returns a new slice header; writing it back to the Sepet permanently requires a pointer receiver.

Append to the end

s.Urunler = ____(s.Urunler, u)

Show answer

s.Urunler = append(s.Urunler, u)

Case and behavior

New product

Added with append
Existing product

Quantity increases
for _, u := range

A copy, doesn't work
*Sepet

Lasting change

Next lesson: Computing total

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