行为异常-变量未正确递增

Question

I have the following code that adds a new element to a slice if it doesnt exist already. If it does exist then the qty property should be incremented of the existing element instead of a new element being added:

package main

import (
    "fmt"
)

type BoxItem struct {
    Id int
    Qty int
}

type Box struct {
    BoxItems []BoxItem
}

func (box *Box) AddBoxItem(boxItem BoxItem) BoxItem {

    // If the item exists already then increment its qty
    for _, item := range box.BoxItems {
        if item.Id == boxItem.Id {
             item.Qty++
             return item
        }
    }

    // New item so append
    box.BoxItems = append(box.BoxItems, boxItem)
    return boxItem
}


func main() {

    boxItems := []BoxItem{}
    box := Box{boxItems}

    boxItem := BoxItem{Id: 1, Qty: 1}

    // Add this item 3 times its qty should be increased to 3 afterwards
    box.AddBoxItem(boxItem)
    box.AddBoxItem(boxItem)
    box.AddBoxItem(boxItem)


    fmt.Println(len(box.BoxItems))  // Prints 1 which is correct

    for _, item := range box.BoxItems {
        fmt.Println(item.Qty)  // Prints 1 when it should print 3
    }
}

The problem is that the qty is never incremented correctly. It always ends in 1 when it should be 3 in the example provided.

I have debugged the code and it does appear that the increment section is reached but the value just isnt persisted to the item.

What is wrong here?

Answer 1

You are incrementing Qty in the copy of the box.BoxItems because range will yield the copy of the elements in the slice. See this example.

So, in for _, item := range box.BoxItems, item is a copy of of the elements in box.BoxItems.

Change your loop to

for i := 0; i < len(box.BoxItems); i++ {
    if box.boxItems[i].Id == boxItem.Id {
         box.boxItems[i].Qty++
         return box.BoxItems[i]
    }
}

Playground