Modify values ​​in a copy of a slice and append to the original slice

In PHP, slice (Slice) is a commonly used data structure that allows us to select a portion of elements from an array or slice. However, sometimes we need to modify the slice and append the modified value to the original slice. This operation is possible in PHP by modifying the value in the slice copy and then appending the modified copy to the original slice using the array_splice function. In this article, we will detail how to use this method to implement slice modification and append operations.

Question content

Write a function that copies a slice and modifies the values ​​of the items in the slice copy, then appends the copy to the original copy. It looks like the code not only modifies the copy of the slice, but also the original slice.

Go to the amusement park

import "fmt"

type item struct {
    name   string
    number int
}

func main() {
    names := []string{"a", "b"}
    numbers := []int{1, 2}

    items := []*item{}

    for _, name := range names {
        item := &item{name: name}
        items = append(items, item)
    }

    for n, i := range numbers {
        if n > 0 {
            fmt.println(n, "make a copy of the items")
            itemcopies := make([]*item, len(items))
            copy(itemcopies, items)

            fmt.println(n, "set the numbers on the copies")
            for _, item := range itemcopies {
                item.number = i
            }

            fmt.println(n, "append the copies to the original")
            items = append(items, itemcopies...)
        } else {

            fmt.println(n, "first pass set all the items to the first number")
            for _, item := range items {
                item.number = i
            }
        }
    }

    for n, i := range items {
        fmt.printf("%d %+v\n", n, *i)
    }
}

What I actually saw

0 {name:a number:2}
1 {name:b number:2}
2 {name:a number:2}
3 {name:b number:2}

What do I expect to see

0 {name:a number:1}
1 {name:b number:1}
2 {name:a number:2}
3 {name:b number:2}

Solution

@jimb is right, you copy the pointer,

I suggest defining a copy function belonging to struct item to create a new copy

package main

import "fmt"

type Item struct {
    name   string
    number int
}

func (it *Item) copy() *Item {
    return &Item{
        name:   it.name,
        number: it.number,
    }
}
func main() {
    names := []string{"a", "b"}
    numbers := []int{1, 2}

    items := []*Item{}

    for _, name := range names {
        item := &Item{name: name}
        items = append(items, item)
    }

    for n, i := range numbers {
        if n > 0 {
            fmt.Println(n, "make a copy of the items")
            itemCopies := make([]*Item, len(items))

            for j := 0; j < len(items); j++ {
                itemCopies[j] = items[j].copy()
            }
            fmt.Println(n, "set the numbers on the copies")
            for _, item := range itemCopies {
                item.number = i
            }

            fmt.Println(n, "append the copies to the original")
            items = append(items, itemCopies...)
        } else {

            fmt.Println(n, "first pass set all the items to the first number")
            for _, item := range items {
                item.number = i
            }
        }
    }

    for n, i := range items {
        fmt.Printf("%d %+v\n", n, *i)
    }
}

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