(Golang) Can anyone tell me why the result is like this

July 14, 2023

import "fmt"

type MyStruct struct {
    Queue []string
}

func main() {

    myStruct := &MyStruct{}
    // Pre-allocate a slice with a capacity of 5
    queue := make([]string, 0, 5)

    myStruct.Queue = queue

    // Add elements to the "end" of the queue
    myStruct.Queue = append(myStruct.Queue, "1")
    myStruct.Queue = append(myStruct.Queue, "2")
    myStruct.Queue = append(myStruct.Queue, "3")

    // queue: [1 2 3]
    fmt.Println(myStruct.Queue)
    fmt.Println(queue)

    // Add another element
    queue = append(queue, "4")
    fmt.Println(queue)

    queue = append(queue, "5")
    fmt.Println(queue)

    // queue: [1 2 3] ? or [1 2 3 4 5] ?
    fmt.Println(myStruct.Queue)
    // [ 4 5 3 ]
}

I expected [1,2,3] or [1,2,3,4,5] but, its [4, 5, 3]
when I changed Capacity of []string as 0, it was [1 ,2, 3]. because its reference pointer address change ([]string length > capacity).

by the way, I don’t understand what its [4, 5, 3]

>Solution :

queue := make([]string, 0, 5)

Makes an anonymous array with a size of 5, and creates the slice queue that points to the beginning of that array, has a capacity of 5, and a length of 0.

myStruct.Queue = queue

Copies the slice-header queue into myStruct.Queue. myStruct.Queue now points to the beginning of the same array, has a capacity of 5, and a length of 0.

myStruct.Queue = append(myStruct.Queue, "1")
myStruct.Queue = append(myStruct.Queue, "2")
myStruct.Queue = append(myStruct.Queue, "3")

Appends three elements to myStruct.Queue. Since space is available, these elements are inserted into the existing array, which now contains ["1" "2" "3" "" ""]. myStruct.Queue‘s length is now 3. queue still points to the same array, but its length is still 0.

queue = append(queue, "4")
queue = append(queue, "5")

Appends two elements to queue. Space is available in the backing array, and queue has a length of zero, so these two elements are inserted into the first two indices of the array, overwriting "1" and "2".

Now the array contains ["4" "5" "3" "" ""]. queue is a view into that array with a length of 2, so it contains ["4" "5"]. myStruct.Queue is a view into the same array with a length of 3, so it contains ["4" "5" "3"].