Golang中的数组子集

Question

I am trying to write the equivalent of the following line in python

H = [e for e in G if condition(e)]

Here is my example code. Basically I am just trying to use a getter function (G.get) to get a subset of G. So I think I want arr2 to be a new array but containing the same object.

package main

import "fmt"

type Object struct {
    x int
}

type Group []Object

func (G *Group) get() (H []Object) {
    for _,v := range *G {
        H = append(H,v)
    }
    return
}

func main() {
    arr := make(Group,1)
    arr[0].x = 1
    fmt.Println(arr)
    arr2 := arr.get()
    arr[0].x = 3
    fmt.Println(arr)
    fmt.Println(arr2)
}

Which compiles and runs and gives me

[{1}]
[{3}]
[{1}]

My question is "Why does arr2 not contain the same instance of an Object as arr?" I believe I understand make only instantiates a Group thing which means it includes one Object in it. But then the for loop shouldn't create a new Object should it?

Thanks for the help!

Answer 1

This simpler snippet of code shows what's going on:

var a Object
a.x = 1
b := a
fmt.Println(a, b) // prints {1} {1}
b.x = 2
fmt.Println(a, b) // prints {1} {2}

playground example

The statement b := a copies the Object value in variable a to the variable b. There are now two copies of the value. Changing one does not change the other.

This is different from Python where assignment copies a reference to a value.

The loop in the question also copies the values:

for _,v := range *G {
    H = append(H,v)
}

The variable v is a copy the slice element from *G. A copy of v is appended to slice H. There are three independent Object values: the value in slice *G, the value in v and the value in slice H.

If you want the slices to all refer to the same Object value, then store pointers to Object values in the slice:

type Group []*Object

arr := make(Group, 1)
arr[0] = &Object{x: 1}
fmt.Println(arr[0].x)  // prints 1
arr2 := arr.get()
arr[0].x = 3
fmt.Println(arr[0].x)  // prints 3
fmt.Println(arr2[0].x) // prints 3

playground example