Adding arrays (from a map) to a slice of slices in Go - arrays

Question

I have a map with arrays for keys.
I want to add all of the keys/arrays to a slice of slices.
However, I am getting unexpected results.
Code:
package main
import "fmt"
func main() {
var finalResult [][]int
set := make(map[[2]int]int)
a, b, c := [2]int{1, 1}, [2]int{2, 2}, [2]int{3, 3}
set[a], set[b], set[c] = 1, 1, 1
fmt.Println("set: ", set)
for x := range set {
fmt.Println("\nfinalResult 0: ", finalResult)
fmt.Println("x: ", x[:])
finalResult = append(finalResult, x[:])
fmt.Println("finalResult 1: ", finalResult)
}
}
Output:
set: map[[1 1]:1 [2 2]:1 [3 3]:1]
finalResult 0: []
x: [1 1]
finalResult 1: [[1 1]]
finalResult 0: [[2 2]]
x: [2 2]
finalResult 1: [[2 2] [2 2]]
finalResult 0: [[3 3] [3 3]]
x: [3 3]
finalResult 1: [[3 3] [3 3] [3 3]]
finalResult appears to be changing during the for loop?
Can someone explain what is happening and how I can work around this issue?

Answer 1

The problem is, that you append the x local variable to the finalResult, which is a pointer, so in every for loop, the x will point to a new array in the memory. When you add this x three times to the finalResult and print it, all the three x will be points to the same memory address. You have to copy the content where x points in every loop to a new variable and add this to the finalResult.
package main
import "fmt"
func main() {
var finalResult [][]int
set := make(map[[2]int]int)
a, b, c := [2]int{1, 1}, [2]int{2, 2}, [2]int{3, 3}
set[a], set[b], set[c] = 1, 1, 1
fmt.Println("set: ", set)
for x := range set {
fmt.Println("\nfinalResult 0: ", finalResult)
fmt.Println("x: ", x[:])
a := make([]int, 2)
copy(a, x[:])
finalResult = append(finalResult, a)
fmt.Println("finalResult 1: ", finalResult)
}
}
But be aware, that ranging over a map will be always in random order, so your final result may change on every run.