this is a case of a variable being reused during a range loop. essentially k[:] is optimized to something resembling &k (a pointer to the memory location of the backing array) which is appended as such to the [][]int variable. the next iteration it is the same variable in memory, however it's pointing to a different backing array. the address of that variable is added to [][]int again. in order to avoid this you will have to redeclare k inside the loop to ensure a new backing array is used when adding to the slice of slices.
maybe the easiest way to explain this is to point you towards this blog post: https://blog.golang.org/go-slices-usage-and-internals and to note that the for loop in your example expands/unrolls to something like this, which breaks what you considered to "work" in your original example: a1 := [3]int{1, 2, 3} a2 := [3]int{4, 5, 6} a3 := [3]int{7, 8, 9} s := make([][]int, 0) a := a1 s = append(s, a[:]) a = a2 s = append(s, a[:]) a = a3 s = append(s, a[:]) fmt.Println(s) On Wed, Nov 21, 2018 at 2:41 PM Sun Frank <elitegobli...@gmail.com> wrote: > > Hi guys, I came across some code that surprised me, but I could not figured > out why it behave like this, > > s := make([][]int, 0) > a1 := [3]int{1, 2, 3} > a2 := [3]int{4, 5, 6} > s = append(s, a1[:]) > s = append(s, a2[:]) > fmt.Println(s) > > mp := make(map[[3]int]bool) > mp[[3]int{1, 2, 3}] = true > mp[[3]int{4, 5, 6}] = true > mp[[3]int{7, 8, 9}] = true > res := make([][]int, 0) > for k := range mp { > fmt.Printf("key is %+v\n", k[:]) > res = append(res, k[:]) > fmt.Printf("after append: %+v\n", res) > } > > or on playground > > the output is: > > [[1 2 3] [4 5 6]] // this is as expected > key is [1 2 3] > after append: [[1 2 3]] > key is [4 5 6] > after append: [[4 5 6] [4 5 6]] // Why it get this not [[1 2 3] [4 5 6]] > key is [7 8 9] > after append: [[7 8 9] [7 8 9] [7 8 9]] // why even this? > > > any thoughts? > > > Thanks! > > > > -- > You received this message because you are subscribed to the Google Groups > "golang-nuts" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to golang-nuts+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
