container/list: the last Element is changed when use array append #31404

sxyluanhai · 2019-04-11T07:30:08Z

What version of Go are you using (`go version`)?

go version go1.12.3 linux/amd64

Does this issue reproduce with the latest release?

YES

What operating system and processor architecture are you using (`go env`)?

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/root/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/sxy/workspace"
GOPROXY=""
GORACE=""
GOROOT="/home/sxy/go"
GOTMPDIR=""
GOTOOLDIR="/home/sxy/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build635210007=/tmp/go-build -gno-record-gcc-switches"

What did you do?

my code

package main
import (

"container/list"

"fmt"

)
func main() {

arr := []int{1,2,3,4}

result := getSubset(arr)
head := 5
for i:=result.Front();i!=nil;i=i.Next() {
    tmp := i.Value.([]int)
    fmt.Println("Begin : " , i.Next())
    tmp = append(tmp,head)
    fmt.Println("End : " , i.Next())
    
}

}
func getSubset(arr []int) (sresult *list.List) {

result := list.New()

if len(arr) == 0 {

b := []int{}

result.PushBack(b)

return result

}

head := arr[0]

subarr := arr[1:len(arr)]
sub := getSubset(subarr)
for i:=sub.Front();i!=nil;i=i.Next() {
    var tmp = i.Value.([]int)
    result.PushBack(tmp)
   
    tmp = append(tmp,head)
    result.PushBack(tmp)
}
return result

}

Output is :

Output

Begin :  &{0xc042066600 0xc0420665a0 0xc0420661e0 [1]}
End :  &{0xc042066600 0xc0420665a0 0xc0420661e0 [1]}
Begin :  &{0xc042066630 0xc0420665d0 0xc0420661e0 [2]}
End :  &{0xc042066630 0xc0420665d0 0xc0420661e0 [2]}
Begin :  &{0xc042066660 0xc042066600 0xc0420661e0 [2 1]}
End :  &{0xc042066660 0xc042066600 0xc0420661e0 [2 1]}
Begin :  &{0xc042066690 0xc042066630 0xc0420661e0 [3]}
End :  &{0xc042066690 0xc042066630 0xc0420661e0 [3]}
Begin :  &{0xc0420666c0 0xc042066660 0xc0420661e0 [3 1]}
End :  &{0xc0420666c0 0xc042066660 0xc0420661e0 [3 1]}
Begin :  &{0xc0420666f0 0xc042066690 0xc0420661e0 [3 2]}
End :  &{0xc0420666f0 0xc042066690 0xc0420661e0 [3 2]}
Begin :  &{0xc042066720 0xc0420666c0 0xc0420661e0 [3 2 1]}
End :  &{0xc042066720 0xc0420666c0 0xc0420661e0 [3 2 1]}
Begin :  &{0xc042066750 0xc0420666f0 0xc0420661e0 [4]}
End :  &{0xc042066750 0xc0420666f0 0xc0420661e0 [4]}
Begin :  &{0xc042066780 0xc042066720 0xc0420661e0 [4 1]}
End :  &{0xc042066780 0xc042066720 0xc0420661e0 [4 1]}
Begin :  &{0xc0420667b0 0xc042066750 0xc0420661e0 [4 2]}
End :  &{0xc0420667b0 0xc042066750 0xc0420661e0 [4 2]}
Begin :  &{0xc0420667e0 0xc042066780 0xc0420661e0 [4 2 1]}
End :  &{0xc0420667e0 0xc042066780 0xc0420661e0 [4 2 1]}
Begin :  &{0xc042066810 0xc0420667b0 0xc0420661e0 [4 3]}
End :  &{0xc042066810 0xc0420667b0 0xc0420661e0 [4 3]}
Begin :  &{0xc042066840 0xc0420667e0 0xc0420661e0 [4 3 1]}
End :  &{0xc042066840 0xc0420667e0 0xc0420661e0 [4 3 1]}
Begin :  &{0xc042066870 0xc042066810 0xc0420661e0 [4 3 2]}
End :  &{0xc042066870 0xc042066810 0xc0420661e0 [4 3 2]}
Begin :  &{0xc0420661e0 0xc042066840 0xc0420661e0 [4 3 2 1]}
End :  &{0xc0420661e0 0xc042066840 0xc0420661e0 [4 3 2 5]}
Begin :  nil
End :  nil

What did you expect to see?

i expect to get
Begin : &{0xc0420661e0 0xc042066840 0xc0420661e0 [4 3 2 1]}
End : &{0xc0420661e0 0xc042066840 0xc0420661e0 [4 3 2 1]}

What did you see instead?

why i get
Begin : &{0xc0420661e0 0xc042066840 0xc0420661e0 [4 3 2 1]}
End : &{0xc0420661e0 0xc042066840 0xc0420661e0 [4 3 2 5]}

when i use append,the last element has changed.

The text was updated successfully, but these errors were encountered:

hengwu0 · 2019-04-11T08:53:23Z

That's because the slice‘s capacity is extended with 2,4,8... and the slice will not extend cap if enough capacity.

1		var tmp = i.Value.([]int)
2		result.PushBack(tmp)

3		tmp = append(tmp, head)
4		result.PushBack(tmp)

when 1:tmp is [4, 3, 2], the cap is 4 actually. Then 3:tmp become [4, 3, 2, 1], and it pushed into result. But if you append the 1:tmp in main(), then the 3:tmp will be changed automaticly.
P.S. 1:tmp means the tmp var in line 1.

sxyluanhai · 2019-04-11T09:38:12Z

@hengwu0
--------if you append the 1:tmp in main(), then the 3:tmp will be changed automaticly.
Q: 1:tmp is [4,3,2],the cap is 4;length is 3. append(tmp,5) then get [4,3,2,5],the length is 4,cap is 4. the cap is 4 in this append. cap not changed.Why 3:tmp will be changed automaticly?
i know 1:tmp and 3:tmp point to the same address.Why only the last tmp has changed

hengwu0 · 2019-04-11T10:21:18Z

@hengwu0
--------if you append the 1:tmp in main(), then the 3:tmp will be changed automaticly.
Q: 1:tmp is [4,3,2],the cap is 4;length is 3. append(tmp,5) then get [4,3,2,5],the length is 4,cap is 4. the cap is 4 in this append. cap not changed.Why 3:tmp will be changed automaticly?
i know 1:tmp and 3:tmp point to the same address.Why only the last tmp has changed

because slice‘s default capacity is 1, 2, 4, 8...

sxyluanhai · 2019-04-11T11:01:12Z

@hengwu0
--------if you append the 1:tmp in main(), then the 3:tmp will be changed automaticly.
Q: 1:tmp is [4,3,2],the cap is 4;length is 3. append(tmp,5) then get [4,3,2,5],the length is 4,cap is 4. the cap is 4 in this append. cap not changed.Why 3:tmp will be changed automaticly?
i know 1:tmp and 3:tmp point to the same address.Why only the last tmp has changed

because slice‘s default capacity is 1, 2, 4, 8...

Sorry，i can not understand capaccity will affect the tmp`s change
My output shows only the last one [4 3 2 1] changed [4 3 2 5],the others do not change.

bcmills · 2019-04-11T21:47:52Z

For usage questions, see the venues in https://golang.org/wiki/Questions. It's not obvious that anything in container/list is working incorrectly in this example.

Please note that code snippets filed to the issue tracker should generally be links to runnable, properly-formatted examples on https://play.golang.org.

ianlancetaylor · 2019-04-12T00:01:24Z

This doesn't have anything to do with container/list. It is just how slices and append work. See https://blog.golang.org/slices.

bcmills changed the title ~~container/list the last Element is changed when use array append~~ container/list: the last Element is changed when use array append Apr 11, 2019

bcmills added the Question label Apr 11, 2019

ianlancetaylor closed this as completed Apr 12, 2019

golang locked and limited conversation to collaborators Apr 11, 2020

gopherbot added the FrozenDueToAge label Apr 11, 2020

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container/list: the last Element is changed when use array append #31404

container/list: the last Element is changed when use array append #31404

sxyluanhai commented Apr 11, 2019 •

edited

hengwu0 commented Apr 11, 2019 •

edited

sxyluanhai commented Apr 11, 2019

hengwu0 commented Apr 11, 2019 •

edited

sxyluanhai commented Apr 11, 2019

bcmills commented Apr 11, 2019

ianlancetaylor commented Apr 12, 2019

container/list: the last Element is changed when use array append #31404

container/list: the last Element is changed when use array append #31404

Comments

sxyluanhai commented Apr 11, 2019 • edited

What version of Go are you using (go version)?

Does this issue reproduce with the latest release?

What operating system and processor architecture are you using (go env)?

What did you do?

What did you expect to see?

What did you see instead?

hengwu0 commented Apr 11, 2019 • edited

sxyluanhai commented Apr 11, 2019

hengwu0 commented Apr 11, 2019 • edited

sxyluanhai commented Apr 11, 2019

bcmills commented Apr 11, 2019

ianlancetaylor commented Apr 12, 2019

sxyluanhai commented Apr 11, 2019 •

edited

What version of Go are you using (`go version`)?

What operating system and processor architecture are you using (`go env`)?

hengwu0 commented Apr 11, 2019 •

edited

hengwu0 commented Apr 11, 2019 •

edited