英文:
Golang append a number to an int array, the value of adjacent element in array changes, why?
问题
我正在尝试使用Golang解决一些动态规划问题。我写了一个类似的函数：
func main() {
fmt.Println(HowSum(5, []int{1, 2, 5}))
}
func HowSum(targetNum int, numbers []int) []int {
retAry = make([][]int, targetNum+1)
retAry[0] = make([]int, 0)
for i := 0; i <= targetNum; i++ {
if retAry[i] != nil {
for _, num := range numbers {
if i+num <= targetNum {
fmt.Print("Before:", i, " round, num =", num, retAry, "\n")
retAry[i+num] = append(retAry[i], num)
fmt.Print("After :", i, " round, num =", num, retAry, "\n\n")
}
}
}
}
return retAry[targetNum]
}
部分结果如下所示：
...
Before:3 round, num =2 [[] [1] [1 1] [1 1 1] [1 1 1 1] [5]]
After :3 round, num =2 [[] [1] [1 1] [1 1 1] [1 1 1 2] [1 1 1 2]]
...
[1 1 1 2 1]
当程序将retAry[5]从[5]替换为[1 1 1 2]时，retAry[4]中的数组从[1 1 1 1]更改为[1 1 1 2]。发生了什么？
然而，如果将retAry[i+num] = append(retAry[i], num)替换为retAry[i+num] = append([]int{num}, retAry[i]...)，我可以得到正确的答案。
...
Before:3 round, num =2 [[] [1] [1 1] [1 1 1] [1 1 1 1] [5]]
After :3 round, num =2 [[] [1] [1 1] [1 1 1] [1 1 1 1] [2 1 1 1]]
...
[1 1 1 1 1]
有人可以帮忙解释一下我犯了什么错误吗？
代码在这里。
英文:
I am trying to solve some dynamic programming problems by using Golang. I wrote a function like
func main() {
fmt.Println(HowSum(5, []int{1, 2, 5}))
}
func HowSum(targetNum int, numbers []int) []int {
retAry = make([][]int, targetNum+1)
retAry[0] = make([]int, 0)
for i := 0; i <= targetNum; i++ {
if retAry[i] != nil {
for _, num := range numbers {
if i+num <= targetNum {
fmt.Print("Before:", i, " round, num =", num, retAry, "\n")
retAry[i+num] = append(retAry[i], num)
fmt.Print("After :", i, " round, num =", num, retAry, "\n\n")
}
}
}
}
return retAry[targetNum]
}
A part of results is as follows.
...
Before:3 round, num =2 [[] [1] [1 1] [1 1 1] [1 1 1 1] [5]]
After :3 round, num =2 [[] [1] [1 1] [1 1 1] [1 1 1 2] [1 1 1 2]]
...
[1 1 1 2 1]
The array in retAry[4] is changed from [1 1 1 1] to [1 1 1 2] when the program replace retAry[5] from [5] to [1 1 1 2]. What happen?
However, if replace
retAry[i+num] = append(retAry[i], num) to retAry[i+num] = append([]int{num}, retAry[i]...) , I can get the correct answer.
...
Before:3 round, num =2 [[] [1] [1 1] [1 1 1] [1 1 1 1] [5]]
After :3 round, num =2 [[] [1] [1 1] [1 1 1] [1 1 1 1] [2 1 1 1]]
...
[1 1 1 1 1]
Could anyone can help to explain what mistake I made?
The code is here
答案1
得分: 0
然而，如果将 retAry[i+num] = append(retAry[i], num) 替换为 retAry[i+num] = append([]int{num}, retAry[i]...)，我可以得到正确的答案。
解释：
retAry[i+num] = append(retAry[i], num) 在 retAry[i+num] 和 retAry[i] 之间共享同一个数组。如果修改一个切片，另一个切片也可能会改变。
func main() {
s1 := append([]int{}, 1, 2, 3) // len: 3, cap 4 []int{1, 2, 3, 0}
// 它将 4 追加到第 4 个索引，并将新的切片赋值给 s2。
// s2 与 s1 共享同一个数组
// s1: len: 3, cap 4 []int{1, 2, 3, 4}
// s2: len: 4, cap 4 []int{1, 2, 3, 4}
s2 := append(s1, 4)
// s3 也与 s1 和 s2 共享同一个数组
// 当你将第 4 个索引追加到 s1，并赋值给 s3 时，s1 和 s2 也会改变
// s1: len: 3, cap 4 []int{1, 2, 3, 5}
// s2: len: 4, cap 4 []int{1, 2, 3, 5}
// s3: len: 4, cap 4 []int{1, 2, 3, 5}
s3 := append(s1, 5)
fmt.Println(s2, s3) // 输出 [1 2 3 5] [1 2 3 5]
}
retAry[i+num] = append([]int{num}, retAry[i]...)
[]int{num} 你使用一个新的数组初始化了一个新的切片，然后将 retAry[i] 中的每个元素追加到新的数组中。retAry[i+num] 和 retAry[i] 之间不再有任何引用关系。
英文:
However, if replace retAry[i+num] = append(retAry[i], num) to retAry[i+num] = append([]int{num}, retAry[i]...) , I can get the correct answer.
Explain:
retAry[i+num] = append(retAry[i], num) is sharing the same array between retAry[i+num] and retAry[i]. If you modify one slice, other slice could be changed as well.
func main() {
s1 := append([]int{}, 1, 2, 3) // len: 3, cap 4 []int{1, 2, 3, 0}
// it appends 4 to 4th index and assign new slice to s2.
// s2 is share the same array with s1
// s1 : len: 3, cap 4 []int{1, 2, 3, 4}
// s2 : len: 4, cap 4 []int{1, 2, 3, 4}
s2 := append(s1, 4)
// s3 is share the same array with s1 and s2 as well
// when you append 4th index to s1, and assign to s3, both s1 and s2 will change as well
// s1 : len: 3, cap 4 []int{1, 2, 3, 5}
// s2 : len: 4, cap 4 []int{1, 2, 3, 5}
// s3 : len: 4, cap 4 []int{1, 2, 3, 5}
s3:= append(s1, 5)
fmt.Println(s2, s3) // output [1 2 3 5] [1 2 3 5]
}
retAry[i+num] = append([]int{num}, retAry[i]...)
[]int{num} you initialize a new slice with a new array, and then append every elements in retAry[i] to the new array. They aren't any reference between retAry[i+num] and retAry[i] anymore.
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