Golang append a number to an int array, the value of adjacent element in array changes, why?-CodePudding

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

CodePudding user response：

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.