Append to slice of slices - golang

Question

I'm trying to implement a golang function to solve the array subset problem. I'm having an issue hard copying a slice of ints to my 2D slice:

// Subsets
func Subsets(nums []int) [][]int {
    // create 2D slice
    sets := make([][]int, 0)
    backtrack(sets, make([]int, 0), nums, 0)
    return sets
}

func backtrack(sets [][]int, tempList []int, nums []int, start int) {
    // insert current list
    sets = append(sets, tempList)           // <---------HERE
    for i := start; i < len(nums); i++ {
        tempList = append(tempList, nums[i])
        backtrack(sets, tempList, nums, i+1)
        tempList = tempList[:len(tempList)-1]
    }
}

I tried replace the append statement with:

    buffer := make([]int, len(tempList))
    copy(buffer, tempList)
    sets = append(sets, buffer)

however I still see the sets slices empty when the program ends. An identical copy of this function works just fine in java with the append line as follow:

    list.add(new ArrayList<>(tempList));

what's the correct way to do this in golang?

can you copy over the code that you think does above? – Omar Odaini Mar 12 '22 at 21:35 — Omar Odaini, Mar 12 '22 at 21:35

score 0 · Accepted Answer · answered Mar 13 '22 at 01:15

0

I am not sure exactly what is the requirement here. But there are couple of things to note: Pass by Value : You are passing the sets to backtrack function. When you do that Go copies it's value and passes it to backtrack function. However, the modified sets is not returned back. See these changes on the go Play ground. https://go.dev/play/p/gULT7UcChI1

answered Mar 13 '22 at 01:15

Shakthi Thillai

61
1

wow thx. so basically slices get passed down by value in golang and by reference in java? – Omar Odaini Mar 13 '22 at 07:52
Go uses "Pass by Value" Semantics across the board. Sometimes the value copied and passed is data and sometimes it is pointer. In Slices and maps the pointer is copied and passed. In your case `backtrack` appends items to `sets` which changes the pointer as the new slice points to different memory address. But in `SubSets` you are referencing the old slice. Please see documentation https://go.dev/blog/slices-intro for great details on this topic. – Shakthi Thillai Mar 13 '22 at 14:08

Append to slice of slices - golang

1 Answers1