Append slice to slice of slices - go

Question

I have data structure:
type PosList []int
type InvertedIndex struct {
Capacity int
Len int
IndexList []PosList
}
I have problem with Add method:
func (ii *InvertedIndex) Add(posList PosList, docId int) {
if ii.Len == ii.Capacity {
newIndexList := make([]PosList, ii.Len, (ii.Capacity+1)*2)
for i := 0; i < ii.Len; i++ {
newIndexList[i] = make([]int, len(ii.IndexList[i]))
copy(newIndexList[i], ii.IndexList[i])
}
ii.IndexList = newIndexList
}
ii.IndexList = ii.IndexList[0 : ii.Len+2]
ii.IndexList[docId] = posList
return
}
Or, i try something like this:
func (ii *InvertedIndex) Add(posList PosList, docId int) {
if ii.Len == ii.Capacity {
newIndexList := make([]PosList, ii.Len, (ii.Capacity+1)*2)
copy(newIndexList, ii.IndexList)
ii.IndexList = newIndexList
}
ii.IndexList = ii.IndexList[0 : ii.Len+2]
ii.IndexList[docId] = posList
return
}
Both of them don't work, may be someone can explain how can i append a slice to structure like this.

Answer 1

You question is confusing. I assume that you are trying to create a typical inverted index. In which case, you probably want to do something like this:
package main
import "fmt"
type DocId int
type Positions []int
type docIndex struct {
docId DocId
positions Positions
}
type InvertedIndex struct {
docIndexes []docIndex
}
func New() *InvertedIndex {
return &InvertedIndex{}
}
func (ii *InvertedIndex) Add(docId DocId, positions Positions) {
for i, di := range (*ii).docIndexes {
if di.docId == docId {
di.positions = append(di.positions, positions...)
(*ii).docIndexes[i] = di
return
}
}
di := docIndex{
docId: docId,
positions: positions,
}
(*ii).docIndexes = append((*ii).docIndexes, di)
}
func main() {
ii := New()
docId := DocId(11)
positions := Positions{42, 7}
ii.Add(docId, positions)
positions = Positions{21, 4}
ii.Add(docId, positions)
docId = DocId(22)
positions = Positions{84, 14}
ii.Add(docId, positions)
fmt.Printf("%+v\n", *ii)
}
Output:
{docIndexes:[{docId:11 positions:[42 7 21 4]} {docId:22 positions:[84 14]}]}
The statement:
di.positions = append(di.positions, positions...)
appends a slice to a slice.
References:
Appending to and copying slices
Arrays, slices (and strings): The mechanics of 'append'
inverted index

Answer 2

I'm not sure I fully understand what you're doing, however something like this should just work fine, replacing the slice with a map :
type PosList []int
type InvertedIndex struct {
Len int
IndexList map[int]PosList
}
func (ii *InvertedIndex) Add(posList PosList, docId int) {
if ii.IndexList == nil {
ii.IndexList = make(map[int]PosList)
}
if _, ok := ii.IndexList[docId]; ok {
ii.IndexList[docId] = append(ii.IndexList[docId], posList...)
} else {
ii.IndexList[docId] = posList
}
ii.Len = len(ii.IndexList)
}