Why this code is generating array index out of bound? - arrays

Question

I am stuck for past 5 - 6 hours in figuring this out that why this code is generating array index out of bound error on run time. I am unable to find out the reason. Can you please tell what modifications are required to correct this code?
spotsArr := make(map[int][]map[int64][]int)
for ind, availableSpot := range availableSpots {
spotsArr[availableSpot.Uid][ind] = make(map[int64][]int)
spotsArr[availableSpot.Uid][ind][availableSpot.Date] = []int{availableSpot.SpotSlug}
}
fmt.Println(spotsArr)
Edit 1: View the full code here https://play.golang.org/p/Smm0BFgtNp
Edit 2: Actually what I need to do is to get output in format something like:
{ uid: { date: {spot_slug, spot_slug} } }
{ 86: { 1536710400: {1000, 1200, 900},
{ 1536105600: {900} } }

Answer 1

The error is, as the error message suggests, because you tried to assign element on the index greater than the slice length. For the sake of getting the error away, you can just initialize the slice to the length, at least, as much as the index you wanted to use :
....
spotsArr[availableSpot.Uid] = make([]map[int64][]int, ind+1, ind+1)
spotsArr[availableSpot.Uid][ind] = make(map[int64][]int)
....
But as you clarified further about the desired output, it seems that you don't need slice in the first place. You need map of Uid where each key has value of map of Date :
spotsArr := make(map[int]map[int64][]int)
for _, availableSpot := range availableSpots {
if _, ok := spotsArr[availableSpot.Uid]; !ok {
spotsArr[availableSpot.Uid] = make(map[int64][]int)
}
spotsArr[availableSpot.Uid][availableSpot.Date] = append(spotsArr[availableSpot.Uid][availableSpot.Date],availableSpot.SpotSlug)
}
fmt.Println(spotsArr)
playground
Given the last two data have the same date, the output is as follows :
map[86:map[1534896000:[900] 1535500800:[900] 1536105600:[900] 1537315200:[900 900]]]

Answer 2

spotsArr is a map of int to an array of maps - map[int][]...
spotsArr := make(map[int][]map[int64][]int)
On this line, you try to assign to an index of that array which has no members yet:
spotsArr[availableSpot.Uid][ind] = make(map[int64][]int)
You're saying set this spot availableSpot.Uid to something (fine) but then set the index ind in an array which doesn't have members to something else (not fine). To fix this I'd recommend trying to do less on each line so that it's much clearer where and what the problem is. You could do this to fix the grammar error:
spotsArr[availableSpot.Uid] = []map[int64][]int{make(map[int64][]int)}
But I can't think why you want to set an index on the map to the index of the Uids you're traversing (your code doing [Ind]). I'd try to make this less complex and confusing if you can and spread it out on several lines to make the intent clear.
PS Give people a code sample which runs (i.e. include all the structs used), it makes it easier to help.
PPS Thanks for code sample, that makes it clearer.