为什么这段代码会超出范围生成数组索引？

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)

<kbd>playground</kbd>

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]]]