在go中转换数据结构：紧急：运行时错误：索引超出范围

Question

I have a data structure in go:

type APIMain struct {
    CodeConv string    `json:"codeConv"`
    Start    time.Time `json:"start"`
    End      time.Time `json:"end"`
    Details  []struct {
        IDPrm string `json:"idPrm"`
        Keys  []struct {
            Timestamp time.Time `json:"timestamp"`
            Value     float64   `json:"value"`
        } `json:"keys"`
    } `json:"details"`
}

that I need to transform to:

type DataGroupedByTS struct {
    CodeConv string    `json:"codeConv"`
    Start    time.Time `json:"start"`
    End      time.Time `json:"end"`
    Details  []struct {
        Timestamp time.Time `json:"timestamp"`
        Keys      []struct {
            IDPrm string  `json:"idPrm"`
            Value float64 `json:"value"`
        } `json:"keys"`
    } `json:"details"`
}

I get:

 panic: runtime error: index out of range

Here is my method but it is failing on the first line of loop:

func groupByTimestamp(apiMain datacheck.APIMain) DataGroupedByTS {
    var dataGrouped DataGroupedByTS
    dataGrouped.CodeConv = apiMain.CodeConv
    dataGrouped.Start = apiMain.Start
    dataGrouped.Start = apiMain.Start
    dataGrouped.End = apiMain.End

    var iDetail = 0
    var iKey = 0
    for _, detail := range apiMain.Details {
        for _, key := range detail.Keys {
            dataGrouped.Details[iDetail].Timestamp = key.Timestamp  // <-- failing here
            dataGrouped.Details[iDetail].Keys[iKey].IDPrm = detail.IDPrm
            dataGrouped.Details[iDetail].Keys[iKey].Value = key.Value
            iKey++
        }
        iDetail++
    }

    return dataGrouped
}

Basically, data originally comes grouped by IDPrm, and I need to group it by timestamp.

How should I do that ? Is there any helpers that could help doing it ?

Answer 1

The problem

The reason is simple:

var dataGrouped DataGroupedByTS

initializes the fields of dataGrouped to the so-called zero value for the type DataGroupedByTS.

The zero value of any compound type T consists of the zero-values for types corresponding to each of the fields of T.

So, for

type DataGroupedByTS struct {
    CodeConv string    `json:"codeConv"`
    Start    time.Time `json:"start"`
    End      time.Time `json:"end"`
    Details  []struct {
        Timestamp time.Time `json:"timestamp"`
        Keys      []struct {
            IDPrm string  `json:"idPrm"`
            Value float64 `json:"value"`
        } `json:"keys"`
    } `json:"details"`
}

the zero value would be

type DataGroupedByTS struct {
    CodeConv: "",
    Start:    time.Time(0),
    End:      time.Time(0),
    Details:  nil,           // watch this!
}

That's because the type of Details is []struct{ ... }, that is, a slice of some structs, and the zero-value of any slice is nil.

You then go on and attempt to write at some index into a non-existing slice (well, the slice without any backing array allocated to hold its data). This reasonably fails with "panic: runtime error: index out of range": an unallocated slice has zero elements, so there is no element at index 0, and there is nothing to assign to.

The fix

Two ways:

1. Preallocate the destination slice:

       var dataGrouped DataGroupedByTS
       // ...
       dataGrouped.Details = make([]struct{...}, len(apiMain.Details))
       for _, detail := range apiMain.Details {
       // ...
   

2. Append to a slice, rather than updating its elements in place:

       var dataGrouped DataGroupedByTS
       // ...
       for _, detail := range apiMain.Details {
           dataGrouped.Details = append(dataGrouped.Details, detail)
           // ...
   

   It's OK to append to a nil slice.