Accessing data in nested arrays & objects with Go

Question

I'm doing my best to unmarshall some json data into usable form in Go, but can't seem to get it. The data is:

{
    "series": [
        {
            "series_id": "PET.EMD_EPD2D_PTE_NUS_DPG.W",
            "name": "U.S. No 2 Diesel Retail Prices, Weekly",
            "units": "Dollars per Gallon",
            "updated": "2013-09-27T07:21:57-0400",
            "data": [
                [
                    "20130923",
                    "3.949"
                ],
                [
                    "20130916",
                    "3.974"
                ]
            ]
        }
    ]
}

I'm trying to get the arrays under data into a variable, so I can loop through them and do something like:

if data[i][0] == "20130923" {
    fuelPrice.Price == data[i][1]
}

I've tried to unmarshall the data into a struct but I can't figure out how to get past series... ie I can't figure out how to do nested arrays. Things like these don't work:

type Series struct {
    SeriesId    string
    Name        string
    Data        [][]string
}

type RawFuelPrice struct {
    Series []Series
    Data []interface{}[]
}

Also if I unmarshal into an interface{}, I can't figure out how to access the data...

I'm definitely a beginner. Thanks for your time and effort.

See e.g. stackoverflow.com/questions/19145202/marshal-of-json-rawmessage stackoverflow.com/questions/19088799/… stackoverflow.com/questions/19081479/xml-decoding-in-go stackoverflow.com/questions/18806325/… and of course golang.org/pkg/encoding/json/#pkg-examples . XML and JSON decoding works pretty much the same, just use json:"xyz" as struct tag. — Volker
– Volker, Commented Oct 2, 2013 at 20:57
@Volker, sorry but I just am not able to figure this out. I've tried this: type EIASeries []struct { SeriesId string json:"series_id"` Data interface{} json:"data" } type EIAResponse struct { Series EIASeries json:"series" }` When I log the var resp EIAResponse, I get a lot of data, but I can't get anything when I log resp.Series or log resp.Series.Data... Thanks. — Andrew Samuelsen
– Andrew Samuelsen, Commented Oct 2, 2013 at 22:03

Henry Finucane · Accepted Answer · 2013-10-02 22:45:17Z

1

Your code is just fine- except for the Data member of the RawFuelPrice struct. I don't think that syntax is valid, and there's no Data attribute at the top level of of the JSON blob.

That said, this is how you'd get the data out:

var rfp RawFuelPrice
json.Unmarshal(input, &rfp)
for _,s := range rfp.Series {
    fmt.Println("Name",s.Name)
    for _,d := range s.Data {
        fmt.Println("\tdate:",d[0])
        fmt.Println("\tprice:",d[1])
    }
    fmt.Println()
}

Although you'd probably want to check that all the data was there.

Go Playground link: http://play.golang.org/p/C47lZJ_L0o

answered Oct 2, 2013 at 22:45

Henry Finucane

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1 Comment

ANisus Over a year ago

The Data []interface{} is redundant. It doesn't match anything in the json data.

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