Unmarshaling unnamed JSON array of unnamed objects

Question

The JSON I am trying to unmarshal with Go is unnamed array of unnamed objects:

[
{
    "date": 1394062029,
    "price": 654.964,
    "amount": 5.61567,
    "tid": 31862774,
    "price_currency": "USD",
    "item": "BTC",
    "trade_type": "ask"
},
{
    "date": 1394062029,
    "price": 654.964,
    "amount": 0.3,
    "tid": 31862773,
    "price_currency": "USD",
    "item": "BTC",
    "trade_type": "ask"
},
{
    "date": 1394062028,
    "price": 654.964,
    "amount": 0.0193335,
    "tid": 31862772,
    "price_currency": "USD",
    "item": "BTC",
    "trade_type": "bid"
}
]

I can successfully unmarshal the object and print the complete tradesResult array as %#v, but when I try to access element of the array I get the following error.

prog.go:41: invalid operation: tradeResult[0] (index of type *TradesResult)

Here is example code you can run to try the problem:

// You can edit this code!
// Click here and start typing.
package main

import (
    "fmt"
    "io/ioutil"
    "net/http"
    "encoding/json"
)

type TradesResultData struct {
    Date     float64 `json:"date"`
    Price    float64 `json:"price"`
    Amount   float64 `json:"amount"`
    Trade    float64 `json:"tid"`
    Currency string  `json:"price_currency"`
    Item     string  `json:"item"`
    Type     string  `json:"trade_type"`
}

type TradesResult []TradesResultData

func main() {
    resp, err := http.Get("https://btc-e.com/api/2/btc_usd/trades")
    if err != nil {
        fmt.Printf("%s\r\n", err)
    }
    json_response, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        fmt.Printf("%s\r\n", err)
    }
    resp.Body.Close()
    fmt.Printf("JSON:\r\n%s\r\n", json_response)
    tradeResult := new(TradesResult)
    err = json.Unmarshal(json_response, &tradeResult)
    if err != nil {
        fmt.Printf("%s\r\n", err)
    }
    // Printing trade result first element Amount
    fmt.Printf("Element 0 Amount: %v\r\n", tradeResult[0].Amount)
}

James Henstridge · Accepted Answer · 2014-03-06 00:24:26Z

5

On this line:

tradeResult := new(TradesResult)

You are declaring the tradeResult variable using the *TradeResult type. That is, a pointer to a slice. The error you received is because you can't use index notation on a pointer to a slice.

One way to fix this would be to change the last line to use (*tradeResult)[0].Amount. Alternatively, you could declare tradeResult as:

var tradeResult TradeResult

The json module will be able to decode into &tradeResult just fine, and you won't need to dereference it to index into the slice.

answered Mar 6, 2014 at 0:24

James Henstridge

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4 Comments

Jeff Over a year ago

can't you also just change new to make?

Tonto Over a year ago

Thanks, seriously. I may be too sleepy. :/

Leo Correa Over a year ago

I figured this same answer out but I wasn't sure of the reason. Thanks for this explanation! The docs seem a bit ambiguous on this explanation golang.org/doc/effective_go.html#allocation_new Specifically

Values of type SyncedBuffer are also ready to use immediately upon allocation or just declaration. In the next snippet, both p and v will work correctly without further arrangement.

James Henstridge Over a year ago

@Jeff: make(TradeResult, 0) would also work. But that's equivalent to a zero initialised slice, so just declaring the variable gives the same result.

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