Golang read byte array from .. to and display result

Question

First of all: I know that this is super basic question and you'd expect to find enough material on the internet and there probably is. I feel pretty stupid right now for not understanding it, so no need to point that out to me - I know^^ From the google Directory API, the response you get when reading a custom Schema is JSON-enocded: https://developers.google.com/admin-sdk/directory/v1/reference/schemas I copy/pasted that response and wanted to read it.

 func main() {
    jsonExample := `
    {
  "kind": "admin#directory#schema",
  "schemaId": "string",
  "etag": "etag",
  "schemaName": "string",
  "displayName": "string",
  "fields": [
    {
      "kind": "admin#directory#schema#fieldspec",
      "fieldId": "string",
      "etag": "etag",
      "fieldType": "string",
      "fieldName": "string",
      "displayName": "string",
      "multiValued": true,
      "readAccessType": "string",
      "indexed": true,
      "numericIndexingSpec": {
        "minValue": 2.0,
        "maxValue": 3.0
      }
    }
  ]
}
`

    var jsonDec schemaExample

    jsonExampleBytes := []byte(jsonExample)    

    m := make(map[string]interface{})
    err := json.Unmarshal([]byte(jsonExample), &m)
    byteStorage := make([]byte,600)
        byteReader := bytes.NewReader(byteStorage)
res, err := byteReader.ReadAt(jsonExampleBytes,50)
fmt.Printf("############Hier : %v Err: \n%v",res,err)
fmt.Printf("Storage: %v\n",byteStorage)



byteStorage := make([]byte,600)
byteReader := bytes.NewReader(byteStorage)
        res, err := byteReader.ReadAt(jsonExampleBytes,50)
        fmt.Printf("Result : %v Err: %v\n",res,err)
        fmt.Printf("Storage: %v\n",byteStorage)

This returns

res : 526 Err: <nil>
Storage: [0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0]

.My question is how to implement a ReadFromTo method, which allows me to read a specific range of bytes from a byte array? And since the storage is empty, I also lack to understand how read that array back at all with the reader functions, only way I know how to pull it off is this:

fmt.Printf("Und die bytes to String: %v",string([]byte(jsonExample)))

This is not a valid json. It is showing the type of value not the value itself. — Himanshu
– Himanshu, Commented Jul 26, 2018 at 9:19
Thanks, I changed it and now those errors are gone - the rest of the question remains thought. — InDaPond
– InDaPond, Commented Jul 26, 2018 at 9:28

Peter · Accepted Answer · 2018-07-26 09:48:44Z

4

From the docs (emphasis mine):

ReaderAt is the interface that wraps the basic ReadAt method.

ReadAt reads len(p) bytes into p starting at offset off in the underlying input source.

type ReaderAt interface {
    ReadAt(p []byte, off int64) (n int, err error)
}

The argument to ReadAt (and Read in general) is the destination. You've got jsonExampleBytes and byteStorage the wrong way around.

package main

import (
        "bytes"
        "fmt"
)

func main() {
        jsonExampleBytes := []byte(`{...}`)

        byteReader := bytes.NewReader(jsonExampleBytes)

        byteStorage := make([]byte, 600)
        n, err := byteReader.ReadAt(byteStorage, 3)

        fmt.Println("Storage:", string(byteStorage[:n]), err) // Storage: .} EOF
}

answered Jul 26, 2018 at 9:48

Peter

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2 Comments

Melroy van den Berg Over a year ago

Small follow-up question: Should I close and/or defer the byteReader? If possible

Peter Over a year ago

@MelroyvandenBerg, bytes.Reader does not have a Close method. In general, you close an io.ReadCloser when it is no longer used, unless documented otherwise. For instance, http.Request.Body does not have to be closed.

gonutz · Accepted Answer · 2018-07-26 09:48:39Z

1

To access a sub-slice of bytes, you can in the most basic case just use the index operator:

array := make([]byte, 100)
bytes5to9 = array[5:10]

note here that the second index is exclusive.

If you need an io.Reader from these bytes, you can use

r := bytes.NewReader(array[5:10])

You can do this again, creating a second read for the same or a different range of the array.

The utility functions in io and ioutil might be of interest to you as well. See for example ioutil.ReadAll, io.Copy, io.CopyBuffer, io.CopyN and io.ReadFull.

answered Jul 26, 2018 at 9:48

gonutz

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