Golang从..到读取字节数组并显示结果

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: 
%v",res,err)
fmt.Printf("Storage: %v
",byteStorage)



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

Answer 1

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
}

Answer 2

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.