How to parse a string (which is an array) in Go using json package?
type JsonType struct{
Array []string
}
func main(){
dataJson = `["1","2","3"]`
arr := JsonType{}
unmarshaled := json.Unmarshal([]byte(dataJson), &arr.Array)
log.Printf("Unmarshaled: %v", unmarshaled)
}
The return value of Unmarshal is an error, and this is what you are printing out:
// Return value type of Unmarshal is error.
err := json.Unmarshal([]byte(dataJson), &arr)
You can get rid of the JsonType as well and just use a slice:
package main
import (
"encoding/json"
"log"
)
func main() {
dataJson := `["1","2","3"]`
var arr []string
_ = json.Unmarshal([]byte(dataJson), &arr)
log.Printf("Unmarshaled: %v", arr)
}
// prints out:
// 2009/11/10 23:00:00 Unmarshaled: [1 2 3]
Code on play: https://play.golang.org/p/GNWlylavam
Background: Passing in a pointer allows Unmarshal to reduce (or get entirely rid of) memory allocations. Also, in a processing context, the caller may reuse the same value to repeatedly - saving allocations as well.
Note: This answer was written before the question was edited. In the original question &arr was passed to json.Unmarshal():
unmarshaled := json.Unmarshal([]byte(dataJson), &arr)
You pass the address of arr to json.Unmarshal() to unmarshal a JSON array, but arr is not an array (or slice), it is a struct value.
Arrays can be unmarshaled into Go arrays or slices. So pass arr.Array:
dataJson := `["1","2","3"]`
arr := JsonType{}
err := json.Unmarshal([]byte(dataJson), &arr.Array)
log.Printf("Unmarshaled: %v, error: %v", arr.Array, err)
Output (try it on the Go Playground):
2009/11/10 23:00:00 Unmarshaled: [1 2 3], error: <nil>
Of course you don't even need the JsonType wrapper, just use a simple []string slice:
dataJson := `["1","2","3"]`
var s []string
err := json.Unmarshal([]byte(dataJson), &s)
Related
I'm new to GO. There is the problem I'm facing.
The function takes a 2D array in arbitrary size as argument:
func PrintArray(a [][]string) {
for _, v1 := range a {
for _, v2 := range v1 {
fmt.Printf("%s ", v2)
}
fmt.Printf("\n")
}
}
As you can see, since the nested loop uses range. The size of the array really doesn't matter.
But when I try to call this function:
a := [3][2]string{
{"line", "tiger"},
{"cat", "dog"},
{"pigeon", "hamster"},
}
PrintArray(a[:])
It complains about:
cannot use a[:] (type [][2]string) as type [][]string in argument to PrintArray
However, it won't compile with a[:][:] either.
What is the correct way to define a multidimensional array in arbitrary size in GO lang?
In Go, array types and slice types are distinct. Pass slices to slices.
For example,
package main
import (
"fmt"
)
func main() {
a := [][]string{
{"line", "tiger"},
{"cat", "dog"},
{"pigeon", "hamster"},
}
PrintSlices(a)
}
func PrintSlices(a [][]string) {
for _, v1 := range a {
for _, v2 := range v1 {
fmt.Printf("%s ", v2)
}
fmt.Printf("\n")
}
}
Playground: https://play.golang.org/p/3mPDTIEUQmT
Output:
line tiger
cat dog
pigeon hamster
To allocate a matrix:
package main
import (
"fmt"
)
func NewMatrix(rows, cols int) [][]int {
m := make([][]int, rows)
for r := range m {
m[r] = make([]int, cols)
}
return m
}
func main() {
m := NewMatrix(3, 2)
fmt.Println(m)
}
Playground: https://play.golang.org/p/qvwQu2giRcP
Output:
[[0 0] [0 0] [0 0]]
Interesting question. This should work
a := [][]string{
{"line", "tiger"},
{"cat", "dog"},
{"pigeon", "hamster"},
}
PrintArray(a[:])
From the error messages, I would guess that arrays defined with size are of different types.
For example, if the PrintArray looks like this:
func PrintArray(a [3][2]string) {
Then you can pass it an array defined as:
a := [3][2]string.
If we vary the numbers in either the PrintArray method or a, so that they differ, we get an error message that says:
cannot use a (type [3][2]string) as type [3][1]string in argument to PrintArray
What's the best way to Unmarshal bson byte[] data into an array of structs, when the array of structs is passed into an interface{} parameter?
For demonstration purposes, in the following code, I use bson.Marshal() on the inStructArr to get the byte[] type of data. This is so I can use bson.Unmarshal(...) to pipe into the outStructArr.
import "gopkg.in/mgo.v2/bson"
type User struct {
Name string
}
func DecodeArrData(inStructArr, outStructArr interface{}) {
inStructArrData, _ := bson.Marshal(inStructArr)
bson.Unmarshal(inStructArrData, outStructArr) // <-- Error happens here
// What's the right way of accomplishing this?
}
func Main() {
outUsers := &[]User{}
inUsers := []User{User{"A"}, User{"B"}}
DecodeArrData(inUsers, outUsers)
}
When I do this, the error-message I get is: Unsupported document type for unmarshalling: []User. What's the right way of doing this?
Thanks in advance!
The Marshal and Unmarshal functions work with BSON documents, not BSON arrays.
Wrap the slices in a struct to provide the document expected by the package:
func DecodeArrData(inStructArr, outStructArr interface{}) error {
in := struct{ Data interface{} }{Data: inStructArr}
inStructArrData, err := bson.Marshal(in)
if err != nil {
return err
}
var out struct{ Data bson.Raw }
if err := bson.Unmarshal(inStructArrData, &out); err != nil {
return err
}
return out.Data.Unmarshal(outStructArr)
}
If you are willing to take advantage of an undocumented feature of the Marshal function and add some BSON format knowledge to your application, then you can omit the wrapper.
The undocumented feature of Marshal is that it encodes slices as BSON arrays. The BSON array can be decoded using a bson.Raw value with Kind set the the BSON code for arrays (the value 4) and Data set to the array data:
func DecodeArrData(inStructArr, outStructArr interface{}) error {
inStructArrData, err := bson.Marshal(inStructArr)
if err != nil {
return err
}
raw := bson.Raw{Kind: 4, Data: inStructArrData}
return raw.Unmarshal(outStructArr)
}
I'm trying to learn Go (or Golang) and can't seem to get it right. I have 2 texts files, each containing a list of words. I'm trying to count the amount of words that are present in both files.
Here is my code so far :
package main
import (
"fmt"
"log"
"net/http"
"bufio"
)
func stringInSlice(str string, list []string) bool {
for _, v := range list {
if v == str {
return true
}
}
return false
}
func main() {
// Texts URL
var list = "https://gist.githubusercontent.com/alexcesaro/c9c47c638252e21bd82c/raw/bd031237a56ae6691145b4df5617c385dffe930d/list.txt"
var url1 = "https://gist.githubusercontent.com/alexcesaro/4ebfa5a9548d053dddb2/raw/abb8525774b63f342e5173d1af89e47a7a39cd2d/file1.txt"
//Create storing arrays
var buffer [2000]string
var bufferUrl1 [40000]string
// Set a sibling counter
var sibling = 0
// Read and store text files
wordList, err := http.Get(list)
if err != nil {
log.Fatalf("Error while getting the url : %v", err)
}
defer wordList.Body.Close()
wordUrl1, err := http.Get(url1)
if err != nil {
log.Fatalf("Error while getting the url : %v", err)
}
defer wordUrl1.Body.Close()
streamList := bufio.NewScanner(wordList.Body)
streamUrl1 := bufio.NewScanner(wordUrl1.Body)
streamList.Split(bufio.ScanLines)
streamUrl1.Split(bufio.ScanLines)
var i = 0;
var j = 0;
//Fill arrays with each lines
for streamList.Scan() {
buffer[i] = streamList.Text()
i++
}
for streamUrl1.Scan() {
bufferUrl1[j] = streamUrl1.Text()
j++
}
//ERROR OCCURRING HERE :
// This code if i'm not wrong is supposed to compare through all the range of bufferUrl1 -> bufferUrl1 values with buffer values, then increment sibling and output FIND
for v := range bufferUrl1{
if stringInSlice(bufferUrl1, buffer) {
sibling++
fmt.Println("FIND")
}
}
// As a testing purpose thoses lines properly paste both array
// fmt.Println(buffer)
// fmt.Println(bufferUrl1)
}
But right now, my build doesn't even succeed. I'm only greeted with this message:
.\hello.go:69: cannot use bufferUrl1 (type [40000]string) as type string in argument to stringInSlice
.\hello.go:69: cannot use buffer (type [2000]string) as type []string in argument to stringInSlice
bufferUrl1 is an array: [4000]string. You meant to use v (each
string in bufferUrl1). But in fact, you meant to use the second
variable—the first variable is the index which is ignored in the code
below using _.
type [2000]string is different from []string. In Go, arrays and slices are not the same. Read Go Slices: usage and internals. I've changed both variable declarations to use slices with the same initial length using make.
These are changes you need to make to compile.
Declarations:
// Create storing slices
buffer := make([]string, 2000)
bufferUrl1 := make([]string, 40000)
and the loop on Line 69:
for _, s := range bufferUrl1 {
if stringInSlice(s, buffer) {
sibling++
fmt.Println("FIND")
}
}
As a side-note, consider using a map instead of a slice for buffer for more efficient lookup instead of looping through the list in stringInSlice.
https://play.golang.org/p/UcaSVwYcIw has the fix for the comments below (you won't be able to make HTTP requests from the Playground).
I want to return a structure that looks like this:
{
results: [
["ooid1", 2.0, "Söme text"],
["ooid2", 1.3, "Åther text"],
]
}
That's an array of arrags that is string, floating point number, unicode character.
If it was Python I'd be able to:
import json
json.dumps({'results': [["ooid1", 2.0, u"Söme text"], ...])
But in Go you can't have an array (or slice) of mixed types.
I thought of using a struct like this:
type Row struct {
Ooid string
Score float64
Text rune
}
But I don't want each to become a dictionary, I want it to become an array of 3 elements each.
We can customize how an object is serialized by implementing the json.Marshaler interface. For our particular case, we seem to have a slice of Row elements that we want to encode as an array of heterogenous values. We can do so by defining a MarshalJSON function on our Row type, using an intermediate slice of interface{} to encode the mixed values.
This example demonstrates:
package main
import (
"encoding/json"
"fmt"
)
type Row struct {
Ooid string
Score float64
Text string
}
func (r *Row) MarshalJSON() ([]byte, error) {
arr := []interface{}{r.Ooid, r.Score, r.Text}
return json.Marshal(arr)
}
func main() {
rows := []Row{
{"ooid1", 2.0, "Söme text"},
{"ooid2", 1.3, "Åther text"},
}
marshalled, _ := json.Marshal(rows)
fmt.Println(string(marshalled))
}
Of course, we also might want to go the other way around, from JSON bytes back to structs. So there's a similar json.Unmarshaler interface that we can use.
func (r *Row) UnmarshalJSON(bs []byte) error {
arr := []interface{}{}
json.Unmarshal(bs, &arr)
// TODO: add error handling here.
r.Ooid = arr[0].(string)
r.Score = arr[1].(float64)
r.Text = arr[2].(string)
return nil
}
This uses a similar trick of first using an intermediate slice of interface{}, using the unmarshaler to place values into this generic container, and then plop the values back into our structure.
package main
import (
"encoding/json"
"fmt"
)
type Row struct {
Ooid string
Score float64
Text string
}
func (r *Row) UnmarshalJSON(bs []byte) error {
arr := []interface{}{}
json.Unmarshal(bs, &arr)
// TODO: add error handling here.
r.Ooid = arr[0].(string)
r.Score = arr[1].(float64)
r.Text = arr[2].(string)
return nil
}
func main() {
rows := []Row{}
text := `
[
["ooid4", 3.1415, "pi"],
["ooid5", 2.7182, "euler"]
]
`
json.Unmarshal([]byte(text), &rows)
fmt.Println(rows)
}
You can read a full example here.
Use []interface{}
type Results struct {
Rows []interface{} `json:"results"`
}
You will then have to use type assertion if you want to access the values stored in []interface{}
for _, row := range results.Rows {
switch r := row.(type) {
case string:
fmt.Println("string", r)
case float64:
fmt.Println("float64", r)
case int64:
fmt.Println("int64", r)
default:
fmt.Println("not found")
}
}
Some clumsy, but you can
type result [][]interface{}
type results struct {
Results result
}
Working example https://play.golang.org/p/IXAzZZ3Dg7
I'm learning Go by writing a simple http server and I need to handle some JSON responses.
With an object response, I can unmarshal it idiomatically with 2 lines of code:
structResult := Foo{}
json.Unmarshal(structBody, &structResult)
I don't know how to do the same for an array response (see the example below). Is there a way to specify (possibly via json tag) that top-level array should go into a given struct field?
package main
import "fmt"
import "encoding/json"
type Foo struct {
Id uint64 `json:"id"`
Name string `json:"name"`
}
type BaseResult struct {
Error string `json:"error"`
}
type FooResult struct {
BaseResult
Foos []Foo
}
func main() {
// Simple and works.
structBody := []byte(`{"id": 1,"name": "foo"}`)
structResult := Foo{}
json.Unmarshal(structBody, &structResult)
fmt.Printf("%#v\n", structResult)
// Doesn't work.
arrayBody := []byte(`[{"id": 1,"name": "foo"},{"id": 2,"name": "bar"},{"id": 3,"name": "foobar"}]`)
arrayResult := FooResult{}
json.Unmarshal(arrayBody, &arrayResult)
fmt.Printf("%#v\n", arrayResult)
}
I know I could make FooResult an array:
type FooResult []Foo
but then I lose the ability to specify base object which I would like to use to store error message and such. I also know that I can unmarshal into &fooResult.Foos directly, but I want the code to work with both objects and arrays.
UPDATE
Implementing UnmarshalJSON as suggested by #dyoo partially solves my problem, but I was hoping that I could use BaseResult to store parse error in case JSON has a different structure:
arrayBody := []byte(`{"error": "foo"}`)
arrayResult := FooResult{}
json.Unmarshal(arrayBody, &arrayResult)
fmt.Printf("%#v\n", arrayResult)
Of course I could implement more complex logic inside UnmarshalJSON - but isn't there a simpler way to do it?
You can implement the json.Unmarshaler interface in your FooResult, to customize exactly how it responds to unmarshaling. (Similarly, there's a json.Marshaler interface.)
Add:
func (f *FooResult) UnmarshalJSON(bs []byte) error {
return json.Unmarshal(bs, &f.Foos)
}
after which your code should otherwise work. http://play.golang.org/p/oMdoB2e-rB
You might try something like:
func (f *FooResult) UnmarshalJSON(bs []byte) error {
err1 := json.Unmarshal(bs, &f.BaseResult)
err2 := json.Unmarshal(bs, &f.Foos)
if err1 != nil && err2 != nil {
// Arbitrarily choose an error.
return err1
}
return nil
}
although even this is beginning to look dubious. Handling union type results is not quite what the json library is designed to handle automatically for you. You'll need to explicitly code the coercion logic if your JSON has dynamic type.
See: How to unmarshall an array of different types correctly? and http://blog.golang.org/json-and-go for related issues.
Just specify Foos when you Unmarshal
package main
import "fmt"
import "encoding/json"
type Foo struct {
Id uint64 `json:"id"`
Name string `json:"name"`
}
type BaseResult struct {
Error string `json:"error"`
}
type FooResult struct {
BaseResult
Foos []Foo
}
func main() {
// Simple and works.
structBody := []byte(`{"id": 1,"name": "foo"}`)
structResult := Foo{}
json.Unmarshal(structBody, &structResult)
fmt.Printf("%#v\n", structResult)
// Doesn't work.
arrayBody := []byte(`[{"id": 1,"name": "foo"},{"id": 2,"name": "bar"},{"id": 3,"name": "foobar"}]`)
arrayResult := FooResult{}
if err := json.Unmarshal(arrayBody, &arrayResult.Foos); err != nil {
arrayResult.BaseResult.Error = string(arrayBody)
}
fmt.Printf("%#v\n", arrayResult)
}