CS 245 - Programming Languages

Lab 6

Go and JSON

JSON (JavaScript Object Notation) is the de-facto standard for data serialization. Its uses goes far beyond JavaScript. In this lab you will learn about JSON and how to marshal (encode) and unmarshal (decode) JSON in Go

JSON

Start the lab by learning about JSON. One of the best resources is https://www.json.org/json-en.html, which is correct and complete but rather hard to read. Other web resources are available.

Write a JSON file -- you can do so in VSC (create a file xxx.json) -- containing data about 3 Bi-Co students (fictional is preferred). The file should contain at least the following information for each student:

  1. First Name
  2. Last Name
  3. Id -- as a string
  4. Age -- as in integer
  5. DateOfBirth (see below)
  6. FavoriteRealNumber -- as a number
The file should be arranged as an array of objects.

Date of birth should itself be an object with the following fields:
  1. Year -- an integer
  2. Month -- a string
  3. Day -- an integer

JSON and Go

Because JSON is a de-facto standard, Go has a package for reading and writing json data. So first, open the file you just made for reading and read the entire contents into an array of bytes. 
        jsonFile, err := os.Open("users.json")
        if err != nil {
            fmt.Println(err)
        }
        fmt.Println("Successfully Opened users.json")
        byteValue, _ := ioutil.ReadAll(jsonFile)
        jsonFile.Close()
My experience with opening files is that if you use the the "run" button in VSC, you need to give the full, absolute path to the file being opened. If you use a terminal window and type "go run xx.go" then you can use a relative path name.

The simplest thing to do is to is to simply unmarshal the byte array into a slice of maps. To do so create two types 
        type jm map[string]interface{}
        type js []jm
then after the read funcion add the following lines: 
        var res js
        json.Unmarshal(byteValue, &res)
Done. You have read in the JSON file. Add a print statement to verify that you have succeeded.

Unmarshalling into structs

Read the web page https://www.sohamkamani.com/golang/json/ down to the header "primitive types". This gives a good overiew of unmarshalling JSON directly into an array of Go structs.

Create structs as needed to read your student data file. You may want to update the representation in your JSON to make this easier. Then adjust your code (create, probably two structs, etc) to read the JSON into a slice of structs.

One important thing when you are creating structs for unmarshalling. The variable names inside the structs MUST have an initial capital letter. Why?

Marshall back to JSON

Finally, take the result of reading your student data into structs and produce new JSON by marshalling. 
        data, _ := json.Marshal(students)
	    fmt.Println(string(data))
The resulting JSON should be fairly similar to your student file.

What to hand in

Send your JSON student data and your final go program to gtowell245@cs.brynmawr.edu