JSON library basics in Ruby

Introduction

JSON helps us save and share data in a simple text format. Ruby's JSON library makes it easy to turn Ruby data into JSON and back.

When you want to save data from your Ruby program to a file in a format other programs can read.

When you need to send data from your Ruby app to a web service or API that uses JSON.

When you receive JSON data from the internet and want to use it inside your Ruby program.

When you want to convert Ruby hashes or arrays into a string to store or send.

When you want to read JSON strings and turn them into Ruby objects to work with.

Syntax

Ruby

require 'json'

# Convert Ruby object to JSON string
json_string = JSON.generate(ruby_object)

# Convert JSON string to Ruby object
ruby_object = JSON.parse(json_string)

Use require 'json' to load the JSON library before using it.

JSON.generate turns Ruby data into a JSON string.

JSON.parse turns a JSON string back into Ruby data.

Examples

This converts a Ruby hash into a JSON string and prints it.

Ruby

require 'json'

ruby_hash = {"name" => "Alice", "age" => 30}
json_str = JSON.generate(ruby_hash)
puts json_str

This converts a JSON string into a Ruby hash and prints the value for "name".

Ruby

require 'json'

json_str = '{"name":"Bob","age":25}'
ruby_hash = JSON.parse(json_str)
puts ruby_hash["name"]

This converts a Ruby array into a JSON string and prints it.

Ruby

require 'json'

ruby_array = [1, 2, 3, "four"]
json_str = JSON.generate(ruby_array)
puts json_str

Sample Program

This program shows how to convert a Ruby hash to a JSON string and then back to a Ruby hash. It prints both the JSON string and the Ruby hash values.

Ruby

require 'json'

# Create a Ruby hash
person = {"name" => "Emma", "city" => "Paris", "age" => 28}

# Convert Ruby hash to JSON string
json_data = JSON.generate(person)
puts "JSON string:"
puts json_data

# Convert JSON string back to Ruby hash
ruby_data = JSON.parse(json_data)
puts "\nRuby hash from JSON:" 
puts ruby_data
puts "Name: #{ruby_data['name']}"

Important Notes

JSON keys are always strings, so Ruby symbols become strings in JSON.

When parsing JSON, Ruby creates hashes with string keys, not symbols.

Use JSON.pretty_generate if you want nicely formatted JSON output.

Summary

Ruby's JSON library helps convert Ruby data to JSON strings and back.

Use JSON.generate to create JSON strings from Ruby objects.

Use JSON.parse to read JSON strings into Ruby objects.

Practice

1.

What does the Ruby JSON.parse method do?

easy

A. Reads a file from disk

B. Converts a Ruby object into a JSON string

C. Converts a JSON string into a Ruby object

D. Writes a JSON string to a file

Solution

1. Step 1: Understand JSON.parse purpose

   The method JSON.parse reads a JSON formatted string and converts it into a Ruby data structure like a hash or array.

2. Step 2: Compare with other JSON methods

   JSON.generate does the opposite by converting Ruby objects into JSON strings.

3. Final Answer:

   Converts a JSON string into a Ruby object -> Option C

4. Quick Check:

   JSON.parse = JSON string to Ruby object [OK]

Hint: Remember: parse means read JSON into Ruby [OK]

Common Mistakes:

• Confusing parse with generate
• Thinking parse writes files
• Assuming parse converts Ruby to JSON

2.

Which of the following is the correct way to convert a Ruby hash {"name" => "Alice"} to a JSON string?

easy

A. JSON.generate({"name" => "Alice"})

B. JSON.dump({"name" => "Alice"}, $stdout)

C. JSON.load({"name" => "Alice"})

D. JSON.parse({"name" => "Alice"})

Solution

1. Step 1: Identify method to convert Ruby object to JSON string

   JSON.generate converts Ruby objects like hashes into JSON strings.

2. Step 2: Check other methods

   JSON.parse reads JSON strings, JSON.load is similar to parse but less common, JSON.dump also converts Ruby objects but requires a string or IO target.

3. Final Answer:

   JSON.generate({"name" => "Alice"}) -> Option A

4. Quick Check:

   generate = Ruby object to JSON string [OK]

Hint: Use generate to make JSON strings from Ruby hashes [OK]

Common Mistakes:

• Using parse instead of generate
• Confusing dump with parse
• Passing Ruby hash directly to parse

3.

What is the output of this Ruby code?

require 'json'
json_str = '{"age":30,"city":"NY"}'
ruby_obj = JSON.parse(json_str)
puts ruby_obj["city"]

medium

A. SyntaxError

B. 30

C. nil

D. NY

Solution

1. Step 1: Parse JSON string into Ruby hash

   The string {"age":30,"city":"NY"} is parsed into a Ruby hash with keys "age" and "city".

2. Step 2: Access the value for key "city"

   Using ruby_obj["city"] returns the string "NY".

3. Final Answer:

   NY -> Option D

4. Quick Check:

   JSON.parse + key access = value [OK]

Hint: Parse JSON then access keys like a hash [OK]

Common Mistakes:

• Expecting numeric output instead of string
• Using symbol keys instead of string keys
• Forgetting to require 'json'

4.

Identify the error in this Ruby code snippet:

require 'json'
json_str = '{"name": "Bob", "age": 25}'
ruby_obj = JSON.generate(json_str)
puts ruby_obj

medium

A. Using JSON.generate on a JSON string instead of a Ruby object

B. Missing require 'json' statement

C. Incorrect JSON string format

D. Using puts instead of print

Solution

1. Step 1: Check method usage

   JSON.generate expects a Ruby object (like a hash), but here it is given a JSON string.

2. Step 2: Understand correct usage

   To convert JSON string to Ruby object, use JSON.parse. To convert Ruby object to JSON string, use JSON.generate.

3. Final Answer:

   Using JSON.generate on a JSON string instead of a Ruby object -> Option A

4. Quick Check:

   generate needs Ruby object, not JSON string [OK]

Hint: Use parse for strings, generate for Ruby objects [OK]

Common Mistakes:

• Confusing generate and parse
• Passing string to generate
• Ignoring error messages

5.

You have a Ruby array [1, 2, 3]. You want to convert it to a JSON string and then back to a Ruby array. Which sequence of methods correctly does this?

hard

A. JSON.dump(JSON.parse([1, 2, 3]))

B. JSON.load(JSON.dump([1, 2, 3]))

C. JSON.generate(JSON.parse([1, 2, 3]))

D. JSON.dump(JSON.load([1, 2, 3]))

Solution

1. Step 1: Convert Ruby array to JSON string

   JSON.dump([1, 2, 3]) converts the Ruby array into a JSON string.

2. Step 2: Convert JSON string back to Ruby array

   JSON.load reads the JSON string and returns the Ruby array.

3. Step 3: Verify other options

   JSON.dump(JSON.parse([1, 2, 3])) tries to parse a Ruby array (invalid); JSON.generate(JSON.parse([1, 2, 3])) tries to parse a Ruby array (invalid); JSON.dump(JSON.load([1, 2, 3])) tries to load a Ruby array (invalid).

4. Final Answer:

   JSON.load(JSON.dump([1, 2, 3])) -> Option B

5. Quick Check:

   dump then load = Ruby array round-trip [OK]

Hint: Use dump then load for Ruby object to JSON and back [OK]

Common Mistakes:

• Parsing Ruby objects instead of JSON strings
• Using generate with parse incorrectly
• Confusing dump/load with generate/parse