JSON library basics in Ruby - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Ruby code using JSON?

Consider this Ruby code that uses the JSON library to parse a JSON string. What will be printed?

Ruby

require 'json'
json_str = '{"name":"Alice","age":30}'
obj = JSON.parse(json_str)
puts obj["name"]

A SyntaxError

B 30

C nil

D Alice

💡 Hint

Think about what JSON.parse returns and how to access values in the resulting object.

JSON.parse converts the JSON string into a Ruby hash. Accessing obj["name"] returns the string "Alice".

❓ Predict Output

intermediate

What does this Ruby code output when generating JSON?

This Ruby code creates a hash and converts it to a JSON string. What is the output?

Ruby

require 'json'
hash = {"city" => "Paris", "country" => "France"}
puts JSON.generate(hash)

A {"city":"Paris","country":"France"}

B {city: "Paris", country: "France"}

C {"city":Paris,"country":France}

D SyntaxError

💡 Hint

Remember JSON keys and string values must be in double quotes.

JSON.generate converts the Ruby hash into a JSON string with keys and string values quoted properly.

🔧 Debug

advanced

What error does this Ruby code raise when parsing JSON?

Look at this Ruby code snippet. What error will it raise when run?

Ruby

require 'json'
json_str = '{name: "Bob", age: 25}'
obj = JSON.parse(json_str)

A NoMethodError

B JSON::ParserError

C TypeError

D SyntaxError

💡 Hint

Check if the JSON string is valid JSON format.

The JSON string uses unquoted keys, which is invalid JSON. JSON.parse raises JSON::ParserError.

🧠 Conceptual

advanced

Which Ruby method converts a Ruby object to a JSON string?

Which method from the JSON library converts a Ruby object like a hash or array into a JSON formatted string?

A JSON.generate

B JSON.load

C JSON.parse

D JSON.read

💡 Hint

Think about the method that creates JSON text from Ruby objects.

JSON.generate takes a Ruby object and returns a JSON string representation.

🚀 Application

expert

How many keys are in the Ruby hash after parsing this JSON string?

Given this Ruby code, how many keys does the resulting hash have?

Ruby

require 'json'
json_str = '{"a":1,"b":2,"c":{"d":4}}'
obj = JSON.parse(json_str)
puts obj.keys.size

A 1

B 4

C 3

D 2

💡 Hint

Count only the top-level keys in the parsed hash.

The JSON string has three top-level keys: "a", "b", and "c". The nested keys inside "c" do not count at the top level.

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