JSON with jsonlite in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this JSON conversion?

Consider this R code using jsonlite:

library(jsonlite)
my_list <- list(name = "Alice", age = 30, scores = c(90, 85, 88))
json_text <- toJSON(my_list, auto_unbox = TRUE)
cat(json_text)

What is the exact output printed by cat(json_text)?

R Programming

library(jsonlite)
my_list <- list(name = "Alice", age = 30, scores = c(90, 85, 88))
json_text <- toJSON(my_list, auto_unbox = TRUE)
cat(json_text)

A ["Alice",30,[90,85,88]]

B {"name":"Alice","age":30,"scores":[90,85,88]}

C {"name":["Alice"],"age":[30],"scores":[90,85,88]}

D {"name":"Alice","age":"30","scores":[90,85,88]}

💡 Hint

Remember that auto_unbox = TRUE removes array brackets for single values.

The toJSON function with auto_unbox = TRUE converts single values like name and age to JSON scalars, not arrays. The scores vector remains an array.

❓ Predict Output

intermediate

What does this JSON parsing produce?

Given this R code:

library(jsonlite)
json_str <- '{"fruit":"apple","count":5,"colors":["red","green"]}'
parsed <- fromJSON(json_str)
str(parsed)

What is the structure printed by str(parsed)?

R Programming

library(jsonlite)
json_str <- '{"fruit":"apple","count":5,"colors":["red","green"]}'
parsed <- fromJSON(json_str)
str(parsed)

A

List of 3
 $ fruit : chr "apple"
 $ count : int 5
 $ colors: chr [1:2] "red" "green"

B

List of 3
 $ fruit : chr [1:1] "apple"
 $ count : chr "5"
 $ colors: list of 2

C

List of 3
 $ fruit : chr "apple"
 $ count : chr "5"
 $ colors: chr [1:2] "red" "green"

D

List of 3
 $ fruit : chr "apple"
 $ count : int 5
 $ colors: list of 1

💡 Hint

Check how fromJSON converts JSON arrays and numbers.

fromJSON converts JSON strings to character vectors, numbers to integers, and JSON arrays to vectors or lists depending on content. Here colors is a character vector of length 2.

🔧 Debug

advanced

Why does this JSON conversion fail?

This R code throws an error:

library(jsonlite)
my_data <- list(a = 1, b = function(x) x + 1)
toJSON(my_data)

What is the cause of the error?

R Programming

library(jsonlite)
my_data <- list(a = 1, b = function(x) x + 1)
toJSON(my_data)

A The list is missing names for elements, causing a syntax error.

B toJSON requires numeric vectors only, so the integer 1 causes an error.

C jsonlite cannot convert functions to JSON, causing an error.

D The function argument x is undefined, causing a runtime error.

💡 Hint

Think about what data types JSON supports.

JSON format cannot represent functions. jsonlite throws an error when trying to convert a function object.

📝 Syntax

advanced

Which option correctly parses nested JSON?

Given this JSON string:

{"person":{"name":"Bob","age":25},"hobbies":["reading","swimming"]}

Which R code correctly parses it into a list with nested elements?

A fromJSON('{"person":{"name":"Bob","age":25},"hobbies":reading,swimming}')

B fromJSON('{person:{name:"Bob",age:25},hobbies:["reading","swimming"]}')

C fromJSON('{"person":{"name":"Bob","age":25},"hobbies":[reading,swimming]}')

D fromJSON('{"person":{"name":"Bob","age":25},"hobbies":["reading","swimming"]}')

💡 Hint

Check JSON syntax rules for strings and keys.

Option D uses valid JSON syntax with double quotes around keys and string values. Others have missing quotes or invalid JSON.

🚀 Application

expert

How many elements are in the parsed JSON array?

This R code parses a JSON array:

library(jsonlite)
json_arr <- '[{"id":1,"val":10},{"id":2,"val":20},{"id":3,"val":30}]'
parsed <- fromJSON(json_arr)
length(parsed)

What is the value of length(parsed)?

R Programming

library(jsonlite)
json_arr <- '[{"id":1,"val":10},{"id":2,"val":20},{"id":3,"val":30}]'
parsed <- fromJSON(json_arr)
length(parsed)

A 2

B 3

C 6

D 1

💡 Hint

Check the structure of parsed data and what length() returns for data frames.

fromJSON converts the JSON array of objects into a data frame with 3 rows and 2 columns. length() on a data frame returns the number of columns, which is 2.

Practice

1. What is the main purpose of the jsonlite package in R?

easy

A. To manage databases

B. To create graphical plots

C. To perform statistical tests

D. To convert R objects to JSON format and back

Solution

1. Step 1: Understand jsonlite's functionality

   The jsonlite package is designed to handle JSON data in R, allowing conversion between R objects and JSON text.

2. Step 2: Identify the correct purpose

   Among the options, only converting R objects to JSON and back matches jsonlite's main use.

3. Final Answer:

   To convert R objects to JSON format and back -> Option D

4. Quick Check:

   jsonlite = convert R and JSON [OK]

Hint: jsonlite = R to JSON and back conversion [OK]

Common Mistakes:

• Confusing jsonlite with plotting packages
• Thinking jsonlite manages databases
• Assuming jsonlite performs statistical tests

2. Which of the following is the correct syntax to convert an R list my_list to JSON text using jsonlite?

easy

A. jsonlite::toJSON(my_list)

B. jsonlite::fromJSON(my_list)

C. toJSON(my_list, jsonlite)

D. fromJSON(my_list, jsonlite)

Solution

1. Step 1: Identify the function to convert R to JSON

   The function to convert R objects to JSON in jsonlite is toJSON().

2. Step 2: Check correct usage with namespace

   Using jsonlite::toJSON(my_list) correctly calls the function from the package.

3. Final Answer:

   jsonlite::toJSON(my_list) -> Option A

4. Quick Check:

   toJSON converts R to JSON [OK]

Hint: Use toJSON() to convert R data to JSON [OK]

Common Mistakes:

• Using fromJSON() to convert R to JSON
• Passing package name as argument
• Mixing function names and arguments

3. What will be the output of the following R code?

library(jsonlite)
my_list <- list(name = "Anna", age = 25)
json_text <- toJSON(my_list, auto_unbox = TRUE)
cat(json_text)

medium

A. {"name":"Anna","age":25}

B. ["Anna",25]

C. {"name":["Anna"],"age":[25]}

D. Error: object 'toJSON' not found

Solution

1. Step 1: Understand toJSON with auto_unbox

   The toJSON() function converts the list to JSON. The auto_unbox=TRUE option removes array brackets for single values.

2. Step 2: Analyze the output format

   The list has named elements "name" and "age" with single values, so JSON will be an object with those keys and unboxed values.

3. Final Answer:

   {"name":"Anna","age":25} -> Option A

4. Quick Check:

   toJSON with auto_unbox = unboxed JSON object [OK]

Hint: auto_unbox=TRUE removes brackets for single values [OK]

Common Mistakes:

• Ignoring auto_unbox and expecting arrays
• Confusing JSON arrays with objects
• Forgetting to load jsonlite package

4. Identify the error in this R code snippet using jsonlite:

library(jsonlite)
json_text <- '{"x":10, "y":20}'
my_data <- toJSON(json_text)

medium

A. Variable my_data is not assigned correctly

B. Missing library(jsonlite) call

C. toJSON() is used instead of fromJSON() to parse JSON text

D. JSON text is not properly formatted

Solution

1. Step 1: Check function usage for JSON parsing

   The code tries to convert JSON text to R data. The correct function for this is fromJSON(), not toJSON().

2. Step 2: Confirm other parts are correct

   The JSON text is valid, and the library is loaded, so the main error is the wrong function call.

3. Final Answer:

   toJSON() is used instead of fromJSON() to parse JSON text -> Option C

4. Quick Check:

   Use fromJSON() to read JSON text [OK]

Hint: Use fromJSON() to read JSON text, not toJSON() [OK]

Common Mistakes:

• Using toJSON() to read JSON text
• Not loading jsonlite package
• Incorrect JSON string format

5. You have a JSON string representing a list of users:

json_users <- '[{"name":"John","age":30},{"name":"Jane","age":25}]'

How can you convert this JSON into an R data frame with columns name and age using jsonlite?

hard

A. df <- toJSON(json_users)

B. df <- fromJSON(json_users)

C. df <- as.data.frame(toJSON(json_users))

D. df <- as.list(fromJSON(json_users))

Solution

1. Step 1: Identify function to parse JSON to R

   The fromJSON() function converts JSON text into R objects, including data frames when JSON is an array of objects.

2. Step 2: Confirm output type matches requirement

   Since the JSON is a list of user objects, fromJSON() returns a data frame with columns for each key.

3. Final Answer:

   df <- fromJSON(json_users) -> Option B

4. Quick Check:

   fromJSON() parses JSON array to data frame [OK]

Hint: Use fromJSON() to convert JSON array to data frame [OK]

Common Mistakes:

• Using toJSON() instead of fromJSON()
• Converting JSON to list instead of data frame
• Trying to convert JSON string directly with as.data.frame()