readRDS and saveRDS in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this R code using saveRDS and readRDS?

Consider the following R code that saves and reads an object:

my_list <- list(a = 1, b = 2)
saveRDS(my_list, file = "temp.rds")
new_list <- readRDS("temp.rds")
print(new_list$b)

What will be printed?

R Programming

my_list <- list(a = 1, b = 2)
saveRDS(my_list, file = "temp.rds")
new_list <- readRDS("temp.rds")
print(new_list$b)

A 2

B NULL

C Error: object 'new_list' not found

D 1

💡 Hint

Remember that saveRDS saves the object exactly and readRDS reads it back as is.

The object my_list is saved to a file and then read back into new_list. Accessing new_list$b returns 2, the value stored under key 'b'.

🧠 Conceptual

intermediate

What does saveRDS() do in R?

Which of the following best describes the purpose of saveRDS() in R?

A Converts an R object to a CSV file

B Saves an R object to a file in a binary format for later use

C Reads an R object from a file and loads it into the global environment

D Saves all variables in the current environment to a text file

💡 Hint

Think about what saveRDS() stores and how it differs from save().

saveRDS() saves a single R object to a file in a binary format. It does not save the entire environment or convert to text formats.

🔧 Debug

advanced

Why does this code cause an error when reading an RDS file?

Look at this R code:

saveRDS(42, file = "number.rds")
num <- readRDS("number.txt")
print(num)

What error will occur and why?

A Error: object 'num' not found

B No error, prints 42

C Error: cannot open the connection to 'number.txt' because the file does not exist

D Error: invalid R object in 'number.txt'

💡 Hint

Check the file names used in saving and reading.

The file was saved as 'number.rds' but the code tries to read 'number.txt', which does not exist, causing a file connection error.

📝 Syntax

advanced

Which option correctly saves and reads an R object?

Which of the following code snippets correctly saves an object df and reads it back into df2?

A

saveRDS(df, "data.rds")
df2 <- readRDS("data.rds")

B

saveRDS(df)
df2 <- readRDS()

C

saveRDS(df, "data.txt")
df2 <- readRDS("data.txt")

D

saveRDS(df, file = "data.rds")
df2 <- readRDS(file = "data.rds")

💡 Hint

Check the correct argument names and file extensions.

Option D uses the correct argument file= and proper file extension .rds. Option D misses the argument name, which is allowed but less clear. Option D uses wrong extension, and B misses file arguments.

🚀 Application

expert

How many objects are saved and restored using saveRDS and readRDS?

Given the following R code:

a <- 10
b <- 20
saveRDS(a, file = "a.rds")
saved_obj <- readRDS("a.rds")

How many objects are saved and restored by saveRDS and readRDS?

A One object: only 'a' is saved and restored

B Two objects: both 'a' and 'b' are saved and restored

C No objects are saved because 'b' is not saved explicitly

D All objects in the environment are saved and restored

💡 Hint

Remember what saveRDS saves compared to save.

saveRDS saves only the single object passed to it, here 'a'. The object 'b' is not saved or restored.

Practice

1. What does the saveRDS() function do in R?

easy

A. It creates a new R script file.

B. It reads data from a CSV file.

C. It saves a single R object to a file.

D. It deletes an R object from memory.

Solution

1. Step 1: Understand the purpose of saveRDS()

   The saveRDS() function is used to save one R object to a file in a binary format.

2. Step 2: Compare with other options

   The other options describe unrelated actions: creating a new R script file, reading data from a CSV file, or deleting an R object from memory.

3. Final Answer:

   It saves a single R object to a file. -> Option C

4. Quick Check:

   saveRDS() = saves object [OK]

Hint: saveRDS saves one object to a file in R [OK]

Common Mistakes:

• Confusing saveRDS() with read.csv()
• Thinking saveRDS() saves multiple objects at once
• Assuming saveRDS() creates scripts

2. Which of the following is the correct syntax to save an object my_data to a file named data.rds?

easy

A. saveRDS(my_data, "data.rds")

B. saveRDS("data.rds", my_data)

C. readRDS(my_data, "data.rds")

D. save(my_data, file = "data.rds")

Solution

1. Step 1: Recall saveRDS() argument order

   The first argument is the object to save, the second is the file name as a string.

2. Step 2: Check each option

   saveRDS(my_data, "data.rds") matches the correct order. saveRDS("data.rds", my_data) reverses arguments. readRDS(my_data, "data.rds") uses readRDS which is for reading, not saving. save(my_data, file = "data.rds") uses save(), which is different.

3. Final Answer:

   saveRDS(my_data, "data.rds") -> Option A

4. Quick Check:

   saveRDS(object, file) = correct syntax [OK]

Hint: Remember: saveRDS(object, "filename") [OK]

Common Mistakes:

• Swapping object and filename arguments
• Using readRDS() instead of saveRDS() to save
• Confusing saveRDS() with save()

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

my_list <- list(a = 1, b = 2)
saveRDS(my_list, "mylist.rds")
loaded_list <- readRDS("mylist.rds")
print(loaded_list$b)

medium

A. Error: object not found

B. 1

C. NULL

D. 2

Solution

1. Step 1: Understand saving and loading

   The list my_list has elements a=1 and b=2. It is saved to a file and then loaded back into loaded_list.

2. Step 2: Access element b of loaded_list

   Accessing loaded_list$b retrieves the value 2, as saved.

3. Final Answer:

   2 -> Option D

4. Quick Check:

   readRDS() loads saved object correctly [OK]

Hint: readRDS loads saved object exactly as saved [OK]

Common Mistakes:

• Expecting print to show 1 instead of 2
• Thinking readRDS returns NULL
• Confusing saveRDS and readRDS usage

4. Identify the error in this R code snippet:

saveRDS(data, "datafile.rds")
new_data <- readRDS(datafile.rds)
print(new_data)

medium

A. Missing quotes around filename in readRDS()

B. Using saveRDS() instead of save()

C. Incorrect object name in saveRDS()

D. No error, code runs fine

Solution

1. Step 1: Check readRDS() argument

   The filename must be a string, so it needs quotes around it.

2. Step 2: Identify the error

   In readRDS(datafile.rds), the filename is unquoted, causing an error.

3. Final Answer:

   Missing quotes around filename in readRDS() -> Option A

4. Quick Check:

   Filename must be a string in readRDS() [OK]

Hint: Always put filename in quotes in readRDS() [OK]

Common Mistakes:

• Forgetting quotes around filename
• Confusing saveRDS() and save()
• Assuming no error when filename is unquoted

5. You saved a data frame df using saveRDS(df, "df.rds"). Later, you want to load it and add a new column status with value "active" for all rows. Which code correctly does this?

hard

A. df <- saveRDS("df.rds")
df$status <- "active"

B. df <- readRDS("df.rds")
df$status <- "active"
saveRDS(df, "df.rds")

C. readRDS(df, "df.rds")
df$status <- "active"

D. df <- readRDS("df.rds")
addColumn(df, status = "active")

Solution

1. Step 1: Load the saved data frame

   Use df <- readRDS("df.rds") to load the saved object into df.

2. Step 2: Add new column and save again

   Add the new column by df$status <- "active" and save the updated object back with saveRDS(df, "df.rds").

3. Final Answer:

   df <- readRDS("df.rds") df$status <- "active" saveRDS(df, "df.rds") -> Option B

4. Quick Check:

   Load, modify, then save again [OK]

Hint: Load with readRDS, modify, then saveRDS to update file [OK]

Common Mistakes:

• Using saveRDS() to load data
• Forgetting to save after modification
• Using incorrect function to add column