readRDS and saveRDS in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the saveRDS() function do in R?

saveRDS() saves a single R object to a file in a binary format. This file can later be loaded back into R with readRDS().

beginner

How do you load an R object saved with saveRDS()?

You use the readRDS() function and provide the file path. It returns the saved object, which you can assign to a variable.

intermediate

What is the difference between saveRDS()/readRDS() and save()/load() in R?

saveRDS() and readRDS() work with a single object and do not restore the object name automatically. save() and load() can save/load multiple objects and restore their names in the environment.

beginner

Write a simple example to save a data frame df to a file called data.rds.

saveRDS(df, file = "data.rds")

beginner

How do you assign the object loaded from data.rds to a variable my_data?

my_data <- readRDS("data.rds")

Which function do you use to save a single R object to a file?

A readRDS()

B saveRDS()

C load()

D write.csv()

saveRDS() saves a single R object to a file in binary format.

What does readRDS() return when you load a file?

A The saved R object

B Nothing, it just loads the object into the environment

C A list of all objects in the file

D A character string of the file name

readRDS() returns the saved object, which you can assign to a variable.

Which statement is true about saveRDS() and save()?

A <code>saveRDS()</code> saves one object; <code>save()</code> can save multiple objects

B <code>saveRDS()</code> saves multiple objects; <code>save()</code> saves one object

C Both save multiple objects

D Neither saves objects to files

saveRDS() saves a single object; save() can save many objects at once.

If you use readRDS() to load an object, what must you do to use it?

A Use <code>load()</code> afterwards

B Nothing, it loads automatically

C Call <code>attach()</code> on it

D Assign it to a variable

readRDS() returns the object; you must assign it to a variable to use it.

Which file extension is commonly used for files saved with saveRDS()?

A .txt

B .csv

C .rds

D .rdata

Files saved with saveRDS() usually have the .rds extension.

Explain how you would save an R object and then load it back later using saveRDS() and readRDS().

Think about saving first, then loading and assigning.

Describe the main difference between saveRDS()/readRDS() and save()/load() in R.

Consider how many objects each can save and how they restore names.

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