readRDS and saveRDS in R Programming - Mini Project: Build & Apply

Using readRDS and saveRDS in R

📖 Scenario: You are working with R data objects and want to save them to a file and then read them back later. This is useful when you want to keep your data safe and load it quickly without recreating it every time.

🎯 Goal: Learn how to save an R object to a file using saveRDS() and then load it back using readRDS().

📋 What You'll Learn

Create a simple R object (a vector) with exact values

Save the object to a file using saveRDS()

Read the object back from the file using readRDS()

Print the loaded object to see the result

💡 Why This Matters

🌍 Real World

Saving and loading R objects is common when working with data analysis projects to avoid repeating expensive computations.

💼 Career

Data scientists and analysts often save intermediate results using <code>saveRDS()</code> and load them later to continue their work efficiently.

1

Create a vector called numbers with values 10, 20, 30, 40, 50

Create a vector called numbers with the exact values 10, 20, 30, 40, 50 using the c() function.

R Programming

# Create the vector numbers with values 10, 20, 30, 40, 50
# Your code here

Hint

Use c(10, 20, 30, 40, 50) to create the vector and assign it to numbers.

2

Save the numbers vector to a file called numbers.rds using saveRDS()

Use saveRDS() to save the numbers vector to a file named "numbers.rds".

R Programming

numbers <- c(10, 20, 30, 40, 50)
# Save the numbers vector to 'numbers.rds'
# Your code here

Hint

Call saveRDS(numbers, "numbers.rds") to save the vector.

3

Read the vector back from the file numbers.rds into a variable called loaded_numbers using readRDS()

Use readRDS() to read the data from the file "numbers.rds" and assign it to a variable called loaded_numbers.

R Programming

numbers <- c(10, 20, 30, 40, 50)
saveRDS(numbers, "numbers.rds")
# Read the data back into loaded_numbers
# Your code here

Hint

Use loaded_numbers <- readRDS("numbers.rds") to load the saved vector.

4

Print the loaded_numbers vector to see the output

Write a print() statement to display the contents of loaded_numbers.

R Programming

numbers <- c(10, 20, 30, 40, 50)
saveRDS(numbers, "numbers.rds")
loaded_numbers <- readRDS("numbers.rds")
# Print loaded_numbers
# Your code here

Hint

Use print(loaded_numbers) to show the vector.

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