readRDS and saveRDS in R Programming - Step-by-Step Execution

Concept Flow - readRDS and saveRDS

Create or have an R object

↓

Use saveRDS(object, file)

↓

Object saved as .rds file

↓

Use readRDS(file)

↓

Object loaded back into R

↓

Use or inspect the loaded object

You save an R object to a file with saveRDS, then later read it back with readRDS to restore the object.

Execution Sample

R Programming

x <- c(10, 20, 30)
saveRDS(x, "data.rds")
y <- readRDS("data.rds")
print(y)

This code saves a vector x to a file and then reads it back into y, printing y.

Execution Table

Step	Action	Object State	File State	Output
1	Create vector x	x = c(10, 20, 30)	No file yet
2	saveRDS(x, "data.rds")	x unchanged	File 'data.rds' created with x data
3	readRDS("data.rds") assigned to y	x unchanged, y = c(10, 20, 30)	File 'data.rds' unchanged
4	print(y)	x unchanged, y unchanged	File unchanged	[1] 10 20 30

💡 Program ends after printing y, which matches original x vector.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4
x	NULL	c(10, 20, 30)	c(10, 20, 30)	c(10, 20, 30)	c(10, 20, 30)
y	NULL	NULL	NULL	c(10, 20, 30)	c(10, 20, 30)

Key Moments - 3 Insights

Why does saveRDS not create a variable in the environment?

saveRDS only writes the object to a file; it does not assign or change variables in R. See step 2 in execution_table where x stays the same.

Does readRDS automatically overwrite existing variables?

No, readRDS returns the object and you must assign it to a variable. In step 3, y is assigned explicitly.

What happens if the file does not exist when using readRDS?

readRDS will give an error because it cannot find the file to load the object.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of y after step 2?

A File path string

B c(10, 20, 30)

C NULL

D Error

💡 Hint

Check the 'Object State' column at step 2 where y is not yet assigned.

At which step is the file 'data.rds' created?

A Step 2

B Step 1

C Step 3

D Step 4

💡 Hint

Look at the 'File State' column to see when the file appears.

If you forget to assign readRDS to a variable, what happens?

A The object is saved to a file

B The object is printed but not saved in a variable

C An error occurs

D The object overwrites x automatically

💡 Hint

Recall that readRDS returns the object but does not assign it unless you do so explicitly.

Concept Snapshot

saveRDS(object, file) saves any R object to a file.
readRDS(file) reads the object back from the file.
You must assign readRDS output to a variable.
saveRDS does not change variables in R.
Files use .rds extension by convention.

Full Transcript

This visual trace shows how saveRDS and readRDS work in R. First, we create a vector x with three numbers. Then, saveRDS writes x to a file named 'data.rds' without changing any variables. Next, readRDS reads the file and returns the saved object, which we assign to y. Finally, printing y shows the original vector. Key points: saveRDS saves to file only, readRDS returns the object and must be assigned. If the file is missing, readRDS errors. This helps save and load R objects easily.

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