read.csv and write.csv in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the read.csv() function do in R?

read.csv() reads data from a CSV file and loads it into R as a data frame, which is like a table you can work with.

beginner

How do you save a data frame to a CSV file in R?

You use the write.csv() function, which writes the data frame to a CSV file on your computer.

intermediate

What argument do you use in read.csv() to prevent R from converting strings to factors?

Use stringsAsFactors = FALSE to keep text as plain strings instead of factors.

intermediate

What does the row.names = FALSE argument do in write.csv()?

It tells R not to write row numbers as a separate column in the CSV file.

intermediate

If you want to read a CSV file with a different separator, should you use read.csv() or another function?

You should use read.table() with the sep argument, because read.csv() assumes commas as separators.

What is the default separator used by read.csv()?

A Semicolon (;)

B Tab (\t)

C Comma (,)

D Space

read.csv() expects commas as the separator by default.

Which argument in write.csv() prevents writing row names to the file?

A row.names = TRUE

B row.names = FALSE

C header = FALSE

D stringsAsFactors = FALSE

Setting row.names = FALSE stops row numbers from being saved as a column.

How do you read a CSV file without converting text columns to factors?

A Use <code>stringsAsFactors = FALSE</code>

B Use <code>stringsAsFactors = TRUE</code>

C Use <code>header = FALSE</code>

D Use <code>sep = ";"</code>

Use stringsAsFactors = FALSE to keep text as strings.

Which function would you use to read a CSV file with semicolon separators?

A read.table(sep = ";")

B write.csv()

C read.csv()

D readLines()

read.table() lets you specify the separator, so use sep = ";" for semicolons.

What type of object does read.csv() return?

A Matrix

B Vector

C List

D Data frame

read.csv() returns a data frame, which is like a table with rows and columns.

Explain how to read a CSV file into R and save it back after making changes.

Think about loading data, modifying it, then writing it back.

Describe the purpose of the stringsAsFactors argument in read.csv().

Consider how R treats text data by default.

Practice

1. What does the read.csv function do in R?

easy

A. Loads data from a CSV file into a data frame

B. Saves a data frame to a CSV file

C. Creates a new CSV file with default data

D. Deletes a CSV file from the disk

Solution

1. Step 1: Understand the purpose of read.csv

   The read.csv function is designed to read data stored in CSV files and load it into R as a data frame.

2. Step 2: Differentiate from write.csv

   write.csv saves data frames to CSV files, so it does the opposite of read.csv.

3. Final Answer:

   Loads data from a CSV file into a data frame -> Option A

4. Quick Check:

   read.csv loads CSV data [OK]

Hint: Remember: read.csv reads files, write.csv writes files [OK]

Common Mistakes:

• Confusing read.csv with write.csv
• Thinking read.csv creates new files
• Assuming read.csv deletes files

2. Which of the following is the correct syntax to save a data frame named df to a CSV file called output.csv?

easy

A. read.csv(df, file = "output.csv")

B. write.csv(df, file = "output.csv")

C. write.csv(file = df, "output.csv")

D. write.csv("output.csv", df)

Solution

1. Step 1: Recall the argument order of write.csv

   The first argument is the data frame to save, the second is the file name with the parameter file=.

2. Step 2: Check each option's argument order

   write.csv(df, file = "output.csv") correctly uses write.csv(df, file = "output.csv"). read.csv(df, file = "output.csv") uses read.csv which is for reading, not writing. Options B and D have wrong argument order.

3. Final Answer:

   write.csv(df, file = "output.csv") -> Option B

4. Quick Check:

   write.csv(data, file=filename) syntax [OK]

Hint: write.csv(data, file=filename) saves data frame [OK]

Common Mistakes:

• Swapping arguments order
• Using read.csv instead of write.csv
• Omitting the file= parameter

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

data <- data.frame(Name = c("Anna", "Bob"), Age = c(25, 30))
write.csv(data, file = "people.csv", row.names = FALSE)
read_data <- read.csv("people.csv")
print(read_data)

medium

A. Error: file not found

B. Name Age
0 Anna 25
1 Bob 30

C. Name Age
1 Anna 25
2 Bob 30
3 NA NA

D. Name Age
1 Anna 25
2 Bob 30

Solution

1. Step 1: Understand the data frame and write.csv call

   The data frame has two rows with names and ages. The write.csv saves it without row names because row.names = FALSE.

2. Step 2: Read the CSV and print

   The read.csv reads the file back, creating a data frame with default row numbers 1 and 2. Printing shows the two rows with correct data.

3. Final Answer:

   Name Age 1 Anna 25 2 Bob 30 -> Option D

4. Quick Check:

   write.csv without row.names keeps data clean [OK]

Hint: Use row.names=FALSE to avoid extra row numbers in CSV [OK]

Common Mistakes:

• Forgetting row.names=FALSE adds extra column
• Expecting zero-based row numbers
• Assuming file not found error

4. Identify the error in this code snippet:

df <- data.frame(x = 1:3, y = 4:6)
write.csv(df, "data.csv")
new_df <- read.csv(data.csv)
print(new_df)

medium

A. print() cannot display data frames

B. write.csv should not have data frame as first argument

C. Missing quotes around file name in read.csv

D. read.csv cannot read CSV files

Solution

1. Step 1: Check the write.csv usage

   write.csv(df, "data.csv") is correct syntax to save the data frame.

2. Step 2: Check the read.csv usage

   The code uses read.csv(data.csv) without quotes around the file name, which causes an error because R treats data.csv as a variable, not a string.

3. Final Answer:

   Missing quotes around file name in read.csv -> Option C

4. Quick Check:

   File names must be strings in quotes [OK]

Hint: Always put file names in quotes in read.csv and write.csv [OK]

Common Mistakes:

• Omitting quotes around file names
• Confusing argument order
• Thinking print() can't show data frames

5. You have a CSV file with missing values represented as empty cells. Which read.csv argument helps you automatically convert these empty cells to NA in R?

hard

A. na.strings = ""

B. stringsAsFactors = TRUE

C. header = FALSE

D. row.names = NULL

Solution

1. Step 1: Understand missing values in CSV

   Empty cells in CSV files often represent missing data, which R can mark as NA if told how to interpret them.

2. Step 2: Use na.strings argument

   The na.strings = "" argument tells read.csv to treat empty strings as NA values automatically.

3. Final Answer:

   na.strings = "" -> Option A

4. Quick Check:

   na.strings="" converts empty cells to NA [OK]

Hint: Use na.strings="" to read blanks as NA [OK]

Common Mistakes:

• Ignoring na.strings argument
• Using stringsAsFactors which affects factor conversion
• Confusing header or row.names with missing data handling