read.csv and write.csv in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this R code reading a CSV?

Consider the CSV file data.csv with content:

name,age
Alice,30
Bob,25

What will be the output of this R code?

R Programming

df <- read.csv('data.csv')
print(df$age[2])

A 25

B 30

C NULL

D Error: object 'data.csv' not found

💡 Hint

Remember that read.csv reads the file into a data frame and $ accesses a column.

The second row's age is 25, so df$age[2] prints 25.

❓ Predict Output

intermediate

What does this code write to a CSV file?

Given this R code:

df <- data.frame(name=c('Tom','Jerry'), score=c(88, 92))
write.csv(df, 'scores.csv', row.names=FALSE)

What will the content of scores.csv be?

A

"name","score"
"Tom",88
"Jerry",92

B

"name","score"
Tom,88
Jerry,92

C

name,score
Tom,88
Jerry,92

D Error: row.names argument invalid

💡 Hint

By default, write.csv adds quotes around strings and includes header.

With row.names=FALSE, row numbers are not written. Strings are quoted by default.

🔧 Debug

advanced

Why does this read.csv call fail?

What error does this code produce and why?

df <- read.csv('data.csv', header=FALSE)
print(df$age)

Assume data.csv has a header row with columns name and age.

R Programming

df <- read.csv('data.csv', header=FALSE)
print(df$age)

A Prints the age column correctly

B Error: object 'age' not found

C NULL because no column named 'age' exists without header

D SyntaxError due to missing comma

💡 Hint

When header=FALSE, column names default to V1, V2, ....

Without header, columns are named V1, V2, so df$age is NULL.

📝 Syntax

advanced

Which option causes a syntax error when writing CSV?

Which of these write.csv calls is syntactically invalid?

A write.csv(df, 'output.csv', row.names=FALSE)

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

C write.csv(df, file='output.csv', row.names=TRUE)

D write.csv(df, file='output.csv' row.names=FALSE)

💡 Hint

Check commas between arguments.

Option D misses a comma between arguments, causing syntax error.

🚀 Application

expert

How many rows will this read.csv produce?

Given a CSV file data.csv with 5 rows of data plus 1 header row, what is the number of rows in df after this code?

df <- read.csv('data.csv', skip=2)

R Programming

df <- read.csv('data.csv', skip=2)

A 4

B 3

C 5

D 6

💡 Hint

The skip argument skips lines before reading header.

skip=2 skips the header and first data row. Then, since header=TRUE (default), the next line is treated as header, leaving 3 data rows.