Accessing columns ($, []) in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the $ operator do in R when working with data frames?

The $ operator extracts a column from a data frame by name, returning it as a vector.

beginner

How do you access multiple columns from a data frame using [] in R?

Use df[, c('col1', 'col2')] to select multiple columns by name, returning a data frame.

intermediate

What is the difference between df$col and df['col'] in R?

df$col returns the column as a vector, while df['col'] returns a data frame with one column.

beginner

Can you use numeric indices with the [] operator to access columns in R?

Yes, df[, 2] accesses the second column of the data frame.

intermediate

Why might you prefer df[['col']] over df$col in R?

df[['col']] allows programmatic access using a variable for the column name, while df$col requires a literal name.

Which operator extracts a single column as a vector from a data frame in R?

A [[]]

B []

C :

D $

The $ operator extracts a column as a vector by name.

What does df[, c('a', 'b')] return in R?

A A vector with columns 'a' and 'b'

B A data frame with columns 'a' and 'b'

C An error

D A list of columns 'a' and 'b'

Using df[, c('a', 'b')] returns a data frame with those columns.

How do you access the third column of a data frame df by position?

A df[, 3]

B df$3

C df[3]

D df[['3']]

df[, 3] accesses the third column by numeric index.

Which method allows using a variable to select a column in R?

A df$var

B df[var]

C df[[var]]

D df[, var]

df[[var]] lets you use a variable holding the column name.

What is the result type of df['col'] in R?

A Data frame

B Vector

C List

D Matrix

df['col'] returns a data frame with one column.

Explain how to access a single column and multiple columns from a data frame in R using $ and [].

Think about how you pick one item vs several items from a list.

Describe the difference between df$col, df['col'], and df[[col]] in R.

Consider how each method handles the column name and output type.

Practice

1. In R, which operator is used to extract a column from a dataframe as a vector by its name?

easy

A. The $ operator

B. The [] operator with column name

C. The [[]] operator with column index

D. The : operator

Solution

1. Step 1: Understand $ operator usage

   The $ operator extracts a column from a dataframe as a vector by its name.

2. Step 2: Compare with other operators

   The [] operator returns a dataframe, [[]] returns a vector but requires double brackets, and : is unrelated.

3. Final Answer:

   The $ operator -> Option A

4. Quick Check:

   $ extracts vector column = C [OK]

Hint: Use $ for quick vector column access by name [OK]

Common Mistakes:

• Using [] returns a dataframe, not a vector
• Confusing [[]] with []
• Using : operator for columns

2. Which of the following is the correct syntax to get a one-column dataframe named 'age' from dataframe df?

easy

A. df['age']

B. df$age

C. df[['age']]

D. df.age

Solution

1. Step 1: Identify syntax for one-column dataframe

   Using single brackets with a column name, like df['age'], returns a one-column dataframe.

2. Step 2: Differentiate from other options

   df$age and df[['age']] return vectors, df.age is invalid syntax in R.

3. Final Answer:

   df['age'] -> Option A

4. Quick Check:

   Single brackets with name = one-column dataframe [OK]

Hint: Use single brackets with quotes for one-column dataframe [OK]

Common Mistakes:

• Using $ returns a vector, not dataframe
• Using [[]] returns a vector, not dataframe
• Using dot notation (df.age) which is invalid

3. Given the dataframe df <- data.frame(name=c('Ann','Bob'), age=c(25,30)), what is the output of class(df['age'])?

medium

A. "integer"

B. "numeric"

C. "data.frame"

D. "vector"

Solution

1. Step 1: Understand df['age'] output type

   Using single brackets with a column name returns a dataframe, so class(df['age']) is "data.frame".

2. Step 2: Differentiate from vector classes

   Using df$age or df[['age']] returns a vector of class "numeric" or "integer" but not a dataframe.

3. Final Answer:

   "data.frame" -> Option C

4. Quick Check:

   Single bracket returns dataframe class = A [OK]

Hint: Single brackets keep dataframe class, double brackets extract vector [OK]

Common Mistakes:

• Assuming df['age'] returns a vector
• Confusing class of vector and dataframe
• Using df$age which returns numeric vector

4. Consider the code: df <- data.frame(x=1:3, y=4:6); col <- df['x'] + 1. What is the error in this code?

medium

A. The column name 'x' is incorrect

B. You cannot add 1 to a dataframe

C. df['x'] returns a vector, so addition is valid

D. The dataframe df is not defined

Solution

1. Step 1: Analyze df['x'] type

   df['x'] returns a one-column dataframe, not a vector.

2. Step 2: Understand addition operation

   Adding 1 to a dataframe causes an error because arithmetic operations expect vectors or numbers.

3. Final Answer:

   You cannot add 1 to a dataframe -> Option B

4. Quick Check:

   Adding number to dataframe causes error = A [OK]

Hint: Use [[]] or $ to get vector before arithmetic [OK]

Common Mistakes:

• Assuming df['x'] is a vector
• Using wrong column name
• Ignoring that df is defined

5. You want to add 5 to every value in the 'score' column of dataframe df and keep the result as a vector. Which code correctly does this?

hard

A. df.score + 5

B. df['score'] + 5

C. df[score] + 5

D. df[['score']] + 5

Solution

1. Step 1: Identify how to get vector column

   df[['score']] extracts the 'score' column as a vector suitable for arithmetic.

2. Step 2: Check other options

   df.score is invalid syntax in R (Python-style dot notation), df['score'] returns a dataframe causing error on addition, df[score] is invalid without quotes around column name.

3. Final Answer:

   df[['score']] + 5 -> Option D

4. Quick Check:

   Double brackets extract vector for math = D [OK]

Hint: Use double brackets to get vector for calculations [OK]

Common Mistakes:

• Using single brackets returns dataframe, causing errors
• Forgetting quotes around column name in []
• Assuming df[score] works without quotes
• Using Python-style dot notation (df.score)