Accessing columns ($, []) in R Programming - Mini Project: Build & Apply

Accessing columns ($, [])

📖 Scenario: You have a small table of fruits and their prices. You want to get the prices using two common ways in R: the $ operator and the [] operator.

🎯 Goal: Learn how to access columns in a data frame using $ and [] in R.

📋 What You'll Learn

Create a data frame called fruits with columns name and price

Create a variable called col_name with the value "price"

Use the $ operator to get the price column and save it in prices_dollar

Use the [] operator with col_name to get the price column and save it in prices_brackets

Print both prices_dollar and prices_brackets

💡 Why This Matters

🌍 Real World

Accessing columns in data frames is a common task when working with data in R, such as analyzing sales, survey results, or scientific data.

💼 Career

Data analysts and scientists frequently use these techniques to manipulate and explore data sets efficiently.

1

Create the data frame

Create a data frame called fruits with two columns: name containing "apple", "banana", "cherry" and price containing 1.2, 0.5, 2.5.

R Programming

# Your code here

Hint

Use data.frame() with name = c(...) and price = c(...).

2

Create the column name variable

Create a variable called col_name and set it to the string "price".

R Programming

fruits <- data.frame(name = c("apple", "banana", "cherry"), price = c(1.2, 0.5, 2.5))
# Your code here

Hint

Use <- "price" to create the variable.

3

Access the price column using $ and []

Use the $ operator on fruits to get the price column and save it in prices_dollar. Then use the [] operator with col_name on fruits to get the same column and save it in prices_brackets.

R Programming

fruits <- data.frame(name = c("apple", "banana", "cherry"), price = c(1.2, 0.5, 2.5))
col_name <- "price"
# Your code here

Hint

Use fruits$price and fruits[[col_name]].

4

Print the results

Print the variables prices_dollar and prices_brackets to show the prices.

R Programming

fruits <- data.frame(name = c("apple", "banana", "cherry"), price = c(1.2, 0.5, 2.5))
col_name <- "price"
prices_dollar <- fruits$price
prices_brackets <- fruits[[col_name]]
# Your code here

Hint

Use print(prices_dollar) and print(prices_brackets).

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)