Accessing columns ($, []) in R Programming - Step-by-Step Execution

Concept Flow - Accessing columns ($, [])

Start with data frame

↓

Choose column name

↓

$ operator

→Extract column as vector

↓

[

↓

[

↓

Use extracted column for analysis

You start with a data frame, pick a column name, then use $ or [] operators to get that column either as a vector or a data frame.

Execution Sample

R Programming

df <- data.frame(name=c("Ann", "Bob"), age=c(25, 30))
age_vector <- df$age
age_df <- df["age"]

This code creates a data frame and extracts the 'age' column as a vector and as a one-column data frame.

Execution Table

Step	Action	Code	Result Type	Value
1	Create data frame	df <- data.frame(name=c("Ann", "Bob"), age=c(25, 30))	data.frame	name: Ann, Bob; age: 25, 30
2	Extract 'age' column with $	age_vector <- df$age	numeric vector	25, 30
3	Extract 'age' column with ["age"]	age_df <- df["age"]	data.frame	age: 25, 30
4	Check type of age_vector	class(age_vector)	character	"numeric"
5	Check type of age_df	class(age_df)	character	"data.frame"

💡 All columns extracted; $ returns vector, ["col"] returns data frame

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
df	NULL	data.frame with 2 rows and 2 columns	same	same	same
age_vector	NULL	NULL	numeric vector c(25, 30)	same	same
age_df	NULL	NULL	NULL	data.frame with 1 column 'age'	same

Key Moments - 2 Insights

Why does df$age return a vector but df["age"] returns a data frame?

Because $ extracts the column as a vector directly (see Step 2), while ["age"] keeps the data frame structure with one column (see Step 3).

Can I use df["age"] to get a vector like df$age?

No, df["age"] always returns a data frame. To get a vector, use df[["age"]] or df$age.

Visual Quiz - 3 Questions

Look at the execution table, what is the type of age_vector after Step 2?

A list

B data frame

C numeric vector

D character vector

💡 Hint

Check the 'Result Type' column in Step 2 of the execution table.

At which step does age_df get assigned a data frame?

A Step 2

B Step 3

C Step 1

D Step 4

💡 Hint

Look at the 'Action' and 'Result Type' columns in the execution table.

If you want to extract the 'age' column as a vector using brackets, which code would you use?

A df[["age"]]

B df["age"]

C df$age

D df[age]

💡 Hint

Recall that df["age"] returns a data frame, but df[["age"]] returns a vector.

Concept Snapshot

Access columns in R data frames:
- Use $ to get a column as a vector: df$col
- Use [["col"]] to get a column as a vector: df[["col"]]
- Use ["col"] to get a one-column data frame: df["col"]
- $ and [[ ]] extract vectors; [ ] extracts data frames
- Choose based on whether you want a vector or data frame

Full Transcript

This lesson shows how to access columns in R data frames using $ and [] operators. Starting with a data frame, you can extract a column as a vector using df$col or df[["col"]]. Using df["col"] extracts the column but keeps it as a data frame with one column. The execution table traces creating a data frame, extracting the 'age' column as a vector and as a data frame, and checking their types. Key moments clarify why $ returns a vector and ["col"] returns a data frame. The quiz tests understanding of these differences. Remember, use $ or [[ ]] for vectors, and [ ] for data frames.