Row and column indexing in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you select the first row of a data frame named df in R?

Use df[1, ]. The number before the comma selects the row, and leaving the column part empty selects all columns.

beginner

What does df[, 3] return in R?

It returns the third column of the data frame df, including all rows.

intermediate

How to select rows 2 to 4 and columns 1 and 3 from a data frame df?

Use df[2:4, c(1, 3)]. This picks rows 2, 3, 4 and columns 1 and 3.

beginner

What happens if you use df[ , ] in R?

It returns the entire data frame df because no rows or columns are excluded.

beginner

How to select a column by name, for example column age, from a data frame df?

Use df["age"] or df$age. Both return the age column.

What does df[3, ] select in R?

A The third row with all columns

B The third column with all rows

C The third row and third column

D All rows and third column

df[3, ] selects the third row and all columns.

How do you select the second and fourth columns of df?

A df[2:4, ]

B df[c(2, 4), ]

C df[, c(2, 4)]

D df[c(2, 4)]

df[, c(2, 4)] selects columns 2 and 4 for all rows.

What does df[ , ] return?

A An error

B The entire data frame

C Only the first row

D Only the first column

Empty row and column indices return the whole data frame.

How to select the column named height from df?

A df$height

B df[height]

C df[, height]

D df[height, ]

df$height selects the column named height.

Which of these selects rows 1 to 3 and columns 2 and 4?

A df[2:4, c(1,3)]

B df[c(1,3), 2:4]

C df[c(2,4), 1:3]

D df[1:3, c(2,4)]

df[1:3, c(2,4)] selects rows 1 to 3 and columns 2 and 4.

Explain how to select specific rows and columns from a data frame in R.

Think about how you slice a table by rows and columns.

Describe the difference between df[ , 2] and df[2, ] in R.

Remember the order: rows first, columns second.

Practice

1. In R, what does data[2, 3] select from a data frame named data?

easy

A. All rows and columns

B. The entire 2nd row

C. The entire 3rd column

D. The value in the 2nd row and 3rd column

Solution

1. Step 1: Understand the indexing format

   In R, data[row, column] selects the element at the specified row and column.

2. Step 2: Apply the given indices

   Here, row = 2 and column = 3, so it selects the value at 2nd row and 3rd column.

3. Final Answer:

   The value in the 2nd row and 3rd column -> Option D

4. Quick Check:

   data[2, 3] = value at row 2, column 3 [OK]

Hint: Use data[row, column] to pick exact cell [OK]

Common Mistakes:

• Confusing row and column order
• Thinking it selects entire row or column
• Using parentheses instead of brackets

2. Which of the following is the correct syntax to select all rows and the 4th column from a data frame df in R?

easy

A. df[, 4]

B. df[4, ]

C. df[4]

D. df[4, 4]

Solution

1. Step 1: Recall indexing rules for rows and columns

   Leaving the row index blank selects all rows; specifying a column index selects that column.

2. Step 2: Match syntax to select all rows and 4th column

   df[, 4] means all rows, 4th column, which is correct.

3. Final Answer:

   df[, 4] -> Option A

4. Quick Check:

   Blank row index means all rows [OK]

Hint: Blank row means all rows; specify column number [OK]

Common Mistakes:

• Using df[4, ] selects 4th row, not column
• Using df[4] selects 4th column but drops dimensions
• Using df[4, 4] selects single cell, not all rows

3. Given the data frame df <- data.frame(A=1:3, B=4:6, C=7:9), what is the output of df[1:2, 2]?

medium

A. [1] 7 8

B. [1] 4 5

C. [1] 1 2

D. [1] 4 5 6

Solution

1. Step 1: Understand the data frame structure

   df has columns A=1,2,3; B=4,5,6; C=7,8,9.

2. Step 2: Select rows 1 and 2, column 2

   Rows 1 and 2 of column B (2nd column) are 4 and 5.

3. Final Answer:

   [1] 4 5 -> Option B

4. Quick Check:

   df[1:2, 2] = c(4, 5) [OK]

Hint: Rows 1:2, column 2 picks values 4 and 5 [OK]

Common Mistakes:

• Confusing column numbers with letters
• Selecting wrong rows or columns
• Expecting output as a data frame instead of vector

4. What is wrong with this R code snippet to select the 3rd row and all columns from data frame df?

df[3]

medium

A. It selects the 3rd column, not the 3rd row

B. It causes a syntax error

C. It selects the 3rd row but drops columns

D. It returns all rows and columns

Solution

1. Step 1: Understand single bracket indexing with one index

   Using one index like df[3] selects the 3rd column, not row.

2. Step 2: Identify correct way to select 3rd row

   To select 3rd row and all columns, use df[3, ].

3. Final Answer:

   It selects the 3rd column, not the 3rd row -> Option A

4. Quick Check:

   df[3] = 3rd column, not row [OK]

Hint: Use df[row, ] to select rows, not df[row] [OK]

Common Mistakes:

• Using single index to select rows
• Expecting df[3] to select 3rd row
• Confusing data frame and vector indexing

5. You have a data frame df with 5 rows and 4 columns named A, B, C, D. How do you select rows 2 to 4 and columns B and D using row and column indexing?

hard

A. df[2:4, c(2, 4)]

B. df[c(2,3,4), 2:4]

C. df[2:4, c("B", "D")]

D. df[2:4, c("2", "4")]

Solution

1. Step 1: Identify rows and columns to select

   Rows 2 to 4 means indices 2, 3, 4; columns B and D are selected by names "B" and "D".

2. Step 2: Use correct syntax for row and column indexing

   Use df[2:4, c("B", "D") ] to select rows 2-4 and columns named B and D.

3. Final Answer:

   df[2:4, c("B", "D") ] -> Option C

4. Quick Check:

   Row indices and column names combined correctly [OK]

Hint: Use row range and column names vector inside brackets [OK]

Common Mistakes:

• Using numeric column indices instead of names
• Passing column names as numbers in quotes
• Selecting wrong rows or columns