Row and column indexing in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of selecting rows and columns by index

What is the output of the following R code?

R Programming

df <- data.frame(A = 1:4, B = 5:8, C = 9:12)
result <- df[2:3, c(1,3)]
print(result)

A

  A  C
2 2 10
3 3 11

B

  A  C
1 1 9
4 4 12

C

  B  C
2 6 10
3 7 11

D

  A  B
2 2 6
3 3 7

💡 Hint

Remember that df[row_indices, column_indices] selects rows and columns by their positions.

The code selects rows 2 and 3, and columns 1 and 3 from the data frame. Column 1 is 'A' and column 3 is 'C'. Rows 2 and 3 have values 2,3 in 'A' and 10,11 in 'C'.

❓ Predict Output

intermediate

Output of logical indexing on rows and columns

What will be the output of this R code?

R Programming

df <- data.frame(X = c(10,20,30), Y = c(5,15,25), Z = c(1,2,3))
result <- df[df$X > 15, c(TRUE, FALSE, TRUE)]
print(result)

A

   X  Y  Z
2 20 15  2
3 30 25  3

B

   X  Z
2 20  2
3 30  3

C

   Y
2 15
3 25

D

   X  Z
1 10  1
2 20  2
3 30  3

💡 Hint

Logical vectors can be used to select columns; TRUE means select that column.

Rows where X > 15 are rows 2 and 3. Columns selected are the first and third columns (X and Z) because the logical vector is TRUE, FALSE, TRUE.

🔧 Debug

advanced

Identify the error in row and column indexing

What error does this R code produce?

R Programming

df <- data.frame(A = 1:3, B = 4:6)
result <- df[1:4, 1]
print(result)

A Error: subscript out of bounds

B No error, prints 1 2 3 4

C Error: object 'result' not found

D No error, prints 1 2 3 NA

💡 Hint

Check how R handles indexing beyond the number of rows in a data frame.

R allows indexing beyond existing rows and fills missing rows with NA values. So row 4 does not exist, but R returns NA for that row without error.

❓ Predict Output

advanced

Output of negative indexing on rows and columns

What is the output of this R code?

R Programming

df <- data.frame(M = 1:5, N = 6:10, O = 11:15)
result <- df[-(2:4), -2]
print(result)

A

  M  O
1 1 11
5 5 15

B

  N  O
1 6 11
5 10 15

C

  M  N
1 1 6
5 5 10

D

  M  O
2 2 12
3 3 13
4 4 14

💡 Hint

Negative indices exclude the specified rows or columns.

Rows 2 to 4 are excluded, so only rows 1 and 5 remain. Column 2 (N) is excluded, so columns M and O remain.

🧠 Conceptual

expert

Number of items after complex indexing

Given the data frame below, how many elements are in the result of df[1:3, c(FALSE, TRUE, TRUE)]?

R Programming

df <- data.frame(A = 1:5, B = 6:10, C = 11:15)

A 3

B 9

C 6

D 15

💡 Hint

Count rows selected times columns selected.

Rows 1 to 3 means 3 rows. The logical vector selects columns 2 and 3 (B and C), so 2 columns. Total elements = 3 * 2 = 6.

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