Handling missing values (na.rm, na.omit) in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of sum() with na.rm parameter

What is the output of the following R code?

R Programming

x <- c(1, 2, NA, 4)
sum(x, na.rm = TRUE)

A 7

B NA

C Error: missing value

D 6

💡 Hint

The na.rm = TRUE argument tells sum() to ignore missing values.

The sum function adds all numbers ignoring NA when na.rm = TRUE. So 1 + 2 + 4 = 7.

❓ Predict Output

intermediate

Effect of na.omit() on a vector

What is the output of this R code?

R Programming

y <- c(3, NA, 5, NA, 7)
na.omit(y)

A 3 5 7

B 3 NA 5 NA 7

C Error: cannot omit NA

D NA NA NA

💡 Hint

na.omit() removes all NA values from the vector.

na.omit() returns the vector without any NA values, so only 3, 5, and 7 remain.

❓ Predict Output

advanced

Sum of vector with NA without na.rm

What happens when you run this R code?

R Programming

z <- c(10, NA, 20)
sum(z)

A 30

B Error: missing value

C 10

D NA

💡 Hint

sum() returns NA if there is any missing value and na.rm is not TRUE.

Without na.rm = TRUE, sum() returns NA if any element is NA.

🧠 Conceptual

advanced

Behavior of na.omit() on data frame

Given the data frame below, what will na.omit(df) return?

R Programming

df <- data.frame(a = c(1, NA, 3), b = c(NA, 2, 3))
na.omit(df)

A Rows with NA in column 'a' removed, but rows with NA in 'b' kept

B All rows kept because na.omit only works on vectors

C Rows with any NA values removed, so only the last row remains

D Error: na.omit cannot be applied to data frames

💡 Hint

na.omit removes any row with at least one NA in any column.

na.omit removes rows with any missing values, so only the row with no NAs remains.

❓ Predict Output

expert

Count of non-NA elements using sum and is.na

What is the output of this R code?

R Programming

v <- c(NA, 4, NA, 7, 9)
sum(!is.na(v))

A 2

B 3

C NA

D Error: invalid argument

💡 Hint

is.na(v) returns TRUE for NA values, !is.na(v) is TRUE for non-NA values, sum counts TRUE as 1.

There are 3 non-NA values (4, 7, 9), so sum(!is.na(v)) returns 3.

Practice

1. What does the argument na.rm = TRUE do when used inside functions like sum() or mean() in R?

easy

A. It tells the function to ignore missing values (NA) during calculation.

B. It replaces missing values with zero before calculation.

C. It stops the function and returns an error if there are missing values.

D. It counts the number of missing values in the data.

Solution

1. Step 1: Understand the role of na.rm argument

   The na.rm argument stands for "NA remove" and controls whether missing values should be ignored.

2. Step 2: Effect of setting na.rm = TRUE

   When set to TRUE, the function skips over any NA values and calculates the result using only the available data.

3. Final Answer:

   It tells the function to ignore missing values (NA) during calculation. -> Option A

4. Quick Check:

   na.rm = TRUE means ignore NA values [OK]

Hint: Remember: na.rm = TRUE means "remove NA" during calculation [OK]

Common Mistakes:

• Thinking na.rm replaces NA with zero
• Assuming na.rm causes an error on NA
• Confusing na.rm with counting NAs

2. Which of the following is the correct syntax to remove missing values from a vector x using na.omit() in R?

easy

A. clean_x <- na.remove(x)

B. clean_x <- omit.na(x)

C. clean_x <- na.omit(x)

D. clean_x <- remove.na(x)

Solution

1. Step 1: Recall the function name for removing NAs

   The correct function in R to remove missing values is na.omit().

2. Step 2: Check the syntax usage

   It is used as na.omit(x) where x is the vector or data frame.

3. Final Answer:

   clean_x <- na.omit(x) -> Option C

4. Quick Check:

   Use na.omit() to remove NAs [OK]

Hint: Use na.omit() exactly as written to remove missing values [OK]

Common Mistakes:

• Swapping function name parts like omit.na
• Using non-existent functions like na.remove
• Confusing na.omit with na.rm argument

3. What is the output of the following R code?

x <- c(2, NA, 5, NA, 8)
sum(x)
sum(x, na.rm = TRUE)

medium

A. [1] 15 and [1] 15

B. [1] NA and [1] 15

C. [1] 15 and [1] NA

D. [1] NA and [1] NA

Solution

1. Step 1: Calculate sum without removing NAs

   Calling sum(x) with NA values returns NA because missing values cause the sum to be undefined.

2. Step 2: Calculate sum with na.rm = TRUE

   Calling sum(x, na.rm = TRUE) ignores NAs and sums only 2 + 5 + 8 = 15.

3. Final Answer:

   [1] NA and [1] 15 -> Option B

4. Quick Check:

   sum() returns NA if NA present unless na.rm=TRUE [OK]

Hint: sum() returns NA if NA present; use na.rm=TRUE to ignore [OK]

Common Mistakes:

• Assuming sum ignores NA by default
• Mixing output order of the two sums
• Thinking sum with na.rm=TRUE returns NA

4. You run this code in R:

data <- c(4, NA, 7, NA, 10)
mean(data)

But it returns NA. How can you fix it to get the mean ignoring missing values?

medium

A. Both A and C are correct

B. Use mean(data, na.rm = TRUE)

C. Use mean(na.omit(data))

D. Replace NA with zero manually before mean

Solution

1. Step 1: Understand why mean returns NA

   By default, mean() returns NA if any missing values exist in the data.

2. Step 2: Ways to ignore missing values

   You can either set na.rm = TRUE inside mean() or remove NAs first using na.omit() and then calculate mean.

3. Step 3: Check options

   Both mean(data, na.rm = TRUE) and mean(na.omit(data)) will give the correct mean ignoring NAs.

4. Final Answer:

   Both A and C are correct -> Option A

5. Quick Check:

   Use na.rm=TRUE or na.omit() to ignore NAs in mean [OK]

Hint: Use na.rm=TRUE or na.omit() to ignore missing values [OK]

Common Mistakes:

• Forgetting to add na.rm=TRUE
• Thinking mean automatically ignores NA
• Replacing NA with zero without reason

5. Given a data frame df with numeric column score containing some NAs, which code correctly calculates the average score excluding missing values and also removes rows with NA in score before calculating the mean?

hard

A. mean(df$score, na.rm = FALSE); df_clean <- na.omit(df); mean(df_clean$score)

B. mean(na.omit(df)); df_clean <- na.omit(df); mean(df_clean$score)

C. mean(df$score); df_clean <- na.omit(df$score); mean(df_clean)

D. mean(df$score, na.rm = TRUE); df_clean <- na.omit(df); mean(df_clean$score)

Solution

1. Step 1: Calculate mean ignoring NAs in the column

   Using mean(df$score, na.rm = TRUE) calculates average excluding NAs directly.

2. Step 2: Remove rows with NA in entire data frame

   na.omit(df) removes all rows with any NA values, including in score.

3. Step 3: Calculate mean on cleaned data

   Then mean(df_clean$score) calculates mean on data without missing values.

4. Final Answer:

   mean(df$score, na.rm = TRUE); df_clean <- na.omit(df); mean(df_clean$score) -> Option D

5. Quick Check:

   Use na.rm=TRUE for mean and na.omit() to clean data frame [OK]

Hint: Use na.rm=TRUE for mean and na.omit() to clean data frame [OK]

Common Mistakes:

• Using na.omit on vector but not on data frame
• Forgetting na.rm=TRUE in mean calculation
• Using na.omit incorrectly on a vector instead of data frame