Useful vector functions (length, sum, mean) in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this R code using length()?

Consider the vector v <- c(3, 7, 2, 9, 4). What does length(v) return?

R Programming

v <- c(3, 7, 2, 9, 4)
length(v)

A 4

B 9

C 5

D Error: object 'v' not found

💡 Hint

The length() function returns how many elements are in the vector.

The vector v has 5 numbers, so length(v) returns 5.

❓ Predict Output

intermediate

What is the sum of elements in this vector?

Given nums <- c(10, 20, 30, 40), what does sum(nums) return?

R Programming

nums <- c(10, 20, 30, 40)
sum(nums)

A Error: object 'nums' not found

B 40

C 10

D 100

💡 Hint

sum() adds all numbers in the vector.

Adding 10 + 20 + 30 + 40 equals 100, so sum(nums) returns 100.

❓ Predict Output

advanced

What is the mean of this vector with NA values?

Given data <- c(5, NA, 15, 20), what does mean(data, na.rm = TRUE) return?

R Programming

data <- c(5, NA, 15, 20)
mean(data, na.rm = TRUE)

A Error: missing value and na.rm not set to TRUE

B 13.33333

C 15

D NA

💡 Hint

na.rm = TRUE tells mean() to ignore NA values.

Ignoring NA, the mean is (5 + 15 + 20) / 3 = 13.33333.

❓ Predict Output

advanced

What error does this code produce?

What happens when you run sum(c(1, 2, '3')) in R?

R Programming

sum(c(1, 2, '3'))

A 6

B Error: invalid 'type' (character) of argument

C 3

D NA

💡 Hint

sum() expects numeric values, but '3' is a character here.

The vector mixes numbers and a character, so sum() cannot add them and throws a type error.

🧠 Conceptual

expert

How many elements are in the vector after this operation?

If v <- c(1, 2, 3, 4, 5) and then v <- v[-c(2, 4)] is run, what is length(v)?

R Programming

v <- c(1, 2, 3, 4, 5)
v <- v[-c(2, 4)]
length(v)

A 3

B 4

C 2

D 5

💡 Hint

Removing elements 2 and 4 means those positions are gone from the vector.

Removing elements at positions 2 and 4 leaves elements 1, 3, and 5, so length is 3.

Practice

1. What does the length() function do when applied to a vector in R?

easy

A. It calculates the sum of all elements in the vector.

B. It sorts the elements in the vector.

C. It finds the average value of the elements in the vector.

D. It returns the number of elements in the vector.

Solution

1. Step 1: Understand the purpose of length()

   The length() function counts how many items are inside a vector.

2. Step 2: Compare with other functions

   Unlike sum() or mean(), it does not add or average values, just counts them.

3. Final Answer:

   It returns the number of elements in the vector. -> Option D

4. Quick Check:

   length() = count elements [OK]

Hint: Remember length() counts items, sum() adds, mean() averages [OK]

Common Mistakes:

• Confusing length() with sum() or mean()
• Thinking length() returns the last element
• Assuming length() sorts the vector

2. Which of the following is the correct syntax to calculate the mean of a vector v in R?

easy

A. mean(v)

B. mean = v

C. mean[v]

D. mean v()

Solution

1. Step 1: Recall function call syntax in R

   Functions are called with parentheses and the vector inside, like mean(v).

2. Step 2: Check each option

   mean(v) uses correct syntax. Options B, C, and D are invalid or incorrect syntax.

3. Final Answer:

   mean(v) -> Option A

4. Quick Check:

   Function call uses parentheses: mean(v) [OK]

Hint: Use parentheses to call functions with arguments in R [OK]

Common Mistakes:

• Using square brackets instead of parentheses
• Assigning mean instead of calling it
• Omitting parentheses after function name

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

v <- c(2, 4, 6, 8)
sum(v) + length(v)

medium

A. 20

B. 22

C. 24

D. 26

Solution

1. Step 1: Calculate sum(v)

   The vector v is c(2,4,6,8). Sum is 2+4+6+8 = 20.

2. Step 2: Calculate length(v) and add

   Length is number of elements = 4. Adding sum + length = 20 + 4 = 24.

3. Final Answer:

   24 -> Option C

4. Quick Check:

   sum=20 + length=4 = 24 [OK]

Hint: Add sum and length values separately, then combine [OK]

Common Mistakes:

• Adding elements incorrectly
• Confusing length with sum
• Forgetting to add length to sum

4. The following R code is intended to calculate the mean of vector x. What is wrong?

x <- c(5, 10, 15)
mean = mean(x)
print(mean)

medium

A. The print() function cannot print variables.

B. The variable name 'mean' overwrites the function mean(), causing errors later.

C. The vector x is not defined correctly.

D. mean() function requires two arguments.

Solution

1. Step 1: Analyze variable assignment

   Assigning mean = mean(x) overwrites the built-in mean() function with a number.

2. Step 2: Understand consequences

   After this, calling mean() again will cause errors because mean is no longer a function.

3. Final Answer:

   The variable name 'mean' overwrites the function mean(), causing errors later. -> Option B

4. Quick Check:

   Don't name variables same as functions [OK]

Hint: Avoid naming variables same as function names [OK]

Common Mistakes:

• Thinking print() cannot print variables
• Believing mean() needs two arguments
• Ignoring variable shadowing of functions

5. You have a numeric vector nums with some missing values (NA). Which R code correctly calculates the mean ignoring the missing values?

hard

A. mean(nums, na.rm = TRUE)

B. mean(nums, remove.na = TRUE)

C. sum(nums) / length(nums)

D. mean(nums, na.ignore = TRUE)

Solution

1. Step 1: Understand NA handling in mean()

   To ignore missing values, use the argument na.rm = TRUE inside mean().

2. Step 2: Check each option

   mean(nums, na.rm = TRUE) uses correct argument na.rm. Options B and D use incorrect argument names. sum(nums) / length(nums) does not handle NA and will return NA.

3. Final Answer:

   mean(nums, na.rm = TRUE) -> Option A

4. Quick Check:

   Use na.rm=TRUE to ignore NA in mean() [OK]

Hint: Use na.rm=TRUE to skip NA values in mean() [OK]

Common Mistakes:

• Using wrong argument names like remove.na or na.ignore
• Calculating mean manually without handling NA
• Ignoring that NA causes mean() to return NA