Ifelse vectorized function in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of ifelse with numeric vector

What is the output of the following R code using ifelse?

R Programming

x <- c(2, 5, 8, 1)
result <- ifelse(x > 4, x * 2, x + 1)
print(result)

A [1] 3 10 16 2

B [1] 2 5 8 1

C [1] 3 5 8 2

D [1] 4 10 16 2

💡 Hint

Remember that ifelse applies the condition element-wise and returns a vector.

The condition x > 4 is checked for each element: for 2 and 1 it is FALSE, so we add 1; for 5 and 8 it is TRUE, so we multiply by 2.

🧠 Conceptual

intermediate

Understanding ifelse with NA values

What will be the output of this R code using ifelse with NA values?

R Programming

x <- c(NA, 3, 7, NA)
result <- ifelse(is.na(x), 0, x)
print(result)

A [1] 0 3 7 0

B [1] NA 3 7 NA

C [1] 0 NA NA 0

D [1] 0 0 0 0

💡 Hint

Check how is.na(x) returns TRUE or FALSE for each element.

The is.na(x) returns TRUE for NA elements, so ifelse replaces those with 0, and keeps other values as is.

🔧 Debug

advanced

Identify the error in ifelse usage

What error does the following R code produce?

R Programming

x <- c(1, 2, 3)
result <- ifelse(x > 2, "High", 0)
print(result)

A Error: non-numeric argument to binary operator

B Warning: Incompatible types in ifelse condition

C No error; output is [1] "0" "0" "High"

D No error; output is [1] 0 0 "High"

💡 Hint

Check how R handles mixed types in ifelse output vectors.

R coerces numeric 0 to character "0" because the other output is a character string. So no error occurs, and output is character vector.

📝 Syntax

advanced

Syntax error in ifelse usage

Which option contains a syntax error in using ifelse in R?

A ifelse(x > 0, "Positive", "Negative")

B ifelse(x > 0 "Positive", "Negative")

C )"evitageN" ,"evitisoP" ,0 > x(eslefi

D felse(x > 0, "Positive", "Negative")

💡 Hint

Look for missing commas or parentheses.

Option B is missing a comma between the condition and the true value, causing a syntax error.

🚀 Application

expert

Using ifelse to categorize numeric vector

Given scores <- c(45, 67, 89, 55, 72), which code correctly assigns grades: "Fail" for scores below 50, "Pass" for 50 to 75, and "Distinction" for above 75?

R Programming

scores <- c(45, 67, 89, 55, 72)

A grades <- ifelse(scores < 50, "Fail", ifelse(scores >= 75, "Pass", "Distinction"))

B grades <- ifelse(scores < 50, "Fail", ifelse(scores > 75, "Distinction", "Pass"))

C grades <- ifelse(scores <= 50, "Fail", ifelse(scores < 75, "Pass", "Distinction"))

D grades <- ifelse(scores < 50, "Fail", ifelse(scores <= 75, "Pass", "Distinction"))

💡 Hint

Think about the order of conditions and inclusive ranges.

Option D correctly nests ifelse to assign "Fail" if below 50, then "Pass" if 50 to 75 inclusive, else "Distinction".

Practice

1. What does the ifelse function in R do?

easy

A. It checks a condition for each element and returns values accordingly.

B. It creates a loop to iterate over elements.

C. It sorts a vector in ascending order.

D. It combines two vectors into one.

Solution

1. Step 1: Understand the purpose of ifelse

   The ifelse function tests a condition for each element in a vector.

2. Step 2: Identify the output behavior

   It returns a new vector with values chosen from the 'yes' or 'no' options based on the condition.

3. Final Answer:

   It checks a condition for each element and returns values accordingly. -> Option A

4. Quick Check:

   ifelse = condition-based selection [OK]

Hint: Think: condition check for each item returns chosen values [OK]

Common Mistakes:

• Confusing ifelse with loops
• Thinking it sorts or merges vectors
• Assuming it modifies original vector in place

2. Which of the following is the correct syntax for using ifelse in R?

easy

A. ifelse(condition, yes, no)

B. ifelse(yes, no, condition)

C. ifelse(condition; yes; no)

D. ifelse(condition -> yes, no)

Solution

1. Step 1: Recall the function signature

   The ifelse function takes three arguments: a condition, a value if true, and a value if false.

2. Step 2: Match the correct order and separators

   The correct syntax uses commas to separate arguments: ifelse(condition, yes, no).

3. Final Answer:

   ifelse(condition, yes, no) -> Option A

4. Quick Check:

   Syntax = ifelse(condition, yes, no) [OK]

Hint: Remember: condition first, then yes, then no, separated by commas [OK]

Common Mistakes:

• Swapping order of arguments
• Using semicolons instead of commas
• Using arrows or other symbols incorrectly

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

nums <- c(5, 10, 15, 20)
result <- ifelse(nums > 10, "big", "small")
print(result)

medium

A. ["big", "big", "small", "small"]

B. ["small", "small", "big", "big"]

C. [5, 10, 15, 20]

D. [TRUE, TRUE, FALSE, FALSE]

Solution

1. Step 1: Evaluate the condition for each element

   Check if each number in nums is greater than 10: 5 > 10 is FALSE, 10 > 10 is FALSE, 15 > 10 is TRUE, 20 > 10 is TRUE.

2. Step 2: Assign values based on condition

   For FALSE, assign "small"; for TRUE, assign "big". So the result vector is ["small", "small", "big", "big"].

3. Final Answer:

   ["small", "small", "big", "big"] -> Option B

4. Quick Check:

   Condition >10 gives small or big [OK]

Hint: Check condition element-wise, assign yes/no values [OK]

Common Mistakes:

• Confusing > with >=
• Returning original numbers instead of labels
• Mixing up true and false outputs

4. Identify the error in this R code snippet:

values <- c(1, 2, 3, 4)
result <- ifelse(values > 2, "high" "low")
print(result)

medium

A. Incorrect vector assignment syntax.

B. Using > operator instead of >=.

C. Using print() instead of cat() to display output.

D. Missing comma between 'high' and 'low' in ifelse arguments.

Solution

1. Step 1: Check the ifelse argument syntax

   The ifelse function requires three arguments separated by commas: condition, yes, no.

2. Step 2: Identify missing comma

   In the code, "high" and "low" are not separated by a comma, causing a syntax error.

3. Final Answer:

   Missing comma between 'high' and 'low' in ifelse arguments. -> Option D

4. Quick Check:

   Arguments must be comma-separated [OK]

Hint: Check commas between all ifelse arguments [OK]

Common Mistakes:

• Omitting commas between arguments
• Confusing operators > and >=
• Thinking print() causes error here

5. You have a numeric vector temps <- c(-5, 0, 10, 20, 30). Use ifelse to create a new vector labels that labels temperatures below 0 as "freezing", between 0 and 20 (inclusive) as "cold", and above 20 as "warm". Which code correctly does this?

hard

A. labels <- ifelse(temps <= 0, "freezing", ifelse(temps < 20, "cold", "warm"))

B. labels <- ifelse(temps < 0, "freezing", ifelse(temps < 20, "cold", "warm"))

C. labels <- ifelse(temps < 0, "freezing", ifelse(temps <= 20, "cold", "warm"))

D. labels <- ifelse(temps < 0, "freezing", ifelse(temps >= 20, "warm", "cold"))

Solution

1. Step 1: Understand the labeling rules

   Label "freezing" if temperature is below 0, "cold" if between 0 and 20 inclusive, and "warm" if above 20.

2. Step 2: Check nested ifelse conditions

   The first ifelse checks temps < 0 for "freezing". The second nested ifelse checks if temps <= 20 for "cold", else "warm".

3. Final Answer:

   labels <- ifelse(temps < 0, "freezing", ifelse(temps <= 20, "cold", "warm")) -> Option C

4. Quick Check:

   Nested ifelse with correct bounds [OK]

Hint: Use nested ifelse for multiple conditions with correct bounds [OK]

Common Mistakes:

• Using < instead of <= for 20 boundary
• Swapping warm and cold labels
• Not nesting ifelse properly