If-else statements in R Programming - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of nested if-else in R

What is the output of this R code?

R Programming

x <- 7
if (x > 10) {
  print("Greater than 10")
} else if (x > 5) {
  print("Between 6 and 10")
} else {
  print("5 or less")
}

A [1] "Between 6 and 10"

B [1] "Greater than 10"

C [1] "5 or less"

D Error in if (x > 10) { : missing value where TRUE/FALSE needed

💡 Hint

Check the conditions in order and see which one matches the value of x.

The variable x is 7, which is not greater than 10, so the first if fails. The else if checks if x is greater than 5, which is true, so it prints "Between 6 and 10".

❓ Predict Output

intermediate

Output of if-else with vector condition

What happens when you run this R code?

R Programming

x <- c(3, 7, 12)
if (x > 5) {
  print("Some values are greater than 5")
} else {
  print("No values greater than 5")
}

A Warning: the condition has length > 1 and only the first element will be used

B [1] "No values greater than 5"

C [1] "Some values are greater than 5"

D NULL

💡 Hint

Remember that if expects a single TRUE or FALSE, not a vector.

The condition x > 5 returns a vector of logicals. Using it directly in if causes a warning and only the first element is used.

🔧 Debug

advanced

Identify the error in this if-else code

What error does this R code produce?

R Programming

x <- 4
if x > 3 {
  print("x is greater than 3")
} else {
  print("x is 3 or less")
}

A Error: object 'x' not found

B Error in if (x > 3) { : missing value where TRUE/FALSE needed

C SyntaxError: unexpected symbol in "if x"

D No error, prints "x is greater than 3"

💡 Hint

Check the syntax of the if statement in R.

In R, the condition in if must be inside parentheses. Writing 'if x > 3' without parentheses causes a syntax error.

🧠 Conceptual

advanced

Behavior of if-else with NA condition

What is the output of this R code?

R Programming

x <- NA
if (x) {
  print("True")
} else {
  print("False")
}

A [1] "True"

B NULL

C [1] "False"

D Error in if (x) { : missing value where TRUE/FALSE needed

💡 Hint

Think about how R treats NA in logical conditions.

NA is an unknown logical value. Using it directly in if causes an error because R cannot decide TRUE or FALSE.

❓ Predict Output

expert

Output of complex nested if-else with multiple variables

What is the output of this R code?

R Programming

a <- 5
b <- 10
if (a > 3) {
  if (b < 5) {
    print("a > 3 and b < 5")
  } else if (b < 15) {
    print("a > 3 and b between 5 and 15")
  } else {
    print("a > 3 and b >= 15")
  }
} else {
  print("a <= 3")
}

A [1] "a > 3 and b < 5"

B [1] "a > 3 and b between 5 and 15"

C [1] "a > 3 and b >= 15"

D [1] "a <= 3"

💡 Hint

Check the values of a and b and follow the nested conditions carefully.

a is 5 which is > 3, so the outer if is true. Then b is 10, which is not < 5 but is < 15, so the second condition matches and prints "a > 3 and b between 5 and 15".

Practice

1. What does an if-else statement do in R?

easy

A. It chooses which code to run based on a condition.

B. It repeats code multiple times.

C. It stores data in a list.

D. It creates a new function.

Solution

1. Step 1: Understand the purpose of if-else

   An if-else statement checks a condition and runs code depending on whether the condition is TRUE or FALSE.

2. Step 2: Compare options with the definition

   Only It chooses which code to run based on a condition. describes choosing code based on a condition, which matches if-else behavior.

3. Final Answer:

   It chooses which code to run based on a condition. -> Option A

4. Quick Check:

   If-else chooses code path = D [OK]

Hint: If-else picks code based on TRUE or FALSE condition [OK]

Common Mistakes:

• Confusing if-else with loops
• Thinking if-else stores data
• Mixing if-else with function creation

2. Which of the following is the correct syntax for an if-else statement in R?

easy

A. if (x > 0) { print("Positive") } else { print("Non-positive") }

B. if x > 0 then print("Positive") else print("Non-positive")

C. if x > 0 { print("Positive") } else { print("Non-positive") }

D. if (x > 0) print "Positive" else print "Non-positive"

Solution

1. Step 1: Recall R if-else syntax

   In R, conditions go inside parentheses, and code blocks inside curly braces.

2. Step 2: Check each option

   if (x > 0) { print("Positive") } else { print("Non-positive") } uses parentheses and braces correctly. Options A, C, and D miss parentheses or braces or use wrong keywords.

3. Final Answer:

   if (x > 0) { print("Positive") } else { print("Non-positive") } -> Option A

4. Quick Check:

   Correct if-else syntax uses ( ) and { } = B [OK]

Hint: Use parentheses for condition and braces for code blocks [OK]

Common Mistakes:

• Omitting parentheses around condition
• Missing curly braces for code blocks
• Using 'then' keyword which R does not use

3. What is the output of this R code?

score <- 75
if (score >= 60) {
  print("Pass")
} else {
  print("Fail")
}

medium

A. "Fail"

B. "Pass"

C. Error

D. No output

Solution

1. Step 1: Evaluate the condition

   The variable score is 75. The condition checks if 75 >= 60, which is TRUE.

2. Step 2: Determine which block runs

   Since the condition is TRUE, the code inside the if block runs, printing "Pass".

3. Final Answer:

   "Pass" -> Option B

4. Quick Check:

   75 >= 60 is TRUE, so output is "Pass" [OK]

Hint: Check if condition is TRUE to pick output [OK]

Common Mistakes:

• Confusing >= with >
• Thinking else block runs when condition is TRUE
• Ignoring quotes in printed output

4. Find the error in this R code snippet:

num <- 10
if num > 5 {
  print("Greater")
} else {
  print("Smaller")
}

medium

A. Using else without if

B. Missing curly braces around print statements

C. Variable num is not defined

D. Missing parentheses around the condition in if statement

Solution

1. Step 1: Check if statement syntax

   In R, the condition in if must be inside parentheses. Here, it's written as if num > 5 without parentheses.

2. Step 2: Verify other parts

   Curly braces are present, else is correctly used, and num is defined as 10.

3. Final Answer:

   Missing parentheses around the condition in if statement -> Option D

4. Quick Check:

   If condition needs ( ) in R [OK]

Hint: Always put condition inside parentheses in if [OK]

Common Mistakes:

• Omitting parentheses around condition
• Confusing curly braces placement
• Thinking else can stand alone

5. You want to write an R program that prints "Even" if a number is divisible by 2, "Odd" if divisible by 3 but not 2, and "Other" otherwise. Which code correctly uses if-else statements?

hard

A. if (num %% 2 == 0) {
print("Odd")
} else {
if (num %% 3 == 0) {
print("Even")
} else {
print("Other")
}
}

B. if (num %% 3 == 0) {
print("Odd")
} else if (num %% 2 == 0) {
print("Even")
} else {
print("Other")
}

C. if (num %% 2 == 0) {
print("Even")
} else if (num %% 3 == 0) {
print("Odd")
} else {
print("Other")
}

D. if (num %% 2 == 0) {
print("Odd")
} else if (num %% 3 == 0) {
print("Even")
} else {
print("Other")
}

Solution

1. Step 1: Understand the conditions order

   Check divisibility by 2 first to print "Even". If not divisible by 2, check divisibility by 3 to print "Odd". Otherwise print "Other".

2. Step 2: Match code with logic

   if (num %% 2 == 0) { print("Even") } else if (num %% 3 == 0) { print("Odd") } else { print("Other") } checks num %% 2 == 0 first, then num %% 3 == 0, then else. This matches the requirement exactly.

3. Final Answer:

   if (num %% 2 == 0) { print("Even") } else if (num %% 3 == 0) { print("Odd") } else { print("Other") } -> Option C

4. Quick Check:

   Check 2 first, then 3, else other = A [OK]

Hint: Check conditions in correct priority order [OK]

Common Mistakes:

• Checking divisibility by 3 before 2
• Swapping print messages for even and odd
• Missing nested else for other cases