If-else statements in R Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of an if-else statement in R?

An if-else statement lets the program choose between two paths based on a condition. If the condition is true, it runs one block of code; if false, it runs another.

beginner

How do you write a simple if statement in R?

Use the syntax: if (condition) { code } where 'condition' is a test that returns TRUE or FALSE, and 'code' runs only if the condition is TRUE.

beginner

What happens if the condition in an if statement is FALSE and there is no else?

The code inside the if block is skipped, and the program continues with the next lines after the if statement.

beginner

How do you write an if-else statement in R?

Use: if (condition) { code if TRUE } else { code if FALSE }. This runs one block if the condition is TRUE, otherwise the other block.

intermediate

Can you have multiple conditions checked in R using if-else?

Yes, by using else if between if and else, you can check multiple conditions in order.

What will this R code print? if (5 > 3) { print("Yes") } else { print("No") }

A Yes

B No

C 5

D Error

Since 5 is greater than 3, the condition is TRUE, so it prints "Yes".

Which keyword is used to check an additional condition after an if in R?

A elseif

B else if

C elif

D elseif()

In R, the correct syntax is 'else if' with a space, not 'elseif' or 'elif'.

What happens if the condition in an if statement is FALSE and there is no else?

A The code inside if is skipped

B The code inside if runs anyway

C The program crashes

D The else block runs

If the condition is FALSE and no else is present, the if block is skipped.

Which of these is the correct syntax for an if-else statement in R?

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

B if (x > 0) { 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") }

Option D uses correct braces and else block syntax in R.

How do you write multiple conditions in R to check if x is positive, zero, or negative?

A if (x > 0) {...} else if x == 0 {...} else {...}

B if (x > 0) {...} elseif (x == 0) {...} else {...}

C if (x > 0) {...} else if (x == 0) {...} else {...}

D if (x > 0) {...} else (x == 0) {...} else {...}

Option C uses the correct 'else if' syntax with parentheses.

Explain how an if-else statement controls the flow of a program in R.

Think about how the program chooses which code to run based on a test.

Write an example of an if-else statement in R that prints "Even" if a number is even, and "Odd" if it is odd.

Use the modulo operator %% to check if a number is divisible by 2.

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