If-else statements in R Programming

Introduction

If-else statements help your program make choices. They let your code do different things based on conditions.

Deciding if a number is positive or negative.

Checking if a user is old enough to access a website.

Choosing what message to show based on the time of day.

Running different code when a button is clicked or not.

Syntax

R Programming

if (condition) {
  # code to run if condition is TRUE
} else {
  # code to run if condition is FALSE
}

The condition is a test that is either TRUE or FALSE.

Use curly braces { } to group the code that runs for each case.

Examples

This checks if x is greater than zero. If yes, it prints "Positive number"; otherwise, it prints "Not positive".

R Programming

x <- 5
if (x > 0) {
  print("Positive number")
} else {
  print("Not positive")
}

This checks if age is 18 or more. It prints "Adult" if true, else "Minor".

R Programming

age <- 17
if (age >= 18) {
  print("Adult")
} else {
  print("Minor")
}

Sample Program

This program checks the temperature. If it is above 25, it says it's hot; otherwise, it says it's not hot.

R Programming

temperature <- 30
if (temperature > 25) {
  print("It's hot outside")
} else {
  print("It's not hot outside")
}

Important Notes

You can add more conditions using else if for multiple choices.

Make sure your conditions are clear and use logical operators like && (and), || (or) if needed.

Summary

If-else lets your program choose between two paths.

Use conditions that are TRUE or FALSE to decide what runs.

Curly braces group the code for each choice.

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