If-else statements in R Programming - Step-by-Step Execution

Concept Flow - If-else statements

Evaluate condition

↓

Is condition TRUE?

No→Execute else block

Yes↓

Execute if block

↓

End

The program checks a condition. If true, it runs the 'if' part; if false, it runs the 'else' part, then continues.

Execution Sample

R Programming

x <- 7
if (x > 5) {
  result <- "big"
} else {
  result <- "small"
}

Checks if x is greater than 5; assigns 'big' if true, otherwise 'small'.

Execution Table

Step	Condition (x > 5)	Result	Branch Taken	Variable 'result'
1	7 > 5	TRUE	if block	"big"
2	End of if-else	-	-	"big"

💡 Condition is TRUE, so 'if' block runs and 'result' is set to 'big'.

Variable Tracker

Variable	Start	After Step 1	Final
x	7	7	7
result	undefined	"big"	"big"

Key Moments - 2 Insights

Why does the else block not run when x is 7?

Because the condition x > 5 is TRUE at step 1, so the if block runs and the else block is skipped.

What happens if the condition is FALSE?

The else block runs instead, setting 'result' to 'small' as shown by the branch taken in the execution table.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of 'result' after step 1?

A "big"

B "small"

C undefined

D 7

💡 Hint

Check the 'Variable result' column in row for step 1 in the execution table.

At which step does the program decide which branch to take?

A Step 2

B Step 1

C Before Step 1

D After Step 2

💡 Hint

Look at the 'Condition' and 'Branch Taken' columns in the execution table.

If x was 3 instead of 7, what would 'result' be after step 1?

A undefined

B "big"

C "small"

D 3

💡 Hint

Think about the condition x > 5 and which branch runs when it is FALSE.

Concept Snapshot

If-else statements in R:
if (condition) {
  # code if TRUE
} else {
  # code if FALSE
}
Runs one block based on condition truth.
Useful for decision making.

Full Transcript

This visual execution shows how an if-else statement works in R. The program checks if x is greater than 5. Since x is 7, the condition is TRUE, so it runs the 'if' block and sets result to 'big'. The else block is skipped. Variables x and result are tracked before and after execution. Key moments clarify why the else block does not run and what happens if the condition is false. The quiz tests understanding of variable values and branch decisions based on the execution table.

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