If–else statement in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of an if–else statement in C++?

An if–else statement lets the program choose between two paths: one if a condition is true, and another if it is false.

beginner

Write the basic syntax of an if–else statement in C++.

if (condition) {
// code if condition is true
} else {
// code if condition is false
}

beginner

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

The program skips the if block and continues with the next code after it.

intermediate

Can you have multiple else if blocks in C++? What is their use?

Yes, multiple else if blocks let you check several conditions one after another, running the code for the first true condition.

beginner

Explain with a real-life example how if–else works.

Imagine deciding what to wear: if it is raining, wear a raincoat; else, wear a t-shirt. The if–else helps choose based on weather.

What does the else block do in an if–else statement?

A Runs code when the if condition is true

B Checks another condition

C Runs code when the if condition is false

D Ends the program

The else block runs only when the if condition is false.

Which keyword is used to check multiple conditions after an if in C++?

A elif

B elseif

C else

D else if

C++ uses 'else if' as two words to check multiple conditions.

What happens if the if condition is true and there is an else block?

A Both if and else blocks run

B Only the if block runs

C Only the else block runs

D Neither block runs

Only the if block runs when the condition is true; else block is skipped.

Which of these is a correct if statement syntax in C++?

A if (condition) { }

B if condition then { }

C if condition { }

D if: (condition) { }

The correct syntax requires parentheses around the condition.

Can an if statement exist without an else block?

A Yes, else is optional

B No, else is required

C Only if the condition is true

D Only in loops

The else block is optional; you can have just an if statement.

Describe how an if–else statement controls the flow of a program.

Think about how the program chooses what to do next based on a question.

Write a simple if–else statement in C++ that checks if a number is positive or not.

Use if (number > 0) and else blocks.

Practice

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

easy

A. It defines a function.

B. It repeats a block of code multiple times.

C. It chooses between two paths based on a condition.

D. It declares a variable.

Solution

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

   An if-else statement lets the program decide which block of code to run based on a condition.

2. Step 2: Compare with other options

   Repeating code is done by loops, functions define reusable code, and variables store data, so these are not correct.

3. Final Answer:

   It chooses between two paths based on a condition. -> Option C

4. Quick Check:

   If-else = choose path [OK]

Hint: If condition true, run if-block; else run else-block [OK]

Common Mistakes:

• Confusing if-else with loops
• Thinking if-else defines functions
• Mixing variable declaration with control flow

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

easy

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

B. if x > 0 then cout << "Positive" else cout << "Non-positive"

C. if (x > 0) cout << "Positive" else cout << "Non-positive";

D. if x > 0 { cout << "Positive" } else { cout << "Non-positive" }

Solution

1. Step 1: Check C++ if-else syntax rules

   C++ requires parentheses around the condition and braces around blocks for multiple statements.

2. Step 2: Analyze each option

   if (x > 0) { cout << "Positive"; } else { cout << "Non-positive"; } uses correct parentheses and braces. Options A and B miss parentheses or use incorrect keywords. if (x > 0) cout << "Positive" else cout << "Non-positive"; misses semicolon after else block.

3. Final Answer:

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

4. Quick Check:

   Correct syntax uses parentheses and braces [OK]

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

Common Mistakes:

• Omitting parentheses around condition
• Missing braces for multiple statements
• Using 'then' keyword (not in C++)

3. What will be the output of this code?

int x = 5;
if (x < 3) {
  cout << "Less";
} else {
  cout << "More or equal";
}

medium

A. Less

B. More or equal

C. No output

D. Compilation error

Solution

1. Step 1: Evaluate the condition x < 3

   Variable x is 5, so 5 < 3 is false.

2. Step 2: Determine which block runs

   Since condition is false, the else block runs, printing "More or equal".

3. Final Answer:

   More or equal -> Option B

4. Quick Check:

   Condition false -> else block output [OK]

Hint: Check condition truth to pick if or else output [OK]

Common Mistakes:

• Assuming if-block runs when condition is false
• Confusing output strings
• Ignoring else block

4. Find the error in this code snippet:

int n = 10;
if n > 5 {
  cout << "Big";
} else {
  cout << "Small";
}

medium

A. Missing parentheses around condition

B. Missing semicolon after cout statements

C. Incorrect else keyword

D. Variable n not declared

Solution

1. Step 1: Check if statement syntax

   C++ requires parentheses around the condition in if statements.

2. Step 2: Identify the error

   The code uses if n > 5 without parentheses, which is invalid syntax.

3. Final Answer:

   Missing parentheses around condition -> Option A

4. Quick Check:

   if condition needs parentheses [OK]

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

Common Mistakes:

• Omitting parentheses in if condition
• Forgetting semicolons (though not the main error here)
• Misreading else keyword

5. You want to write a program that prints "Even" if a number is divisible by 2, otherwise prints "Odd". Which code correctly uses if-else to do this?

hard

A. if (num % 2 = 0) { cout << "Even"; } else { cout << "Odd"; }

B. if (num / 2 == 0) { cout << "Even"; } else { cout << "Odd"; }

C. if (num % 2 != 0) { cout << "Even"; } else { cout << "Odd"; }

D. if (num % 2 == 0) { cout << "Even"; } else { cout << "Odd"; }

Solution

1. Step 1: Understand divisibility by 2

   A number is divisible by 2 if remainder when divided by 2 is zero, i.e., num % 2 == 0.

2. Step 2: Check each option's condition

   if (num % 2 == 0) { cout << "Even"; } else { cout << "Odd"; } correctly uses num % 2 == 0. if (num / 2 == 0) { cout << "Even"; } else { cout << "Odd"; } uses division instead of modulo. if (num % 2 = 0) { cout << "Even"; } else { cout << "Odd"; } uses assignment (=) instead of comparison (==). if (num % 2 != 0) { cout << "Even"; } else { cout << "Odd"; } reverses the logic.

3. Final Answer:

   if (num % 2 == 0) { cout << "Even"; } else { cout << "Odd"; } -> Option D

4. Quick Check:

   Use modulo == 0 for even check [OK]

Hint: Use modulo (%) and == 0 to check even numbers [OK]

Common Mistakes:

• Using assignment (=) instead of comparison (==)
• Using division (/) instead of modulo (%)
• Reversing if and else blocks