If–else statement in C++ - Step-by-Step Execution

Concept Flow - If–else statement

Start

↓

Evaluate condition

↓

Execute if-block

↓

End

↓

Execute else-block

↓

End

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

Execution Sample

C++

#include <iostream>
using namespace std;

int main() {
  int x = 10;
  if (x > 5) {
    cout << "Big";
  } else {
    cout << "Small";
  }
  return 0;
}

Checks if x is greater than 5; prints 'Big' if true, otherwise 'Small'.

Execution Table

Step	Condition (x > 5)	Result	Branch Taken	Output
1	10 > 5	true	if-block	"Big"
2	-	-	End	-

💡 Condition true, executed if-block, then program ends.

Variable Tracker

Variable	Start	After Step 1	Final
x	10	10	10

Key Moments - 2 Insights

Why does the program print "Big" and not "Small"?

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

What happens if the condition is false?

The else-block runs instead, printing "Small" as shown in the branch taken column if condition was false.

Visual Quiz - 3 Questions

Look at the execution table, what is the output at step 1?

A "Small"

B "Big"

C No output

D Error

💡 Hint

Check the Output column at step 1 in the execution_table.

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

A Step 2

B Before Step 1

C 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 10, what would the output be?

A "Small"

B "Big"

C No output

D Error

💡 Hint

Refer to the key_moments explanation about condition false leading to else-block.

Concept Snapshot

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

Checks condition once; runs if-block if true, else-block if false.

Full Transcript

This example shows how an if-else statement works in C++. The program starts by checking if x is greater than 5. Since x is 10, the condition is true, so it runs the code inside the if-block and prints "Big". If the condition were false, it would run the else-block and print "Small". The execution table shows each step: evaluating the condition, choosing the branch, and printing the output. The variable tracker shows that x stays 10 throughout. This helps beginners see how the program decides which code to run based on a condition.

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