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

Concept Flow - If–else statement

Start

↓

Evaluate condition

Yes No↓

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 <stdio.h>

int main() {
    int x = 10;
    if (x > 5) {
        printf("Greater\n");
    } else {
        printf("Smaller or equal\n");
    }
    return 0;
}

Checks if x is greater than 5 and prints a message accordingly.

Execution Table

Step	Condition (x > 5)	Result	Branch Taken	Output
1	10 > 5	True	if-block	"Greater\n" printed
2	End of if-else	-	-	Program ends

💡 Condition true, so if-block executed; else-block skipped.

Variable Tracker

Variable	Start	After Condition Check	Final
x	10	10	10

Key Moments - 2 Insights

Why does the else-block not run even though it exists?

Because the condition (x > 5) is true at step 1 in the execution_table, so only the if-block runs.

What happens if the condition is false?

The else-block runs instead, as shown by the 'Branch Taken' column in the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output at step 1?

A No output

B "Greater\n" printed

C "Smaller or equal\n" printed

D Error message

💡 Hint

Check the 'Output' column in the first row of 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 execution_table row 1.

If x was 3 instead of 10, which branch would be taken?

A else-block

B if-block

C Both if and else blocks

D Neither block

💡 Hint

Refer to the condition (x > 5) and what happens when it is false.

Concept Snapshot

if (condition) {
  // code runs if condition true
} else {
  // code runs if condition false
}
Checks condition once, runs one branch only.

Full Transcript

This example shows how an if-else statement works in C. The program sets x to 10, then checks if x is greater than 5. Since 10 is greater than 5, it prints "Greater" and skips the else part. The flow diagram shows the decision path. The execution table tracks the condition check and output. Variables remain unchanged. Key points clarify why only one branch runs. The quiz tests understanding of output, decision step, and behavior if x changes.

Practice

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

easy

A. It declares a variable.

B. It repeats a block of code multiple times.

C. It defines a new function.

D. It chooses between two actions based on a condition.

Solution

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

   An if-else statement lets the program pick one of two paths depending on whether a condition is true or false.

2. Step 2: Match the description to the options

   Only It chooses between two actions based on a condition. describes choosing between two actions based on a condition, which is exactly what if-else does.

3. Final Answer:

   It chooses between two actions based on a condition. -> Option D

4. Quick Check:

   If-else = choose between two paths [OK]

Hint: If you want two choices, use if-else [OK]

Common Mistakes:

• Confusing if-else with loops
• Thinking if-else declares variables
• Mixing if-else with function definitions

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

easy

A. if (condition) then { /* code */ } else { /* code */ }

B. if condition then { /* code */ } else { /* code */ }

C. if (condition) { /* code */ } else { /* code */ }

D. if condition { /* code */ } else { /* code */ }

Solution

1. Step 1: Recall C syntax rules for if-else

   In C, conditions must be inside parentheses and braces {} are used for code blocks.

2. Step 2: Check each option

   Only if (condition) { /* code */ } else { /* code */ } uses parentheses around the condition and braces correctly without extra keywords like 'then'.

3. Final Answer:

   if (condition) { /* code */ } else { /* code */ } -> Option C

4. Quick Check:

   Correct syntax uses parentheses and braces [OK]

Hint: Remember: if(condition) with braces {} [OK]

Common Mistakes:

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

3. What will be the output of this C code?

#include 
int main() {
    int x = 10;
    if (x > 5) {
        printf("Greater\n");
    } else {
        printf("Smaller\n");
    }
    return 0;
}

medium

A. Greater

B. Smaller

C. 10

D. No output

Solution

1. Step 1: Evaluate the condition x > 5

   Variable x is 10, which is greater than 5, so the condition is true.

2. Step 2: Determine which block runs

   Since the condition is true, the code inside the if block runs, printing "Greater".

3. Final Answer:

   Greater -> Option A

4. Quick Check:

   10 > 5 is true, so prints "Greater" [OK]

Hint: Check if condition is true or false to pick output [OK]

Common Mistakes:

• Confusing which block runs on true condition
• Ignoring the newline character in printf
• Assuming else block runs when condition is true

4. Find the error in this if-else code snippet:

int a = 3;
if a > 0
    printf("Positive\n");
else
    printf("Non-positive\n");

medium

A. Missing semicolon after if statement

B. Missing parentheses around condition in if statement

C. Missing braces around printf statements

D. Incorrect else keyword

Solution

1. Step 1: Check syntax of if condition

   In C, the condition in an if statement must be inside parentheses (). Here, it is missing.

2. Step 2: Verify other parts

   Semicolons after printf are present, braces are optional for single statements, and else keyword is correct.

3. Final Answer:

   Missing parentheses around condition in if statement -> Option B

4. Quick Check:

   if condition needs parentheses ( ) [OK]

Hint: Always put condition inside parentheses ( ) [OK]

Common Mistakes:

• Forgetting parentheses in if condition
• Thinking braces are always required for single statements
• Misplacing else keyword

5. You want to write a program that prints "Even" if a number is even, and "Odd" if it is odd. Which if-else code correctly does this in C?

hard

A. if (num % 2 == 0) { printf("Even\n"); } else { printf("Odd\n"); }

B. if (num / 2 == 0) { printf("Even\n"); } else { printf("Odd\n"); }

C. if (num % 2 = 0) { printf("Even\n"); } else { printf("Odd\n"); }

D. if (num % 2 != 0) { printf("Even\n"); } else { printf("Odd\n"); }

Solution

1. Step 1: Understand how to check even numbers

   A number is even if dividing by 2 leaves remainder 0, which is checked by num % 2 == 0.

2. Step 2: Analyze each option

   if (num % 2 == 0) { printf("Even\n"); } else { printf("Odd\n"); } correctly uses modulus operator and equality check. if (num / 2 == 0) { printf("Even\n"); } else { printf("Odd\n"); } uses division instead of modulus. if (num % 2 = 0) { printf("Even\n"); } else { printf("Odd\n"); } uses assignment (=) instead of comparison (==). if (num % 2 != 0) { printf("Even\n"); } else { printf("Odd\n"); } reverses the logic.

3. Final Answer:

   if (num % 2 == 0) { printf("Even\n"); } else { printf("Odd\n"); } -> Option A

4. Quick Check:

   Use modulus == 0 to check even [OK]

Hint: Use num % 2 == 0 to test even numbers [OK]

Common Mistakes:

• Using division instead of modulus
• Using = instead of == in condition
• Reversing even and odd logic