Control Flow Fundamentals

Making Decisions in C

By default, C statements execute sequentially from top to bottom. Branching statements allow your code to choose different execution paths based on runtime conditions.

In C, what numerical value represents a true condition in an if statement?

branching_overview.cDecision Logic

#include <stdio.h>

int main(void) {
    int score = 85;

    // Condition evaluated: (score >= 50)
    if (score >= 50) {
        printf("Result: PASSED!\n");
    } else {
        printf("Result: FAILED!\n");
    }
    return 0;
}

One-Way Branching

The Simple `if` Statement

An if statement executes a block of code only if the specified condition evaluates to true (non-zero). If false, the block is completely skipped.

Syntax & Mechanics

if (condition) { // Code executes if condition is TRUE }

If there is only a single statement inside the if block, curly braces {} are optional, but using them is best practice to prevent bugs.

Enter Program

Condition?

TRUE → [ Execute Block ] FALSE → [ Skip ]

Interactive Lab: Temperature Alert

Test how a single if statement responds to input values.

Current Temperature (°C):

if (temp > 35) {
    printf("HEAT WAVE WARNING!");
}
printf("Have a nice day.");

Two-Way Branching

The `if-else` Statement

Use if-else when you have two mutually exclusive choices: execute Action A if the condition is true, or Action B if the condition is false.

Interactive Code Tracer: Voting Eligibility

Enter Age:

Click evaluate to run the tracer.

voting_check.cExecution Trace

int age = 19;
if (age >= 18) {
    printf("Eligible to vote!");
} else {
    printf("Not eligible yet.");
}

Hierarchical Logic

Nested `if-else` & The Dangling Else

An if or else block can contain another complete if-else structure. Beware of the Dangling Else ambiguity!

⚠️ The Dangling Else Trap

In C, an else clause is always paired with the nearest preceding if that does not already have an else, regardless of indentation!

// Misleading Indentation! if (a > 0) if (b > 0) printf("Both positive"); else printf("Is this for (a>0)?"); // NO! It pairs with (b>0)!

Fix: Always wrap blocks in curly braces {} to enforce clear scope!

Interactive Admission Test

Total Marks (%):

Math Score (%):

Adjust scores and click evaluate.

Multi-Way Branching

The `if-else` Ladder

When testing a single variable or expression against multiple sequential ranges, an if-else if-else ladder evaluates conditions top-to-bottom until a true match is found.

Interactive Grade Calculator

Student Marks: 82

Condition: marks >= 90 Grade A+

Condition: marks >= 80 Grade A

Condition: marks >= 70 Grade B

Condition: marks >= 60 Grade C

Default else Grade F

grade_ladder.cSequential Evaluation

if (marks >= 90) {
    grade = 'A+';
} else if (marks >= 80) {
    grade = 'A';  <-- Stops here if 82
} else if (marks >= 70) {
    grade = 'B';
} else if (marks >= 60) {
    grade = 'C';
} else {
    grade = 'F';
}

Best Practices

Common Branching Pitfalls in C

Watch out for these frequent logical bugs that compile cleanly but produce wrong behavior!

🚨

Assignment vs Equality

Accidentally writing if (x = 5) instead of if (x == 5) assigns 5 to x, which evaluates as TRUE!

if (x = 5) { // BUG! Always true printf("Oops!"); }

🛑

Extra Semicolon Bug

Placing a semicolon right after if(...) ; creates an empty statement, causing the following block to ALWAYS run!

if (score > 90); { // BUG! ';' ends if printf("Passed!"); // Runs always! }

💡

Best Practice: Braces

Always use curly braces {} even for single-line blocks to improve code readability and maintainability.

if (isReady) { execute(); }

Knowledge Check

Test Your Branching Mastery

Question 1 of 5

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