Module 06: Control Flow & Decision Making

1. if-else Logic 1 / 4

Conditional Branching & Dangling Else

How C evaluates truthiness (0 = False, Non-zero = True)

In C, there is no separate boolean hardware type in original C89; any integer other than 0 is considered True, and 0 is False.

dangling_else_resolved.c

// Dangling Else Rule: An 'else' always binds to the closest unmatched preceding 'if'
if (score >= 50) {
    if (score >= 90) {
        printf("Grade: A*\\n");
    }
} else {
    printf("Grade: Fail\\n"); // Explicit braces guarantee correct binding
}

2. switch-case 2 / 4

switch-case Statement Rules & Fall-Through

Constant expressions, integer/char types, and break statements

⚠️

Switch Expression Rules

The switch expression MUST evaluate to an integer type (int, char, short, enum). Floating point (float, double) and strings are illegal in C switches.
Case labels MUST be compile-time constants (e.g. case 1: or case 'A':). Variables like case x: are forbidden.

switch_rules.c

switch (operator_char) {
    case '+':
        result = a + b;
        break; // Essential: Prevents falling through into subtraction!
    case '-':
        result = a - b;
        break;
    default:
        printf("Invalid operator!\\n");
        break;
}

3. Jump Tables 3 / 4

How Compilers Optimize switch: Jump Tables

Why switch is O(1) constant time while if-else is O(n)

When you have a switch statement with consecutive integer cases, the compiler compiles an internal array of code memory addresses called a Jump Table:

Jump Table Assembly Translation

Jump_Table:
    .quad Case_0_Address
    .quad Case_1_Address
    .quad Case_2_Address
    .quad Case_3_Address

# CPU executes: jmp *Jump_Table(,%rax,8) -> O(1) Instant direct branch!

4. Intentional Fallthrough 4 / 4

Intentional Fallthrough & C23 [[fallthrough]]

Grouping common cases together cleanly

fallthrough_c23.c

switch (letter) {
    case 'a':
    case 'e':
    case 'i':
    case 'o':
    case 'u':
        printf("Vowel\\n");
        break;
    default:
        printf("Consonant\\n");
        break;
}

Module 06: Control Flow & Decision Making Structures

Conditional Branching & Dangling Else

switch-case Statement Rules & Fall-Through

How Compilers Optimize switch: Jump Tables

Intentional Fallthrough & C23 [[fallthrough]]

1. Choosing between if-else and switch-case

2. Practice Exercises & Solutions