5. Flow Control

5.1 Conditional Statements

5.1.1 if Statement

Python vs C Syntax:

Python:

if condition:
    statement1
    statement2
elif another_condition:
    statement3
else:
    statement4

C:

if (condition) {
    statement1;
    statement2;
} else if (another_condition) {
    statement3;
} else {
    statement4;
}

Key Differences:

• C requires parentheses around conditions
• C uses curly braces {} instead of indentation
• C requires semicolons after statements

5.1.2 Relational Operators

Operator	Meaning	Python	C
==	Equal to	a == b	a == b
!=	Not equal to	a != b	a != b
<	Less than	a < b	a < b
>	Greater than	a > b	a > b
<=	Less than or equal	a <= b	a <= b
>=	Greater than or equal	a >= b	a >= b

5.1.3 Logical Operators

Operator	Meaning	Python	C
&&	Logical AND	and or &	&&
||	Logical OR	or or |	||
!	Logical NOT	not or ~	!

Examples in C :

// Python: if age >= 18 and score > 80:
if (age >= 18 && score > 80) {
    printf("Eligible for scholarship\n");
}

// Python: if not (x < 0 or x > 100):
if (!(x < 0 || x > 100)) {
    printf("Valid percentage\n");
}

5.1.4 Executing Code in if Conditions

While primarily used for conditions, C allows expressions that evaluate to a non-zero value (true) or zero (false) within the parentheses. This means you can sometimes perform assignments or function calls directly within the condition, though it's often discouraged for readability.

int x = 10;
if (x = 5) { // Assigns 5 to x, then evaluates to 5 (true)
    printf("x is now 5 and this code runs.\n");
}

5.1.5 Single Statement if:

If an if or else block contains only a single statement, the curly braces {} are optional. However, it's good practice to always use them to avoid ambiguity and potential errors when adding more statements later.

if (score > 90)
    printf("Excellent!\n");
else
    printf("Keep trying.\n");

5.1.6 switch Statement

C provides switch as an alternative to multiple if-else statements:

switch (variable) {
    case value1:
        // statements
        break;
    case value2:
        // statements
        break;
    default:
        // statements
        break;
}

Example:

int grade;
printf("Enter grade (1-5): ");
scanf("%d", &grade);

switch (grade) {
    case 5:
        printf("Excellent!\n");
        break;
    case 4:
        printf("Very Good!\n");
        break;
    case 3:
        printf("Good!\n");
        break;
    case 2:
        printf("Fair!\n");
        break;
    case 1:
        printf("Poor!\n");
        break;
    default:
        printf("Invalid grade!\n");
        break;
}

Understanding break and Fall-through: In C's switch statement, the break keyword is essential. If break is omitted from a case block, execution will "fall through" to the next case block (and subsequent ones) until a break is encountered or the end of the switch statement is reached. This "fall-through" behavior can be intentionally used for specific logic, but it's a common source of bugs if not intended.

Example of Fall-through:

int day = 2; // Monday
switch (day) {
    case 1:
        printf("Weekend!\n");
        break;
    case 2:
    case 3:
    case 4:
    case 5:
        printf("Weekday.\n"); // Execution falls through from case 2, 3, 4 to 5
        break;
    case 6:
        printf("Weekend!\n");
        break;
    default:
        printf("Invalid day.\n");
        break;
}
// Output for day = 2: Weekday.

This example shows how case 2, case 3, case 4, and case 5 all execute the same printf("Weekday.\n"); because there are no break statements between them.

5.2 Iteration Statements (Loops)

5.2.1 for Loop

Python vs C Syntax:

Python:

for i in range(5):
    print(i)

for i in range(1, 10, 2):
    print(i)

C:

// Basic for loop
for (int i = 0; i < 5; i++) {
    printf("%d\n", i);
}

// Step by 2
for (int i = 1; i < 10; i += 2) {
    printf("%d\n", i);
}

For Loop Structure:

for (initialization; condition; increment/decrement) {
    // loop body
}

5.2.2 while Loop

Python vs C:

Python:

i = 0
while i < 5:
    print(i)
    i += 1

C:

int i = 0;
while (i < 5) {
    printf("%d\n", i);
    i++;
}

5.2.3 do-while Loop

C provides do-while loop (not available in Python):

int choice;
do {
    printf("Enter choice (1-3): ");
    scanf("%d", &choice);
    
    if (choice < 1 || choice > 3) {
        printf("Invalid choice! Try again.\n");
    }
} while (choice < 1 || choice > 3);

Key Difference: do-while executes the loop body at least once, even if the condition is initially false.

5.2.4 Loop Control Statements

Statement	Python	C	Purpose
break	break	break;	Exit loop immediately
continue	continue	continue;	Skip to next iteration

Example:

for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
        continue;  // Skip even numbers
    }
    if (i > 7) {
        break;     // Stop when i > 7
    }
    printf("%d ", i);  // Output: 1 3 5 7
}