Statement Categories

C programs consist of several statement types:

1. Expression statements
2. Function call statements
3. Control flow statements
4. Compound statements (blocks)
5. Null statements

Control flow statements include decision-making (branching) and iteration (looping) constructs. Branching relies on if and switch, while loops are implemented via while, do-while, and for.

Branching with if

The if keyword evaluates a condition. In C, zero represents false, and any non-zero value represenst true.

if (condition)
    single_statement;

if (condition)
    true_branch;
else
    false_branch;

// Multiple conditions
if (cond1)
    action1;
else if (cond2)
    action2;
else
    default_action;

Example of boolean evaluaiton:

#include <stdio.h>

int main() {
    if (0) printf("Visible?\n"); // Skipped
    if (1) printf("Always prints\n"); // Executed

    int x = 10, y = 20;
    if (x > y) {
        printf("x is larger\n");
    } else {
        printf("y is larger or equal\n");
    }
    return 0;
}

Logical operators:

• && (AND): Result is true only if both operands are true.
• || (OR): Result is true if at least one operand is true.

Example using input and logic:

#include <stdio.h>

int main() {
    int num;
    scanf("%d", &num);

    if (num > 0 && num < 50) {
        if (num % 2 == 0) {
            printf("%d is even\n", num);
        } else {
            printf("%d is odd\n", num);
        }
    }
    return 0;
}

switch Statement

The switch statement allows multi-way branching based on an integer value. The break keyword exits the switch block; without it, execution falls through to subsequent cases. The default label handles unmatched values.

#include <stdio.h>

int main() {
    int choice;
    scanf("%d", &choice);

    switch (choice) {
        case 1:
            printf("Option A\n");
            break;
        case 2:
            printf("Option B\n");
            break;
        case 3:
            printf("Option C\n");
            break;
        default:
            printf("Invalid selection\n");
            break;
    }
    return 0;
}

Nested switch example:

int m = 1, n = 5;
switch (m) {
    case 1:
        n += 2; // n becomes 7
        // no break, fall through
    case 2:
        m += 1; // m becomes 2
        switch (m) {
            case 2:
                n++; // n becomes 8
                m++; // m becomes 3
                break;
        }
        // no break
    case 3:
        n += 1; // n becomes 9
        break;
}
// Final: m = 3, n = 9

Loop Constructs

while Loop

while (condition) {
    // code block
}

• break: Exits the loop immediately.
• continue: Skips the rest of the current iteration and re-evaluates the condition.

Printing even numbers up to 20:

int val = 0;
while (val <= 20) {
    if (val % 2 == 0) {
        printf("%d ", val);
    }
    val++;
}

for Loop

for (initialization; test_condition; update) {
    // loop body
}

• Initialization: Executed once at the start.
• Test condition: Checked before each iteration.
• Update: Executed after each iteration.

Nested loop pitfall (missing re-initialization):

int i, j;
for (i = 0; i < 2; i++) {
    for (j = 0; j < 3; j++) {
        printf("Hi ");
    }
    // j remains 3 here; if the inner loop lacked initialization,
    // it would not run again after the first outer iteration.
}

do-while Loop

Executes the body at least once before checking the condition.

int counter = 1;
do {
    if (counter == 3) {
        counter++;
        continue; // Skips printf for 3
    }
    printf("%d ", counter);
    counter++;
} while (counter <= 5);

Using break to exit early:

int k = 0;
do {
    if (k == 4) break;
    printf("%d ", k);
    k++;
} while (k < 10);