Control Structures in C

Control structures (also called decision control statements) let a C program choose what to do next based on conditions, and repeat actions using loops. They are essential for building real programs that respond to user input, data, and changing situations.

C control flow mainly includes:

• Decision making: if, if-else, nested if, else-if ladder, switch, ternary ?:
• Loops: for, while, do-while
• Jump statements: break, continue, goto, return

---

1) Why Do We Need Conditional Statements?

In programming, you often need logic like:

• If marks ≥ 40 → pass, else fail
• If age ≥ 18 → eligible, else not
• If choice is 1/2/3 → do different actions

Conditional statements help you implement that decision-making cleanly.

---

2) Decision Making in C

2.1 if Statement

Executes a block only if the condition is true.

Syntax

if (condition) {
    // executes if condition is true
}

Example

#include <stdio.h>

int main() {
    int x = 10;

    if (x > 5) {
        printf("x is greater than 5\n");
    }

    printf("Program continues...\n");
    return 0;
}

---

2.2 if-else Statement

Provides two paths:

• if condition is true → run if block
• else → run else block

Syntax

if (condition) {
    // true block
} else {
    // false block
}

Example (Even/Odd)

#include <stdio.h>

int main() {
    int num = 4956;

    if (num % 2 == 0) {
        printf("%d is Even\n", num);
    } else {
        printf("%d is Odd\n", num);
    }

    return 0;
}

---

2.3 Nested if / Nested if-else

An if (or if-else) inside another if block for more complex decisions.

Syntax

if (condition1) {
    if (condition2) {
        // both true
    } else {
        // condition1 true, condition2 false
    }
} else {
    // condition1 false
}

Example

#include <stdio.h>

int main() {
    int x = 15;

    if (x > 10) {
        if (x < 20) {
            printf("x is between 10 and 20\n");
        } else {
            printf("x is 20 or more\n");
        }
    } else {
        printf("x is 10 or less\n");
    }

    return 0;
}

---

2.4 if - else if - else Ladder

Used when you have multiple conditions to test in order.
The first true condition runs, and the rest are skipped.

Syntax

if (condition1) {
    // ...
} else if (condition2) {
    // ...
} else if (condition3) {
    // ...
} else {
    // none matched
}

Example (Positive/Negative/Zero)

#include <stdio.h>

int main() {
    int number = 0;

    if (number > 0) {
        printf("The number is Positive\n");
    } else if (number < 0) {
        printf("The number is Negative\n");
    } else {
        printf("The number is Zero\n");
    }

    return 0;
}

✅ Tip: Ladder is different from nested if because it checks conditions sequentially at the same level.

---

2.5 switch Statement

Best when you compare one expression against many constant values.

Syntax

switch (expression) {
    case value1:
        // ...
        break;
    case value2:
        // ...
        break;
    default:
        // ...
}

Example

#include <stdio.h>

int main() {
    int day = 3;

    switch (day) {
        case 1: printf("Sunday"); break;
        case 2: printf("Monday"); break;
        case 3: printf("Tuesday"); break;
        default: printf("Invalid day");
    }

    return 0;
}

Important: If you forget break, C will fall through into the next case.

---

2.6 Ternary Operator ?:

Short form of if-else when assigning/returning a value.

Syntax

condition ? expr_if_true : expr_if_false;

Example

#include <stdio.h>

int main() {
    int x = 5;
    int result = (x > 0) ? 1 : 0;
    printf("Result: %d\n", result);
    return 0;
}

---

3) Looping Control Structures in C

Loops help repeat tasks efficiently.

3.1 for Loop

Best when number of iterations is known.

for (int i = 1; i <= 5; i++) {
    printf("HelloWorld\n");
}

---

3.2 while Loop

Runs while condition remains true (entry-controlled loop).

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

---

3.3 do-while Loop

Runs at least once because the condition is checked after the body.

#include <stdbool.h>
#include <stdio.h>

int main() {
    bool condition = false;

    do {
        printf("This is the loop body.\n");
    } while (condition);

    return 0;
}

---

4) Jump Statements (Flow Control Helpers)

• break → exits loop/switch immediately
• continue → skips to next loop iteration
• return → exits the function
• goto → jumps to a labeled statement (rarely recommended)

Example (break)

for (int i = 1; i <= 10; i++) {
    if (i == 6) break;
    printf("%d ", i);
}

Output: 1 2 3 4 5

---

Summary

• Use if for simple decisions
• Use if-else when there are two choices
• Use nested if for multi-level checks
• Use else-if ladder for multiple conditions in order
• Use switch for clean multi-option branching
• Use loops (for/while/do-while) for repetition
• Use jump statements (break/continue/return) to control execution flow