Input/Output & Control Structures in C: Functions, Decisions & Loops

TAKEAWAYS:

• Master 8 core I/O functions and their syntax, including formatted (scanf, printf) and character-level (getchar, putchar) operations.
• Understand 3 control structures (selection, iteration, branching) and their use cases, with visual flowcharts for each.
• Apply nested conditions and switch-case for multi-way branching, avoiding common pitfalls like missing break.
• Trace loop execution (for, while, do-while) step-by-step using variable tables, including infinite-loop detection.
• Solve real problems like menu-driven calculators (eSewa-style), password validation (Khalti), and transaction logs (NTC billing).
• Debug logical errors in control flow using printf debugging and rubber-ducking techniques.

1. Input/Output Functions in C

Input/Output (I/O) functions allow programs to interact with users or external devices. C provides standard library functions in <stdio.h> for this purpose.

1.1 Key I/O Functions

1.2 Worked Example: Menu-Driven Calculator (eSewa-Style)

Problem: Write a program that mimics an eSewa payment menu, where the user selects an operation (add, subtract, multiply, divide) and inputs two numbers.

#include <stdio.h>
int main() {
    int choice, num1, num2, result;
    printf("eSewa Payment Menu:\n");
    printf("1. Add\n2. Subtract\n3. Multiply\n4. Divide\n");
    printf("Enter choice (1-4): ");
    scanf("%d", &choice);
    printf("Enter two numbers: ");
    scanf("%d %d", &num1, &num2);

    switch(choice) {
        case 1: result = num1 + num2; break;
        case 2: result = num1 - num2; break;
        case 3: result = num1 * num2; break;
        case 4:
            if (num2 != 0) result = num1 / num2;
            else { printf("Error: Division by zero!\n"); return 1; }
            break;
        default: printf("Invalid choice!\n"); return 1;
    }
    printf("Result: %d\n", result);
    return 0;
}

Trace Table for choice = 3, num1 = 5, num2 = 4:

Visual Flowchart:

flowchart TD
    A["Start"] --> B["Display Menu"]
    B --> C["Read choice"]
    C --> D{"choice == 1?"}
    D -->|"Yes"| E["Add: result = num1 + num2"]
    D -->|"No"| F{"choice == 2?"}
    F -->|"Yes"| G["Subtract: result = num1 - num2"]
    F -->|"No"| H{"choice == 3?"}
    H -->|"Yes"| I["Multiply: result = num1 * num2"]
    H -->|"No"| J{"choice == 4?"}
    J -->|"Yes"| K["Divide: result = num1 / num2"]
    J -->|"No"| L["Invalid choice"]
    E --> M["Print result"]
    G --> M
    I --> M
    K --> M
    L --> N["End"]
    M --> N

2. Control Structures

Control structures dictate the flow of execution in a program. They include:

1. Selection (if, else if, else, switch)
2. Iteration (for, while, do-while)
3. Branching (break, continue, goto)

2.1 Selection Structures

2.1.1 if-else Ladder

Used for multi-way decisions. Example: Khalti transaction validation (check if balance ≥ amount).

#include <stdio.h>
int main() {
    float balance = 5000, amount;
    printf("Enter amount to transfer: ");
    scanf("%f", &amount);
    if (amount > balance) {
        printf("Insufficient funds!\n");
    } else if (amount <= 0) {
        printf("Invalid amount!\n");
    } else {
        balance -= amount;
        printf("Transfer successful. New balance: %.2f\n", balance);
    }
    return 0;
}

Visual:

flowchart TD
    A["Start"] --> B["Read amount"]
    B --> C{"amount > balance?"}
    C -->|"Yes"| D["Print 'Insufficient funds'"]
    C -->|"No"| E{"amount <= 0?"}
    E -->|"Yes"| F["Print 'Invalid amount'"]
    E -->|"No"| G["Update balance"]
    G --> H["Print success message"]
    D --> I["End"]
    F --> I
    H --> I

2.1.2 switch-case

Used for discrete choices (e.g., menu options). Example: NTC billing system (monthly plans).

#include <stdio.h>
int main() {
    int plan;
    printf("NTC Monthly Plans:\n1. Basic (Rs. 500)\n2. Premium (Rs. 1000)\n3. Unlimited (Rs. 1500)\n");
    printf("Choose plan (1-3): ");
    scanf("%d", &plan);
    switch(plan) {
        case 1: printf("Billed: Rs. 500\n"); break;
        case 2: printf("Billed: Rs. 1000\n"); break;
        case 3: printf("Billed: Rs. 1500\n"); break;
        default: printf("Invalid plan!\n");
    }
    return 0;
}

Common Pitfalls:

• Missing break: Causes fall-through (executes all subsequent cases).

  switch(x) {
      case 1: printf("One"); // Missing break → executes case 2 too!
      case 2: printf("Two");
  }
  

• Default case: Should handle invalid inputs.

2.2 Iteration Structures

Loops repeat a block of code until a condition is met.

2.2.1 for Loop

Syntax:

for (initialization; condition; increment) {
    // Loop body
}

Example: Print Ncell call logs (first 10 calls).

#include <stdio.h>
int main() {
    for (int i = 1; i <= 10; i++) {
        printf("Call %d: Duration %.1f minutes\n", i, (float)i * 2.5);
    }
    return 0;
}

Trace Table:

2.2.2 while Loop

Syntax:

while (condition) {
    // Loop body
}

Example: Pathao driver earnings (calculate total until target is met).

#include <stdio.h>
int main() {
    float target = 500, earnings = 0, fare;
    while (earnings < target) {
        printf("Enter fare earned: ");
        scanf("%f", &fare);
        earnings += fare;
    }
    printf("Target achieved! Total earnings: %.2f\n", earnings);
    return 0;
}

Visual:

flowchart TD
    A["Start"] --> B["Initialize earnings = 0"]
    B --> C{"earnings < target?"}
    C -->|"Yes"| D["Read fare"]
    D --> E["earnings += fare"]
    E --> C
    C -->|"No"| F["Print success message"]
    F --> G["End"]

2.2.3 do-while Loop

Executes at least once. Example: Password validation (retry until correct).

#include <stdio.h>
#include <string.h>
int main() {
    char password[20], input[20];
    strcpy(password, "secure123");
    do {
        printf("Enter password: ");
        scanf("%s", input);
        if (strcmp(input, password) != 0) {
            printf("Wrong password! Try again.\n");
        }
    } while (strcmp(input, password) != 0);
    printf("Access granted!\n");
    return 0;
}

Trace Table:

3. Branching Statements

Example: Daraz Order Processing (Skip Out-of-Stock Items)

#include <stdio.h>
int main() {
    int items[5] = {10, 0, 5, 0, 8}; // 0 = out of stock
    for (int i = 0; i < 5; i++) {
        if (items[i] == 0) {
            printf("Item %d: Out of stock. Skipping...\n", i+1);
            continue; // Skip to next iteration
        }
        printf("Processing item %d\n", i+1);
    }
    return 0;
}

Output:

Item 1: Out of stock. Skipping...
Processing item 2
Processing item 3
Item 4: Out of stock. Skipping...
Processing item 5

## In the Real World

1. eSewa/Khalti:

   • Idea Used: switch-case for menu-driven payment options (e.g., mobile recharge, bill payment, transfer).
   • How: The app presents a list of services (like case 1: recharge, case 2: pay bill), and the user’s choice triggers the corresponding logic.

2. NTC Billing System:

   • Idea Used: if-else ladder to validate SIM cards and apply different tariffs (e.g., if (plan == "Basic") { charge = 500; }).
   • How: The system checks the user’s plan type and calculates the bill accordingly, with error handling for invalid inputs.

3. Pathao Driver App:

   • Idea Used: while loop to accumulate earnings until the driver reaches their daily target.
   • How: The app continuously updates the driver’s earnings after each ride (earnings += fare) and stops prompting for new rides once the target is met.

4. Bank Loan Interest Calculation:

   • Idea Used: for loop to compute compound interest over n years.
   • Example Code:

     #include <stdio.h>
     #include <math.h>
     int main() {
         float principal = 100000, rate = 0.05, amount;
         int years;
         printf("Enter loan duration (years): ");
         scanf("%d", &years);
         for (int i = 1; i <= years; i++) {
             amount = principal * pow(1 + rate, i);
             printf("Year %d: Amount = %.2f\n", i, amount);
         }
         return 0;
     }
     

   • Trace for years = 3:

     Year	amount Calculation	Output
     1	100000 * (1.05)^1 = 105000.00	Year 1: Amount = 105000.00
     2	100000 * (1.05)^2 = 110250.00	Year 2: Amount = 110250.00
     3	100000 * (1.05)^3 = 115762.50	Year 3: Amount = 115762.50

## Exam Tip

1. I/O Functions:

   • Always include <stdio.h> for I/O functions.
   • Use %d for integers, %f for floats, %c for characters, and %s for strings in scanf/printf.
   • Common Mistake: Forgetting the & before variables in scanf (e.g., scanf("%d", &x)).

2. Control Structures:

   • if-else: Use else if for multiple conditions. Example:

     if (marks >= 80) printf("A");
     else if (marks >= 60) printf("B");
     else printf("C");
     

   • switch: Ensure every case ends with break unless fall-through is intentional.
   • Loops: Initialize loop variables correctly. Example:

     // Wrong: for (int i; i < 5; i++) → Error!
     // Correct: for (int i = 0; i < 5; i++)
     

3. Debugging:

   • Use printf to print variable values during execution (e.g., printf("i = %d\n", i);).
   • Infinite Loop Check: Ensure loop conditions can eventually become false (e.g., while (x > 0) with x-- inside the loop).

4. Real-World Scenarios:

   • Menu Systems: Always use switch-case for clear, readable options.
   • Validation: Use if-else to validate user inputs (e.g., age ≥ 18, balance ≥ amount).
   • Repetition: Use for for known iterations (e.g., printing patterns), while for unknown iterations (e.g., user input loops).

5. Avoid goto:

   • Examiners dislike goto unless absolutely necessary (e.g., error handling in large programs). Prefer structured loops.

Practice Question: Write a program to simulate a bank ATM where:

1. The user enters a PIN (3 attempts max).
2. If correct, display a menu:
   • Check balance
   • Withdraw cash
   • Transfer money
3. Use switch-case for the menu and if-else for PIN validation. Hint: Use a while loop for PIN attempts and break to exit on success/failure.