Control Structures (Selection): if-else, switch, nested conditions

TAKEAWAYS:

• Understand how if, else-if, and else execute based on boolean conditions.
• Master the switch-case structure for multi-way branching and its limitations.
• Learn to combine conditions using logical operators (&&, ||, !) for complex decisions.
• Visualize how nested conditions work with real-world examples (e.g., bank loan eligibility).
• Avoid common pitfalls like missing break in switch or redundant conditions.
• Apply selection structures to solve problems like menu-driven programs or grade calculators.

1. Introduction to Selection Structures

Selection structures allow a program to execute different blocks of code based on a condition. In C, the primary selection structures are:

• if statement: Executes a block if a condition is true.
• if-else statement: Executes one block if true, another if false.
• else-if (ladder): Checks multiple conditions sequentially.
• switch-case: Efficient for multi-way branching (e.g., menu-driven programs).

Why use selection? Without selection, programs would execute linearly, making them inflexible. For example:

• A bank approves a loan only if the applicant’s credit score > 600 and income > 50,000.
• A traffic light changes color based on sensor input.

2. if Statement

Syntax

if (condition) {
    // Code to execute if condition is true
}

• The condition must evaluate to true (non-zero) or false (zero).
• The block can have one or more statements (enclosed in {}).

Example: Check Even/Odd

#include <stdio.h>
int main() {
    int num = 7;
    if (num % 2 == 0) {
        printf("%d is even.", num);
    }
    return 0;
}

Output:

7 is not even. (No output, since condition is false)

Visualization: Flow of if

flowchart TD
    A["Start"] --> B["Check condition (num % 2 == 0)?"]
    B -->|"True"| C["Print 'even'"]
    B -->|"False"| D["End"]
    C --> D

Real-World Example: Loan Approval (Nepal Bank)

• Scenario: A bank approves a loan if the applicant’s income > 50,000.
• Code:

  float income = 45000;
  if (income > 50000) {
      printf("Loan approved!\n");
  }
  

• Output: No output (loan denied).

3. if-else Statement

Syntax

if (condition) {
    // True block
} else {
    // False block
}

• Executes the else block only if the condition is false.

Example: Positive/Negative Check

int num = -3;
if (num >= 0) {
    printf("%d is positive.", num);
} else {
    printf("%d is negative.", num);
}

Output:

-3 is negative.

Visualization: Flow of if-else

flowchart TD
    A["Start"] --> B["Check condition (num >= 0)?"]
    B -->|"True"| C["Print 'positive'"]
    B -->|"False"| D["Print 'negative'"]
    C --> E["End"]
    D --> E

Real-World Example: Traffic Signal Control (NTC)

• Scenario: A traffic light turns red if no vehicles are detected for 30 seconds.
• Code:

  int vehicleDetected = 0;
  if (vehicleDetected == 1) {
      printf("Keep green light.\n");
  } else {
      printf("Turn red light.\n");
  }
  

4. else-if (Ladder)

Syntax

if (condition1) {
    // Code for condition1
} else if (condition2) {
    // Code for condition2
} else {
    // Default case
}

• Checks conditions sequentially. Only the first true condition executes.

Example: Grade Calculator

int marks = 85;
if (marks >= 90) {
    printf("Grade: A");
} else if (marks >= 80) {
    printf("Grade: B");
} else if (marks >= 70) {
    printf("Grade: C");
} else {
    printf("Grade: F");
}

Output:

Grade: B

Visualization: Flow of else-if

flowchart TD
    A["Start"] --> B["marks >= 90?"]
    B -->|"True"| C["Grade A"]
    B -->|"False"| D["marks >= 80?"]
    D -->|"True"| E["Grade B"]
    D -->|"False"| F["marks >= 70?"]
    F -->|"True"| G["Grade C"]
    F -->|"False"| H["Grade F"]
    C --> I["End"]
    E --> I
    G --> I
    H --> I

Real-World Example: Daraz Order Status

• Scenario: Daraz shows different messages based on order status:
  • status = 1: "Processing"
  • status = 2: "Shipped"
  • status = 3: "Delivered"
• Code:

  int status = 2;
  if (status == 1) {
      printf("Processing\n");
  } else if (status == 2) {
      printf("Shipped\n");
  } else if (status == 3) {
      printf("Delivered\n");
  }
  

Output:

Shipped

5. Nested if-else

Syntax

if (condition1) {
    if (condition2) {
        // Nested true block
    } else {
        // Nested false block
    }
} else {
    // Outer false block
}

• Used for complex decisions (e.g., checking multiple criteria).

Example: Bank Loan Eligibility

float income = 60000;
int creditScore = 650;
if (income > 50000) {
    if (creditScore >= 600) {
        printf("Loan approved!\n");
    } else {
        printf("Low credit score. Loan denied.\n");
    }
} else {
    printf("Income too low. Loan denied.\n");
}

Output:

Loan approved!

Visualization: Nested if-else Flow

flowchart TD
    A["Start"] --> B["income > 50000?"]
    B -->|"False"| C["Income too low"]
    B -->|"True"| D["creditScore >= 600?"]
    D -->|"False"| E["Low credit score"]
    D -->|"True"| F["Loan approved"]
    C --> G["End"]
    E --> G
    F --> G

Real-World Example: Pathao Driver Bonus

• Scenario: Pathao gives a bonus if:
  1. Driver completes > 10 rides and
  2. Average rating > 4.5.
• Code:

  int rides = 12;
  float rating = 4.7;
  if (rides > 10) {
      if (rating > 4.5) {
          printf("Bonus: $20\n");
      }
  }
  

Output:

Bonus: $20

6. Logical Operators

Combine conditions using:

• && (AND): Both conditions must be true.
• || (OR): At least one condition must be true.
• ! (NOT): Inverts the condition.

Example: Login System (Khalti)

char username[20] = "user123";
char password[20] = "pass123";
char inputUser[20] = "user123";
char inputPass[20] = "wrongpass";

if (strcmp(username, inputUser) == 0 && strcmp(password, inputPass) == 0) {
    printf("Login successful!\n");
} else {
    printf("Invalid credentials.\n");
}

Output:

Invalid credentials.

Comparison Table: Logical Operators

7. switch-case Statement

Syntax

switch (expression) {
    case constant1:
        // Code for constant1
        break;
    case constant2:
        // Code for constant2
        break;
    default:
        // Default case
}

• Key Points:
  • expression must be an integer, character, or enum.
  • break prevents "fall-through" (executing next cases unintentionally).
  • default runs if no case matches.

Example: Menu-Driven Program

int choice = 2;
switch (choice) {
    case 1:
        printf("Option 1 selected\n");
        break;
    case 2:
        printf("Option 2 selected\n");
        break;
    default:
        printf("Invalid option\n");
}

Output:

Option 2 selected

Visualization: switch-case Flow

flowchart TD
    A["Start"] --> B["choice = 2"]
    B --> C["case 1?"]
    C -->|"No"| D["case 2?"]
    D -->|"Yes"| E["Option 2 selected"]
    D -->|"No"| F["default"]
    E --> G["End"]
    F --> G

Real-World Example: YouTube Video Controls

• Scenario: YouTube uses switch-case for playback controls:
  • 1: Play
  • 2: Pause
  • 3: Stop
• Code:

  int control = 2;
  switch (control) {
      case 1: printf("Playing...\n"); break;
      case 2: printf("Paused.\n"); break;
      case 3: printf("Stopped.\n"); break;
      default: printf("Invalid control.\n");
  }
  

Output:

Paused.

Common Pitfalls

1. Missing break: Causes fall-through.

   switch (x) {
       case 1: printf("One"); // No break!
       case 2: printf("Two"); // Executes if x=1!
   }
   

2. Non-integer expressions: Compile-time error.

   switch (3.14) { // Error: float not allowed
   

8. Ternary Operator (?:)

A shorthand for simple if-else:

condition ? expr1 : expr2;

• If condition is true, expr1 executes; else, expr2.

Example: Maximum of Two Numbers

int a = 5, b = 10;
int max = (a > b) ? a : b;
printf("Max: %d", max); // Output: 10

9. Comparison with Other Languages

10. Advantages and Disadvantages

In the Real World

1. eSewa (Nepali Digital Payment)

   • Uses if-else to validate transactions:

     if (balance >= amount) {
         deductBalance(amount);
         sendMoney(receiver);
     } else {
         showError("Insufficient funds");
     }
     

   • How it works: Checks if the user’s balance is sufficient before processing a payment.

2. Pathao (Ride-Hailing App)

   • Uses switch-case for driver actions:

     switch (driverAction) {
         case ACCEPT_RIDE: startNavigation(); break;
         case REJECT_RIDE: notifyUser(); break;
         default: logError();
     }
     

   • How it works: Drivers select options (accept/reject), and the app responds accordingly.

3. Nepal Electricity Authority (NEA) Billing

   • Uses nested if-else to calculate bills:

     if (units <= 100) {
         charge = units * 2.5;
     } else if (units <= 200) {
         charge = 100*2.5 + (units-100)*4.0;
     }
     

   • How it works: Different tariffs apply based on usage tiers.

Exam Tip

1. Syntax Matters: Always include {} even for single statements in if-else.

   • ❌ Wrong: if (x > 0) printf("Positive");
   • ✅ Correct: if (x > 0) { printf("Positive"); }

2. Trace Conditions: For questions like "What is the output?", trace each condition step-by-step.

   • Example:

     int x = 1;
     if (x == 1) printf("One");
     else if (x == 2) printf("Two");
     

     Output: One (only the first true condition executes).

3. Avoid Redundancy: Combine conditions logically:

   • ❌ Redundant: if (x > 0 && x < 10) { ... }
   • ✅ Better: if (x > 0 && x < 10) { ... } (same, but ensure no overlapping cases in switch).

4. Practice Common Patterns:

   • Grade calculators.
   • Menu-driven programs.
   • Loan/insurance eligibility checks.

5. Watch for Edge Cases:

   • What if switch has no default?
   • What if if conditions are always true/false?

Final Note: Selection structures are the backbone of decision-making in programs. Master them by practicing with real-world scenarios (e.g., banking, traffic systems, e-commerce) and always visualize the flow of execution.