Decision Making in C: if-else, switch, nested conditions, logical operators

Core Concepts

1. Decision-Making Basics

Decision-making in programming allows a program to execute different blocks of code based on certain conditions. The two primary constructs are:

• Conditional Statements: if, else, else if
• Multi-way Branching: switch-case

How if-else Works

flowchart TD
    A["Condition"] -->|"True"| B["Execute if-block"]
    A -->|"False"| C["Execute else-block"]
    B --> D["End"]
    C --> D

Example: Loan Approval (Bank Scenario)

#include <stdio.h>
int main() {
    float income, loan;
    printf("Enter your annual income: ");
    scanf("%f", &income);
    printf("Enter loan amount: ");
    scanf("%f", &loan);

    if (income >= 50000) {
        if (loan <= 0.8 * income) {
            printf("Loan approved!\n");
        } else {
            printf("Loan denied: Exceeds 80%% of income.\n");
        }
    } else {
        printf("Loan denied: Income too low.\n");
    }
    return 0;
}

Trace Table:

2. Logical Operators

Logical operators combine multiple conditions:

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

Example: Daraz Order Status Check

#include <stdio.h>
int main() {
    int order_id, stock;
    float amount;
    printf("Enter order ID: ");
    scanf("%d", &order_id);
    printf("Enter stock available: ");
    scanf("%d", &stock);
    printf("Enter order amount: ");
    scanf("%f", &amount);

    if (order_id > 0 && stock > 0 && amount <= 10000) {
        printf("Order processed successfully!\n");
    } else {
        printf("Order failed: Invalid input.\n");
    }
    return 0;
}

Trace Table:

3. Nested if-else

Nested if-else allows for hierarchical decision-making. Use it when conditions depend on previous outcomes.

Example: NTC Electricity Bill Calculation

flowchart TD
    A["Check if units > 100"] -->|"Yes"| B["Check if units > 200"]
    A -->|"No"| C["Bill = units * 2"]
    B -->|"Yes"| D["Bill = 200*2 + (units-200)*4"]
    B -->|"No"| E["Bill = 100*2 + (units-100)*3"]

#include <stdio.h>
int main() {
    int units;
    printf("Enter electricity units: ");
    scanf("%d", &units);
    float bill;

    if (units > 200) {
        bill = 200 * 2 + (units - 200) * 4;
    } else if (units > 100) {
        bill = 100 * 2 + (units - 100) * 3;
    } else {
        bill = units * 2;
    }
    printf("Total bill: Rs. %.2f\n", bill);
    return 0;
}

Trace Table:

4. switch-case Statement

Use switch-case for multi-way branching when checking a single variable against multiple constant values.

Example: Menu-Driven Program (Khalti Payment Options)

flowchart TD
    A["Choose option"] --> B["1: UPI"]
    A --> C["2: Debit Card"]
    A --> D["3: Credit Card"]
    A --> E["4: Exit"]
    B --> F["Process UPI"]
    C --> F
    D --> F
    E --> G["End"]

#include <stdio.h>
int main() {
    int choice;
    printf("Khalti Payment Options:\n");
    printf("1. UPI\n2. Debit Card\n3. Credit Card\n4. Exit\n");
    printf("Enter your choice: ");
    scanf("%d", &choice);

    switch (choice) {
        case 1:
            printf("Processing UPI payment...\n");
            break;
        case 2:
            printf("Processing Debit Card payment...\n");
            break;
        case 3:
            printf("Processing Credit Card payment...\n");
            break;
        case 4:
            printf("Exiting...\n");
            break;
        default:
            printf("Invalid choice!\n");
    }
    return 0;
}

Trace Table:

5. Ternary Operator (?:)

A shorthand for simple if-else conditions. Syntax: condition ? expr1 : expr2

Example: Pathao Driver Availability

#include <stdio.h>
int main() {
    int drivers_available = 5;
    char status = (drivers_available > 0) ? 'A' : 'N';
    printf("Driver status: %c\n", status); // Output: A
    return 0;
}

Common Pitfalls and Best Practices

1. Missing break in switch-case: Leads to "fall-through" behavior (unintended execution of subsequent cases).

   switch (x) {
       case 1: printf("One"); // Missing break → falls to case 2
       case 2: printf("Two"); break;
   }
   

2. Uninitialized Variables: Always initialize variables before checking them.

   int age; // Uninitialized!
   if (age > 18) { ... } // Undefined behavior
   

3. Overusing Nested if-else: Can reduce readability. Refactor into functions or use switch-case where applicable.

In the Real World

1. eSewa Payment Gateway:

   • Uses if-else to validate user credentials and process payments.
   • Example: Checks if the user’s bank balance ≥ transaction amount before deducting funds.

   if (balance >= amount) {
       balance -= amount;
       printf("Payment successful!\n");
   } else {
       printf("Insufficient funds.\n");
   }
   

2. Ncell Prepaid Recharge:

   • Uses switch-case to handle different recharge denominations (e.g., Rs. 100, Rs. 500, Rs. 1000).

   switch (denomination) {
       case 100: add_units(10); break;
       case 500: add_units(50); break;
       default: printf("Invalid denomination.\n");
   }
   

3. WhatsApp Status Updates:

   • Logical operators (&&, ||) determine if a status update is visible to all contacts or only specific ones.

   if (is_public && !is_expired) {
       show_status_to_all();
   } else if (!is_public && is_friend) {
       show_status_to_friends();
   }
   

Exam Tip

1. Understand the Flow:

   • Always draw a flowchart or trace table for complex if-else or switch-case logic. Examiners love this!
   • Example: For a question like "Write a program to find the largest of three numbers," show the decision tree:

     flowchart TD
         A["a > b"] -->|"Yes"| B["a > c"]
         A -->|"No"| C["b > c"]
         B -->|"Yes"| D["a is largest"]
         B -->|"No"| E["c is largest"]
         C -->|"Yes"| F["b is largest"]
         C -->|"No"| G["c is largest"]

2. Practice with Real Scenarios:

   • Questions often tie to real-world problems (e.g., "Write a program to calculate NTC bill" or "Check if a user is eligible for a loan").
   • Always include input/output examples in your code comments.

3. Avoid Common Mistakes:

   • Off-by-one errors: E.g., if (i <= 10) vs. if (i < 10).
   • Forgotten break in switch: Leads to partial marks if not handled.
   • Incorrect operator precedence: Use parentheses to clarify logic (e.g., if ((x > 0) && (y > 0))).

4. Code Structure:

   • Indent blocks properly for readability (e.g., if body, else body).
   • Use meaningful variable names (e.g., is_eligible instead of flag).

Visual Summary:

mindmap
  root((Decision Making in C))
    if-else
      syntax["if(condition) { ... } else { ... }"]
      nested["if-else inside if-else"]
      example["Loan approval, NTC bill"]
    switch-case
      syntax["switch(var) { case 1: ... break; }"]
      fall-through["Missing break causes fall-through"]
      example["Khalti payment menu"]
    logical operators
      AND["&& (both true)"]
      OR["|| (either true)"]
      NOT["! (inverts condition)"]
      example["Daraz order validation"]
    ternary
      syntax["condition ? expr1 : expr2"]
      example["Pathao driver status"]