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HSSLIVE Plus One Computer Science Chapter 7: Control Statements Notes

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Control statements provide the logic and decision-making capabilities that allow programs to respond differently based on conditions and to repeat operations. This chapter covers the three fundamental control structures: sequence, selection (if-else constructs and switch statements), and iteration (for, while, and do-while loops). It demonstrates how these structures can be combined and nested to implement complex algorithms, with emphasis on loop design patterns, loop invariants, and techniques for ensuring proper loop termination.

Chapter 7: Control Statements

Control statements direct the flow of program execution based on conditions or for repetitive tasks. They are fundamental building blocks that enable programs to make decisions and repeat actions. C++ provides three main types of control statements: selection (decision-making), iteration (loops), and jump statements.

Selection Statements

Selection statements allow a program to execute different code blocks based on certain conditions.

if Statement

The if statement executes a block of code if a specified condition is true.

Syntax:

cpp

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

Example:

cpp

int age = 18;
if (age >= 18) {
    cout << "You are eligible to vote." << endl;
}

if-else Statement

The if-else statement provides an alternative block of code to execute when the condition is false.

Syntax:

cpp

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

Example:

cpp

int number = 15;
if (number % 2 == 0) {
    cout << number << " is even." << endl;
} else {
    cout << number << " is odd." << endl;
}

if-else if-else Statement

This structure allows checking multiple conditions in sequence.

Syntax:

cpp

if (condition1) {
    // Code for condition1 true
} else if (condition2) {
    // Code for condition2 true
} else if (condition3) {
    // Code for condition3 true
} else {
    // Code for all conditions false
}

Example:

cpp

int marks = 75;

if (marks >= 90) {
    cout << "Grade: A+" << endl;
} else if (marks >= 80) {
    cout << "Grade: A" << endl;
} else if (marks >= 70) {
    cout << "Grade: B+" << endl;
} else if (marks >= 60) {
    cout << "Grade: B" << endl;
} else {
    cout << "Grade: C" << endl;
}

Nested if Statements

An if statement can be placed inside another if or else block.

Example:

cpp

int age = 25;
bool hasID = true;

if (age >= 18) {
    if (hasID) {
        cout << "You can enter the venue." << endl;
    } else {
        cout << "You need to show ID to enter." << endl;
    }
} else {
    cout << "You must be 18 or older to enter." << endl;
}

switch Statement

The switch statement selects one of many code blocks to execute based on the value of an expression.

Syntax:

cpp

switch (expression) {
    case value1:
        // Code for value1
        break;
    case value2:
        // Code for value2
        break;
    // More cases...
    default:
        // Code for when no cases match
}

Example:

cpp

int day = 3;
switch (day) {
    case 1:
        cout << "Monday" << endl;
        break;
    case 2:
        cout << "Tuesday" << endl;
        break;
    case 3:
        cout << "Wednesday" << endl;
        break;
    case 4:
        cout << "Thursday" << endl;
        break;
    case 5:
        cout << "Friday" << endl;
        break;
    case 6:
        cout << "Saturday" << endl;
        break;
    case 7:
        cout << "Sunday" << endl;
        break;
    default:
        cout << "Invalid day number" << endl;
}

Iteration Statements (Loops)

Loops allow a program to execute a block of code repeatedly.

for Loop

The for loop is typically used when the number of iterations is known in advance.

Syntax:

cpp

for (initialization; condition; update) {
    // Code to repeat
}

Example:

cpp

// Print numbers from 1 to 5
for (int i = 1; i <= 5; i++) {
    cout << i << " ";
}
// Output: 1 2 3 4 5

while Loop

The while loop continues executing as long as a specified condition is true.

Syntax:

cpp

while (condition) {
    // Code to repeat
}

Example:

cpp

// Print numbers from 1 to 5
int i = 1;
while (i <= 5) {
    cout << i << " ";
    i++;
}
// Output: 1 2 3 4 5

do-while Loop

The do-while loop is similar to the while loop, but it guarantees that the code block is executed at least once before checking the condition.

Syntax:

cpp

do {
    // Code to repeat
} while (condition);

Example:

cpp

// Print numbers from 1 to 5
int i = 1;
do {
    cout << i << " ";
    i++;
} while (i <= 5);
// Output: 1 2 3 4 5

Nested Loops

Loops can be placed inside other loops to handle multi-dimensional data or complex repetitive tasks.

Example:

cpp

// Print a 3x3 multiplication table
for (int i = 1; i <= 3; i++) {
    for (int j = 1; j <= 3; j++) {
        cout << i * j << "\t";
    }
    cout << endl;
}

Output:

1    2    3
2    4    6
3    6    9

Jump Statements

Jump statements alter the normal flow of program execution.

break Statement

The break statement terminates the innermost loop or switch statement.

Example in a loop:

cpp

// Print numbers from 1 to 10, but stop at 5
for (int i = 1; i <= 10; i++) {
    if (i > 5) {
        break;
    }
    cout << i << " ";
}
// Output: 1 2 3 4 5

continue Statement

The continue statement skips the rest of the current iteration and proceeds to the next iteration of the loop.

Example:

cpp

// Print odd numbers from 1 to 10
for (int i = 1; i <= 10; i++) {
    if (i % 2 == 0) {
        continue;  // Skip even numbers
    }
    cout << i << " ";
}
// Output: 1 3 5 7 9

goto Statement

The goto statement transfers control to a labeled statement within the same function. (Note: Generally discouraged in modern programming due to potential for creating “spaghetti code”)

Example:

cpp

int i = 1;
start:
    if (i <= 5) {
        cout << i << " ";
        i++;
        goto start;
    }
// Output: 1 2 3 4 5

return Statement

The return statement exits the current function and optionally returns a value.

Example:

cpp

int getMax(int a, int b) {
    if (a > b) {
        return a;
    } else {
        return b;
    }
}

Practical Applications of Control Statements

Example 1: Finding the Sum of First n Natural Numbers

cpp

#include <iostream>
using namespace std;

int main() {
    int n, sum = 0;
    
    cout << "Enter a positive integer: ";
    cin >> n;
    
    for (int i = 1; i <= n; i++) {
        sum += i;
    }
    
    cout << "Sum of first " << n << " natural numbers is: " << sum << endl;
    
    return 0;
}

Example 2: Menu-Driven Program

cpp

#include <iostream>
using namespace std;

int main() {
    int choice;
    bool running = true;
    
    while (running) {
        cout << "\nMENU\n";
        cout << "1. Add two numbers\n";
        cout << "2. Check if a number is prime\n";
        cout << "3. Check if a number is even or odd\n";
        cout << "4. Exit\n";
        cout << "Enter your choice: ";
        cin >> choice;
        
        switch (choice) {
            case 1: {
                int a, b;
                cout << "Enter two numbers: ";
                cin >> a >> b;
                cout << "Sum: " << a + b << endl;
                break;
            }
            case 2: {
                int num, i;
                bool isPrime = true;
                
                cout << "Enter a positive integer: ";
                cin >> num;
                
                if (num <= 1) {
                    isPrime = false;
                } else {
                    for (i = 2; i <= num / 2; i++) {
                        if (num % i == 0) {
                            isPrime = false;
                            break;
                        }
                    }
                }
                
                if (isPrime) {
                    cout << num << " is a prime number." << endl;
                } else {
                    cout << num << " is not a prime number." << endl;
                }
                break;
            }
            case 3: {
                int num;
                cout << "Enter an integer: ";
                cin >> num;
                
                if (num % 2 == 0) {
                    cout << num << " is even." << endl;
                } else {
                    cout << num << " is odd." << endl;
                }
                break;
            }
            case 4:
                running = false;
                cout << "Exiting program. Goodbye!" << endl;
                break;
            default:
                cout << "Invalid choice. Please try again." << endl;
        }
    }
    
    return 0;
}

Example 3: Pattern Printing

cpp

#include <iostream>
using namespace std;

int main() {
    int rows;
    
    cout << "Enter number of rows: ";
    cin >> rows;
    
    for (int i = 1; i <= rows; i++) {
        // Print spaces
        for (int j = 1; j <= rows - i; j++) {
            cout << " ";
        }
        
        // Print stars
        for (int k = 1; k <= 2 * i - 1; k++) {
            cout << "*";
        }
        
        cout << endl;
    }
    
    return 0;
}

Output for rows = 5:

    *
   ***
  *****
 *******
*********

Understanding control statements is crucial for developing efficient and effective programs. They enable programs to make decisions, repeat tasks, and control the flow of execution, allowing for the implementation of complex algorithms and solutions to various problems.

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