Conditional Statements and Loops

If-Else Statement

Used to check Boolean conditions (True or False).

Syntax:

if (boolean expression){
    // Body
} else {
    // Do this
}

Example:

public class IfElse {
    public static void main(String[] args) {
        int salary = 25400;
        if (salary > 10000) {
            salary = salary + 2000;
        } else {
            salary = salary + 1000;
        }
        System.out.println(salary);
    }
}

Output: 27400

Multiple If-Else Statement

Executes one condition from multiple statements.

Syntax:

if (condition1) {
    // Code to be executed if condition1 is true
} else if (condition2) {
    // Code to be executed if condition2 is true
} else if (condition3) {
    // Code to be executed if condition3 is true
} else {
    // Code to be executed if all conditions are false
}

Example:

public class MultipleIfElse {
    public static void main(String[] args) {
        int salary = 25400;
        if (salary <= 10000) {
            salary += 1000;
        } else if (salary <= 20000) {
            salary += 2000;
        } else {
            salary += 3000;
        }
        System.out.println(salary);
    }
}

Output: 28400

Loops

For Loop:

Used when the number of iterations is known.

Syntax:

for (initialization; condition; increment/decrement){
    // Body
}

Example 1:

public class ForLoop {
    public static void main(String[] args) {
        for (int num = 1; num <= 5; num += 1){
            System.out.println(num);
        }
    }
}

Output: 1, 2, 3, 4, 5

Example 2:

import java.util.Scanner;
public class ForLoop {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        for (int num = 1; num <= n; num += 1){
            System.out.print(num + " ");
        }
    }
}

Input: 6
Output: 1 2 3 4 5 6

While Loop:

Used when the number of iterations is not known.

Syntax:

while (condition){
    // Code to be executed
    // Increment/Decrement
}

Example:

public class WhileLoop {
    public static void main(String[] args) {
        int num = 1;
        while (num <= 5){
            System.out.println(num);
            num += 1;
        }
    }
}

Output: 1, 2, 3, 4, 5

Do-While Loop:

Used when the loop needs to execute at least once.
Exit-controlled loop.

Syntax:

do {
    // Code to be executed
    // Update statement (increment/decrement)
} while (condition);

Example:

public class DoWhileLoop {
    public static void main(String[] args) {
        int n = 1;
        do {
            System.out.println(n);
            n++;
        } while (n <= 5);
    }
}

Output: 1, 2, 3, 4, 5

Comparison between While Loop and Do-While Loop:

While Loop

Do-While Loop

Used when the number of iterations is not fixed

Used when we want to execute the statement at least once

Entry-controlled loop

Exit-controlled loop

No semicolon required at the end of while (condition)

Semicolon is required at the end of while (condition)

Programs

🎯 Program: Largest of Three Numbers

Problem Statement: "Find largest among three numbers."

Approach 1:

import java.util.Scanner;
public class LargestOfThree {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int a = in.nextInt();
        int b = in.nextInt();
        int c = in.nextInt();
        int max = a;
        if (b > max) {
            max = b;
        }
        if (c > max) {
            max = c;
        }
        System.out.println(max);
    }
}

Approach 2:

import java.util.Scanner;
public class LargestOfThree {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int a = in.nextInt();
        int b = in.nextInt();
        int c = in.nextInt();
        int max = 0;
        if (a > b) {
            max = a;
        } else {
            max = b;
        }
        if (c > max) {
            max = c;
        }
        System.out.println(max);
    }
}

Approach 3 (Using Math.max):

import java.util.Scanner;
public class LargestOfThree {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int a = in.nextInt();
        int b = in.nextInt();
        int c = in.nextInt();
        int max = Math.max(c, Math.max(a, b));
        System.out.println(max);
    }
}

Input: 3 6 5
Output: 6

🎯 Program: Alphabet Case Check

Problem Statement: "Take an input character from the keyboard and check whether it is an uppercase alphabet or lowercase alphabet."

Example:

import java.util.Scanner;
public class AlphabetCaseCheck {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        char ch = in.next().trim().charAt(0);
        if (ch >= 'a' && ch <= 'z') {
            System.out.println("Lowercase");
        } else {
            System.out.println("Uppercase");
        }
    }
}

Input: a
Output: Lowercase
Input: Z
Output: Uppercase

🎯 Program: Fibonacci Numbers

Problem Statement: "Find the nth Fibonacci number."

Example:

import java.util.Scanner;
public class FibonacciNumbers {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int a = 0, b = 1, count = 2;
        while (count <= n) {
            int temp = b;
            b = b + a;
            a = temp;
            count++;
        }
        System.out.println(b);
    }
}

Input: 7
Output: 13

🎯 Program: Counting Occurrence

Problem Statement: "Input two numbers, find how many times the second number's digit is present in the first number."

Example:

import java.util.Scanner;
public class CountingOccurrence {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int count = 0;
        int Fn = in.nextInt();
        int Sn = in.nextInt();
        while (Fn > 0) {
            int rem = Fn % 10;
            if (rem == Sn) {
                count++;
            }
            Fn = Fn / 10;
        }
        System.out.println(count);
    }
}

Input: 45535 5
Output: 3

🎯 Program: Reverse a Number

Problem Statement: "Input a number from the keyboard and show the output as the reverse of that number."

Example:

import java.util.Scanner;
public class ReverseANumber {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int num = in.nextInt();
        int reversedNum = 0;
        while (num > 0) {
            int rem = num % 10;
            num /= 10;
            reversedNum = reversedNum * 10 + rem;
        }
        System.out.println(reversedNum);
    }
}

Input: 458792
Output: 297854

🎯 Program: Calculator Program

Example of a simple calculator program.

Input and output are managed until the user presses 'X' or 'x'.

import java.util.Scanner;
public class Calculator {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int ans = 0;
        while (true) {
            System.out.print("Enter the operator: ");
            char op = in.next().trim().charAt(0);
            if (op == '+' || op == '-' || op == '*' || op == '/' || op == '%') {
                System.out.print("Enter two numbers: ");
                int num1 = in.nextInt();
                int num2 = in.nextInt();
                switch (op) {
                    case '+':
                        ans = num1 + num2;
                        break;
                    case '-':
                        ans = num1 - num2;
                        break;
                    case '*':
                        ans = num1 * num2;
                        break;
                    case '/':
                        if (num2 != 0) {
                            ans = num1 / num2;
                        }
                        break;
                    case '%':
                        ans = num1 % num2;
                        break;
                }
            } else if (op == 'x' || op == 'X') {
                break;
            } else {
                System.out.println("Invalid operation!!");
            }
            System.out.println(ans);
        }
    }
}

Conclusion

To wrap up, diving into Conditional Statements and Loops equips you with essential tools for effective programming. The covered topics, from basic If-Else statements to versatile loops, offer practical solutions for various scenarios. The hands-on examples, like finding the largest number or building a basic calculator, reinforce your skills and prepare you for real-world coding challenges. In a nutshell, this learning journey sets a strong foundation for making informed decisions and handling repetitive tasks in Java programming.

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