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.