4.18
import java.util.Scanner;
public class LabProgram
{
public static void main(String[] args)
{

   //Create the scanner class
Scanner sc = new Scanner(System.in);

//Declare a variable userString of type string.
String userString;

//Declare a boolean variable result.
boolean result = true;


//Take the input from the user.

userString = sc.next();

//converts strings to sequence of characters.
for(char ch: userString.toCharArray())
{

//check whether the specified character is a digit.
if(!Character.isDigit(ch))
{
result = false;
}
}

if(result)
{
System.out.println("yes");
}
else
{
System.out.println("no");
}
}
}
4.19
import java.util.Scanner;

public class LabProgram {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String password = in.nextLine();
        String modified = "";
        for(int i = 0; i < password.length(); ++i) {
            if(password.charAt(i) == 'i') {
                modified += "!";
            } else if(password.charAt(i) == 'a') {
                modified += "@";
            } else if(password.charAt(i) == 'm') {
                modified += "M";
            } else if(password.charAt(i) == 'B') {
                modified += "8";
            } else if(password.charAt(i) == 'o') {
                modified += ".";
            } else {
                modified += password.charAt(i);
            }
        }
        modified += "q*s";
        System.out.println(modified);
    }

}
4.20
import java.util.Scanner;

public class LabProgram {

    public static String RemoveSpaces (String userString) {
        String result = "";
        for(int i = 0; i < userString.length(); ++i) {
            if(userString.charAt(i) != ' ') {
                result += userString.charAt(i);
            }
        }
        return result;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.println(RemoveSpaces(in.nextLine()));
    }

}
4.21
import java.util.Scanner;

public class LabProgram {

    public static String RemoveSpaces (String userString) {
        String result = "";
        for(int i = 0; i < userString.length(); ++i) {
            if(userString.charAt(i) != ' ') {
                result += userString.charAt(i);
            }
        }
        return result;
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        System.out.println(RemoveSpaces(in.nextLine()));
    }

}
4.22
import java.util.Scanner;
public class LabProgram
{
   public static void main(String[] args)
   {
       Scanner keyboard = new Scanner(System.in);


       int n = keyboard.nextInt();


       if(n < 20 || n > 98)
       {
           System.out.print("Input must be 20-98");
           System.out.println("");
       }
       else
       {
           System.out.print(n);
           while(n % 11 != 0)
           {
               n = n - 1;
               System.out.print(" " + n);
           }
           System.out.println(" ");
       }

       keyboard.close();
   }

}
4.23
import java.util.Scanner;

public class LabProgram {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int start = in.nextInt(), end = in.nextInt();
        if (start > end) {
            System.out.println("Second integer can't be less than the first.");
        } else {
            while (start <= end) {
                System.out.print(start + " ");
                start += 10;
            }
            System.out.println();
        }
    }
}
4.24
import java.util.Scanner;

public class LabProgram {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        String s;
        while (true) {
            s = in.nextLine();
            if (s.equalsIgnoreCase("quit") || s.equalsIgnoreCase("q")) {
                break;
            }
            for (int i = s.length()-1; i >= 0; --i) {
                System.out.print(s.charAt(i));
            }
            System.out.println();
        }
    }
}
4.25
import java.util.Scanner;
public class LabProgram {
    public static void main(String args[]){
        Scanner scanner = new Scanner(System.in);

        String s = scanner.nextLine();

        boolean flag = true;

        // i is the starting index of the string s
        // j is the ending index of the string s
        int i = 0, j = s.length()-1;

        // Checking the value if character at index i with the character at index j
        // If characters match, then move i to its next position and j t its previous position
        // If characters does not match then the string is not a palindrome.
        // In the case of does not match update the value of flag to false and break the loop
        while(i<j){
            if(s.charAt(i)==' '){
                i++;
            }
            else if(s.charAt(j)==' '){
                j--;
            }
            else {
                if(s.charAt(i)!=s.charAt(j)){
                    flag = false;
                    break;
                }
                i++;
                j--;
            }
        }

        if (flag) {
            System.out.println(s+" is a palindrome");
        } else {
            System.out.println(s+" is not a palindrome");
        }
    }
}
4.26
import java.util.Scanner;

public class LabProgram {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int x1, y1, n1, x2, y2, n2;
        x1 = in.nextInt();
        y1 = in.nextInt();
        n1 = in.nextInt();

        x2 = in.nextInt();
        y2 = in.nextInt();
        n2 = in.nextInt();

        for (int x = -10; x <= 10; x++) {
            for (int y = -10; y <= 10; y++) {
                if (x1*x + y1*y == n1 && x2*x + y2*y == n2) {
                    System.out.println(x + " " + y);
                    return;
                }
            }
        }
        System.out.println("No solution");
    }
}
4.27
import java.util.Scanner;

//declare class DrawRightTriangle
public class DrawRightTriangle
{
   //main method
   public static void main(String[] args)
   {

       //Creating a scanner object to take the input from the user
       Scanner scnr = new Scanner(System.in);
       //Declaring variable to store the input
       char triangleChar;
       int triangleHeight;

       //Prompting for a character that a user wants to print on the screen
       System.out.println("Enter a character:");
       triangleChar = scnr.next().charAt(0);

       //Prompting for the height of the
       //triangle that a user wants to print
       System.out.println("Enter triangle height:");
       triangleHeight = scnr.nextInt();
       System.out.println("");

       //Use nested for-loops to print the right angle traingle
       ///for the user specified character
       for(int i=0; i<triangleHeight; i++)
       {
           for(int j=0; j<=i; j++)
           {
               //This prints on the same line
               System.out.printf("%s ",triangleChar);
           }
           //print the newline for each row
           System.out.println();
       }
   }
}
4.28
import java.util.Scanner;
public class DrawHalfArrow {
    public static void main(String[] args) {
        Scanner scnr = new Scanner(System.in);
        int arrowBaseHeight = 0;
        int arrowBaseWidth = 0;
        int arrowHeadWidth = 0;

        System.out.println("Enter arrow base height:");
        arrowBaseHeight = scnr.nextInt();

        System.out.println("Enter arrow base width:");
        arrowBaseWidth = scnr.nextInt();

        System.out.println("Enter arrow head width:");
        arrowHeadWidth = scnr.nextInt();

        while (arrowHeadWidth<=arrowBaseWidth) {
            System.out.println("arrow head width is not larger than arrow base width");
            System.out.println("Enter arrow head width:");
            arrowHeadWidth = scnr.nextInt();
        }
        System.out.println();

        StringBuilder row = new StringBuilder();
        for (int x = 1; x <= arrowBaseWidth; x++) {
            row.append("*");
        }
        for (int i = 1; i <= arrowBaseHeight; i++) {
            System.out.println(row);
        }

        int tmp = arrowHeadWidth;
        for (int y = 1; y <= arrowHeadWidth; y++) {
            for (int z = tmp; z > 0; z--) {
                System.out.print("*");
            }
            tmp -= 1;
            System.out.println();
        }
    }
}