Question

JAVA

Implement and test the following static recursive methods.

1) DigitCount – find the sum of the digits in an integer digitCount(12345) would be 5

2) Power2 - Efficiently compute exponentiation for non-negative exponents by the following recursive algorithm:

bx = 1 if x == 0

bx = (b(x/2))2 if x is even bx = b * (b(x-1)) if x is odd

3) Write a recursive method isBackwards that has two String parameters and returns true if the two Strings have the same sequence of characters but in the opposite order (ignoring white space and capitalization), and returns false otherwise. The method should throw an IllegalArgumentException if either String is null.

Test this method with following examples, isBackwards("Fried", "deirf") -> true isBackwards("Sit", "Toes") -> false isBackwards("", "") -> true

isBackwards("I madam", "mad am I") -> true

Answer 1

/*Java test program to test the three functions , DigitCount , power2 and isBackwrards and then print the results on the console output*/

//RecursiveFunctions.java
public class RecursiveFunctions
{
   public static void main(String[] args)
   {

       int n=12345;
       //Calling DigitCount method
       System.out.println("Digit count of "+n+" is "+DigitCount(n));

       int x=5;
       //Calling power2 method
       System.out.println("2 ^"+x+" = "+ power2(x));
      
       String first="Fried";
       String second="deirf";
       System.out.println(first);
       System.out.println(second);
       first=first.toLowerCase().trim();
       second=second.toLowerCase().trim();

       //Calling isBackwards method
       if(isBackwards(first,second))
           System.out.println("Equal.");
       else
           System.out.println("Not eqaul");
      
      
       System.out.println();

       first="I madam";
       second="mad am I";
       System.out.println(first);
       System.out.println(second);
       first=first.toLowerCase().trim();
       second=second.toLowerCase().trim();

       //Calling isBackwards method
       if(isBackwards(first,second))
           System.out.println("Equal.");
       else
           System.out.println("Not eqaul");
      
       System.out.println();

       first="Sit";
       second="Toes";
       System.out.println(first);
       System.out.println(second);
       first=first.toLowerCase().trim();
       second=second.toLowerCase().trim();
      
       if(isBackwards(first,second))
           System.out.println("Equal.");
       else
           System.out.println("Not eqaul");

   }

   /* 3. Recursive method isBackwards that takes left and right two string variables
   * and finds if they are equal in reverse of right is equal to left
   * otherwise returns false*/
   public static boolean isBackwards(String left,String right)
   {
       //Replace whitespaces with "" string
       left = left.toLowerCase().trim().replace(" ", "");
       right = right.toLowerCase().trim().replace(" ", "");
       //Returns true if both are emoty
       if(left.isEmpty()&& right.isEmpty())
           return true;
       //Throws if any of the string is null
       else if(left==null || right==null)
           throw new IllegalArgumentException();
       //Get the first character and last character of right string is equal
       else if(left.charAt(0)==right.charAt(right.length()-1))
       {      
           //call the isBackwards method with substring which includes the string except the first character in left string
           //and the substring of the right which excludes the last character.
           return isBackwards(left.substring(1), right.substring(0, right.length()-1));
       }
       else
           //otherwise return false
           return false;
   }
   /*2. Recursive method that takes a power,x
   * and raise the 2 to the power of x*/
   public static int power2(int x)
   {
       if (x != 0)
           //calling recursive method,power2
           return (2*power2(x-1));
       else
           return 1;
   } //end of the method,power2

   /*1. Recursive method to find the sum of the */
   public static int DigitCount (int n)
   {
       if (n == 0)
           return 0;
       return 1+ DigitCount(n / 10);
   } //end of the method,DigitCount
}

Sample Output:

Digit count of 12345 is 5
2 ^5 = 32
Fried
deirf
Equal.

I madam
mad am I
Equal.

Sit
Toes
Not eqaul