Question

1. Write a method called printReverse() that takes a string and uses recursion to print the contents of the string in reverse order. The string itself should not be reversed; it must be left in its original form.

   The method has the following header:

     void printReverse(String s, int i)

   where s is a reference to the string, and i is an integer parameter that you may use as you see fit. You do not need to code up this method as part of a class – a written version of the method itself is all that is required.

   For full credit, your printReverse() method should consider the first element of the list first – i.e., the first call to printReverse() should be the one that prints the first element, the second call should be the one that prints the second element, and so on. Half credit will be given for methods that consider the last element first. In either case, your method must be recursive; no credit will be given for methods that employ iteration.

2. Write a method called addOddNums() that uses recursion to add all of the odd numbers in an integer array starting from a certain index.

The method has the following header:

   int addOddNums(int[] a, int i)

where a is the array and i is an index into the array.

For example:

           int[] array = new  []{1, 2, 3, 4, 5};
             int sum = addOddNums(array, 0); // This call should return a value 9 (1+3+5)
             int sum = addOddNums(array, 3); // This call should return a value 5.

The method should not do any printing and if the value null is passed in as the array parameter it should return 0 (zero).

3. Write a method called printWithSpaces()that uses recursion to print the individual characters of a string separated by spaces.

The method has the following header:

    void printWithSpaces(String s)

For example, the following call printWithSpaces("spaces") should produce the following output:

s p a c e s

This method should not return a value. The method should not do any printing if the empty string ("") or the value null is passed in as the parameter.

4. Write a method called weave() that uses recursion to return the string that is formed by "weaving" together the characters in the strings s1 and s2 to create a single string.

This method has the following header:

String weave(String s1, String s2)

For example:

         weave("aaaa", "bbbb")should return the string "abababab"

         weave("hello", "world")should return the string "hweolrllod"

If one of the strings is longer than the other, its "extra" characters – the ones with no counterparts in the shorter string – should appear immediately after the "woven" characters (if any) in the returned string.

For example, weave("recurse", "NOW") should return the string  "rNeOcWurse", in which the extra characters from the first string – the characters in "urse" – come after the characters that have been woven together.

This method should not do any printing; it should simply return the resulting string. If null is passed in for either parameter, the method should print an error message. If the empty string ("") is passed in for either string, the method should return the other string. For example, weave("hello", "")   should return  "hello" and weave("", "") should return "".

5. Write a method called multByAdd()that uses recursion to multiply integers together, without using multiplication. You can assume that the second parameter, n, is positive.

The method has the following header:

               int multByAdd(int m, int n)

For example, the following calls should return the following values:

                                 multByAdd(5, 4); //returns 20
  multByAdd(100, 0); //returns
                                    multByAdd(-6, 10); //returns -60

GIVEN STARTER FILE IS AS FOLLOW:

public class RecursionApp {
  
public static void main(String[] args) {
reversePrint(null, 0);
reversePrint("", 0);
reversePrint("reverse", 0);
System.out.println();
reversePrint("!nuf si noisruceR", 0);
System.out.println();
int[] array = new int[]{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
System.out.println("The sum of the odd numbers is: " + countOddNums(array, 0));
System.out.println("The sum of the odd numbers is: " + countOddNums(array, 9));
array = null;
System.out.println("The sum of the odd numbers is: " + countOddNums(array, 0));
array = new int[]{0, 2, 4, 6, 8, 10};
System.out.println("The sum of the odd numbers is: " + countOddNums(array, 0));
printSpaces(null);
printSpaces("");
printSpaces("spaces");
printSpaces("s p a c e s ");
System.out.println(multByAdd(6, 4));
System.out.println(multByAdd(1, 1000));
System.out.println(multByAdd(1000, 0));
System.out.println(multByAdd(0, 1000));
System.out.println(multByAdd(-10, 44));
System.out.println(weave("ace", "bdf"));
System.out.println(weave("Rcrini u!", "euso sfn"));
System.out.println(weave("aab", "abbaaa"));
System.out.println(weave("ccdeeeeeeeeeeeeeeeeeeeee", "cdd"));
System.out.println(weave(null, null));
System.out.println(weave(null, "cat"));
System.out.println(weave("dog", null));
System.out.println(weave("Isn't recursion fun?", ""));
System.out.println(weave("", "Isn't recursion fun?"));
}

public static void reversePrint(String s, int i) {
// put your solution here
  
}
  
public static int countOddNums(int[] a, int i) {
//put your solution here
return 0;
}
  
public static void printSpaces(String s) {
//put your solution here
}
  
public static int multByAdd(int n, int m) {
//put your solution here
return 0;
}
  
public static String weave(String s1, String s2) {
//put your solution here
return "";
}
}

AND THE OUTPUT OF THE PROGRAM SHOULD LOOK LIKE THE OUTPUT:

The string is empty or does not exist. The string is empty or does not exist. Reverse print: esrever Reverse print: Recursion is fun! The sum of the odd numbers is: 25 The sum of the odd numbers is: 9 The sum of the odd numbers is: The sum of the odd numbers is: spaces S P a Ces 24 1000 -440 abcdef Recursion is fun! aaabbbaaa cccdddeeeeeeeeeeeeeeeeeeeee Can't weave two strings if one or both strings are null Can't weave two strings if one or both strings are null Can't weave two strings if one or both strings are null Isn't recursion fun? Isn't recursion fun?

Answer 1

Please find the code below:

JavaRecursions.java

package classess5;
public class JavaRecursions {

   public static void main(String[] args) {
       reversePrint(null, 0);
       reversePrint("", 0);
       reversePrint("reverse", 0);
       System.out.println();
       reversePrint("!nuf si noisruceR", 0);
       System.out.println();
       int[] array = new int[]{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
       System.out.println("The sum of the odd numbers is: " + countOddNums(array, 0));
       System.out.println("The sum of the odd numbers is: " + countOddNums(array, 9));
       array = null;
       System.out.println("The sum of the odd numbers is: " + countOddNums(array, 0));
       array = new int[]{0, 2, 4, 6, 8, 10};
       System.out.println("The sum of the odd numbers is: " + countOddNums(array, 0));
       printSpaces(null);
       printSpaces("");
       printSpaces("spaces");
       printSpaces("s p a c e s ");
       System.out.println(multByAdd(6, 4));
       System.out.println(multByAdd(1, 1000));
       System.out.println(multByAdd(1000, 0));
       System.out.println(multByAdd(0, 1000));
       System.out.println(multByAdd(-10, 44));
       System.out.println(weave("ace", "bdf"));
       System.out.println(weave("Rcrini u!", "euso sfn"));
       System.out.println(weave("aab", "abbaaa"));
       System.out.println(weave("ccdeeeeeeeeeeeeeeeeeeeee", "cdd"));
       System.out.println(weave(null, null));
       System.out.println(weave(null, "cat"));
       System.out.println(weave("dog", null));
       System.out.println(weave("Isn't recursion fun?", ""));
       System.out.println(weave("", "Isn't recursion fun?"));
   }

   public static void reversePrint(String s, int i) {
       // put your solution here
       if(s==null || s.length()==0){
           System.out.println("The string is empty or does not exist.");
       }else{
           if(i>=s.length()){
               return;
           }else{
               if(i==0){
                   System.out.print("Reverse print : ");
               }
               System.out.print(s.charAt(s.length()-i-1));
               reversePrint(s,i+1);
           }
       }
   }

   public static int countOddNums(int[] a, int i) {
       if(a==null){
           return 0;
       }else if(i>=a.length){
           return 0;
       }else{
           if(a[i]%2==0){
               return countOddNums(a,i+1);
           }else {
               return a[i]+countOddNums(a,i+1);
           }
       }
   }

   public static void printSpaces(String s) {
       if(s==null || s.length()==0){
           System.out.println();
           return ;
       }else{
           System.out.print(s.charAt(0)+" ");
           printSpaces(s.substring(1));
       }
       //put your solution here
   }

   public static int multByAdd(int n, int m) {
       if(m<=0){
           return 0;
       }else{
           return n+multByAdd(n, m-1);
       }
   }

   public static String weave(String s1, String s2) {
       if(s1==null || s2==null){
           System.out.println("Can't weave two strings if one or both strings are null");
           return "";
       }else if(s1.length()==0 && s2.length()==0){
           return "";
       }else{
           if(s1.length()>0 && s2.length()>0){
               return   s1.charAt(0)+""+s2.charAt(0)+weave(s1.substring(1),s2.substring(1));
           }else if(s1.length()>0){
               return   s1.charAt(0)+weave(s1.substring(1),s2);
           }else if(s2.length()>0){
               return   s2.charAt(0)+weave(s1,s2.substring(1));
           }else{
               return "";
           }

       }
   }

}

output:

Quick Access : Java EE *' Java * Debug & Console X * * * B SEO, <terminated> JavaRecursions [Java Application] C:\Program Files\Java\jdk1.8.0_121\jre\bin\javaw.exe (Mar 3, The string is empty or does not exist. The string is empty or does not exist. Reverse print : esrever Reverse print : Recursion is fun! The sum of the odd numbers is: 25 The sum of the odd numbers is: 9 The sum of the odd numbers is: 0 The sum of the odd numbers is: 0 e s spaces s p a c 24 1000 ings ar -440 abcdef Recursion is fun! aaabbbaaa cccdddeeeeeeeeeeeeeeeeeeeee Can't weave two strings if one or both strings are null s2.sub Can't weave two strings if one or both strings are null Can't weave two strings if one or both strings are null Isn't recursion fun? Isn't recursion fun?

Answer 2

//Java program

public class RecursionApp {
  
public static void main(String[] args) {
reversePrint(null, 0);
reversePrint("", 0);
reversePrint("reverse", 0);
System.out.println();
reversePrint("!nuf si noisruceR", 0);
System.out.println();
int[] array = new int[]{0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
System.out.println("The sum of the odd numbers is: " + countOddNums(array, 0));
System.out.println("The sum of the odd numbers is: " + countOddNums(array, 9));
array = null;
System.out.println("The sum of the odd numbers is: " + countOddNums(array, 0));
array = new int[]{0, 2, 4, 6, 8, 10};
System.out.println("The sum of the odd numbers is: " + countOddNums(array, 0));
printSpaces(null);
printSpaces("");
printSpaces("spaces");
printSpaces("s p a c e s ");
System.out.println(multByAdd(6, 4));
System.out.println(multByAdd(1, 1000));
System.out.println(multByAdd(1000, 0));
System.out.println(multByAdd(0, 1000));
System.out.println(multByAdd(-10, 44));
System.out.println(weave("ace", "bdf"));
System.out.println(weave("Rcrini u!", "euso sfn"));
System.out.println(weave("aab", "abbaaa"));
System.out.println(weave("ccdeeeeeeeeeeeeeeeeeeeee", "cdd"));
System.out.println(weave(null, null));
System.out.println(weave(null, "cat"));
System.out.println(weave("dog", null));
System.out.println(weave("Isn't recursion fun?", ""));
System.out.println(weave("", "Isn't recursion fun?"));
}

public static void reversePrint(String s, int i) {
   if(s==null || s.length()==0) {
       System.out.println("The string is empty or does not exit");
       return;
   }
   if(i==s.length())return;
   reversePrint(s,i+1);
   System.out.print(s.charAt(i));
  
}
  
public static int countOddNums(int[] a, int i) {
   if(a==null || a.length==0) {
       System.out.println("The string is empty or does not exit");
       return 0;
   }
   if(i==a.length)return 0;
   if(a[i]%2==1) {
       return a[i] + countOddNums(a,i+1);
   }
   return countOddNums(a,i+1);
}
  
public static void printSpaces(String s) {
   if(s==null || s.length()==0) {
       System.out.println("The string is empty or does not exit");
       return;
   }
   if(s.length()==0)return;
   System.out.print(s.charAt(0)+" ");
   printSpaces(s.substring(1));
}
  
public static int multByAdd(int n, int m) {
   if(m==0 || n==0)return 0;
   return n + multByAdd(n, m-1);
}
  
public static String weave(String s1, String s2) {
   if(s1==null || s2==null) {
       System.out.println("Can't weave two strings if one or both strings are null");
       return "";
   }
   if(s1.isEmpty() || s2.isEmpty()) {
   return s1 + s2;
   }
   return s1.substring(0, 1) + s2.substring(0, 1) + weave(s1.substring(1), s2.substring(1));

}
}
//sample output

<terminated> RecursionApp [Java Application] C:\Program Files Java\jdk1.8.0_151\bin\javaw.exe The string is empty or does not exit The string is empty or does not exit egrever Recursion is fun! The sum of the odd numbers is: 25 The sum of the odd numbers is: 9 The string is empty or does not exit The sum of the odd numbers is: 0 The sum of the odd numbers is: 0 The string is empty or does not exit The string is empty or does not exit spaces The string is empty or does not exit 3 P a c e 3 The string is empty or does not exit 24 1000 0 - 440 abcdef Recursion is fun! aaabbbaaa cccdddeeeeeeeeeeeeeeeeeeeee Can't weave two strings if one or both strings are null Can't weave two strings if one or both strings are null Can't weave two strings if one or both strings are null Isn't recursion fun? Isn't recursion fun?