Question

What would be the result of each of the following programs?

1	public class RecursionInJava1{   public static void main(String args[]) {        System.out.println(guess(9));    } Public static int guess(int num){          if (num == 1)                   return 0;          else                   return (guess(num-2)-num); } }
2	public class RecursionInJava2{   public static void main(String args[]) {    System.out.println(sum(10));     }      public static int sum(int number){        //base case   if(number == 7){   return 0;   }   return sum(number-1)+number    } }

Answer 1

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Answer for question 1:

The program will output -24.

Explanation:

Let’s go through each recursive call of the method guess().

Initially the value passed is 9.

guess(9) :- Since 9 is not 1, else block gets executed which will call guess(9-2) and subtract 9 from it before returning it. So the method will return guess(7)-9

guess(7) :- Since 7 is not 1, else block gets executed which will call guess(7-2) and subtract 7 from it before returning it. So the method will return guess(5)-7

guess(5) :- Since 5 is not 1, else block gets executed which will call guess(5-2) and subtract 5 from it before returning it. So the method will return guess(3)-5

guess(3) :- Since 3 is not 1, else block gets executed which will call guess(3-2) and subtract 3 from it before returning it. So the method will return guess(1)-3

guess(1) :- Since 1 is equal to 1, this method simply returns 0

Combining all the return statements, we get

(((((0)-3)-5)-7)-9) => -24

Answer for question 2:

The program will output 27

Explanation:

Let’s go through each recursive call of the method sum().

Initially the value passed to sum() from the main is 10.

sum(10) :- Since 10 is not 7, sum(10-1)+10 or sum(9)+10 is returned

sum(9) :- Since 9 is not 7, sum(9-1)+9 or sum(8)+9 is returned

sum(8) :- Since 8 is not 7, sum(8-1)+8 or sum(7)+8 is returned

sum(7) :- Since 7 equals 7, 0 is returned

Combining all the return statements, we get

((((0)+8)+9)+10) = 27