Welcome to the Java Programming Forums


The professional, friendly Java community. 21,500 members and growing!


The Java Programming Forums are a community of Java programmers from all around the World. Our members have a wide range of skills and they all have one thing in common: A passion to learn and code Java. We invite beginner Java programmers right through to Java professionals to post here and share your knowledge. Become a part of the community, help others, expand your knowledge of Java and enjoy talking with like minded people. Registration is quick and best of all free. We look forward to meeting you.


>> REGISTER NOW TO START POSTING


Members have full access to the forums. Advertisements are removed for registered users.

Results 1 to 7 of 7

Thread: Quick Recursion question involving sum

  1. #1
    Member
    Join Date
    Nov 2012
    Posts
    49
    My Mood
    Amused
    Thanks
    10
    Thanked 0 Times in 0 Posts

    Default Quick Recursion question involving sum

    Hi,

    I have a recursive method to find the sum of a given number that's greater than 10 respectively.

     
        static int sum(int n) {
            if(n < 10) {
                return n;
            } else {
                return sum(n/10) + (n % 10);
            }
        }

    I know this splits the number up into lets say 256 was entered 2 + 5 + 6 = 13 the sum.
    My question is how exactly does it split it up.

    I only got this through keyboard bashing and I want to be able to see recursion in my head properly.

    Many thanks,
    Seán


  2. #2
    Super Moderator Norm's Avatar
    Join Date
    May 2010
    Location
    Eastern Florida
    Posts
    25,139
    Thanks
    65
    Thanked 2,720 Times in 2,670 Posts

    Default Re: Quick Recursion question involving sum

    question is how exactly does it split it up.
    print out the results of the two operators: % and / to see what they do to the number.
    Then compare the results to the original number.
    what is nbr/10
    what is nbr % 10
    If you don't understand my answer, don't ignore it, ask a question.

  3. #3
    Member
    Join Date
    Nov 2012
    Posts
    49
    My Mood
    Amused
    Thanks
    10
    Thanked 0 Times in 0 Posts

    Default Re: Quick Recursion question involving sum

    Quote Originally Posted by Norm View Post
    print out the results of the two operators: % and / to see what they do to the number.
    Then compare the results to the original number.
    what is nbr/10
    what is nbr % 10
    I understand how the div and mod works. What I'm having trouble with is when it comes in for the first time.

    256 / 10 = 25
    256 % 10 = 6

    which is 31.

  4. #4
    Super Moderator Norm's Avatar
    Join Date
    May 2010
    Location
    Eastern Florida
    Posts
    25,139
    Thanks
    65
    Thanked 2,720 Times in 2,670 Posts

    Default Re: Quick Recursion question involving sum

    When I execute sum(256) it returns 13? How do you get 31?
    If you don't understand my answer, don't ignore it, ask a question.

  5. #5
    Member
    Join Date
    Nov 2012
    Posts
    49
    My Mood
    Amused
    Thanks
    10
    Thanked 0 Times in 0 Posts

    Default Re: Quick Recursion question involving sum

    No, when I run this code I get 13. The code is right I'm just finding it hard to understand the logic behind it.
    I understand how it works (kind of) just what happens when it gets passed through first.

  6. #6
    Super Moderator Norm's Avatar
    Join Date
    May 2010
    Location
    Eastern Florida
    Posts
    25,139
    Thanks
    65
    Thanked 2,720 Times in 2,670 Posts

    Default Re: Quick Recursion question involving sum

    Add lots of println statements that show the execution flow and the values of the variables as they are used and passed. That should show you when the method is called and what the args are when it is called and what value it returns for each call. You will have to rework the code so you can print out what the method is going to return before it returns it.
    If you don't understand my answer, don't ignore it, ask a question.

  7. The Following User Says Thank You to Norm For This Useful Post:

    ManInTheMiddle (December 1st, 2012)

  8. #7
    Member
    Join Date
    Nov 2012
    Posts
    49
    My Mood
    Amused
    Thanks
    10
    Thanked 0 Times in 0 Posts

    Default Re: Quick Recursion question involving sum

    Cheers man. I'll give it a shot.

    Much appreciated.

Similar Threads

  1. Question about for loop to print the sum of series of fractions
    By _K_ in forum What's Wrong With My Code?
    Replies: 0
    Last Post: November 17th, 2012, 08:08 AM
  2. Quick question
    By l1nk3 in forum Java Theory & Questions
    Replies: 1
    Last Post: February 24th, 2012, 02:02 PM
  3. Quick Question
    By sird00dguyman in forum Java Theory & Questions
    Replies: 2
    Last Post: August 4th, 2011, 06:14 PM
  4. Another Quick Question
    By Jacksontbh in forum What's Wrong With My Code?
    Replies: 1
    Last Post: July 1st, 2011, 07:18 AM
  5. question about recursion..
    By gonfreecks in forum What's Wrong With My Code?
    Replies: 2
    Last Post: March 16th, 2011, 09:51 AM

Tags for this Thread