Results 1 to 20 of 20
  1. #1
    SapphireSpark is offline Member
    Join Date
    Oct 2008
    Posts
    38
    Rep Power
    0

    Default (Help) Fraction Summation and Exponents

    I'm making a program that's supposed to calculate the sum of this:



    Java Code:
    import java.util.Scanner;
    public class Project5 {
        public static void main (String[] args) {
            Scanner scan = new Scanner (System.in);
            System.out.println ("Enter n: ");
            int n = scan.nextInt();
            double sum = 0.0;
            int x=1;
            //I'm not sure if that's right, because I'm trying to make x be the numerator, so it can keep increasing by 1. Should I do that?
    
            while (x<=n) {
            if (x%2==1) {
               sum += (x/    **how do I say 2 to the power of n?**  );
           }else{
              sum -= (x/       **how do I say 2 to the power of n?**  );
          }
       x++
    }
    And how do I get it to reach that end summation, of (-1) to the power of n+1?

    I'm not even sure if the stuff I have so far can possibly work. I just know that we're not allowed to use "Math.pow". Please help, I can't find any useful examples in my book or notes.
    Last edited by SapphireSpark; 10-08-2008 at 05:52 AM. Reason: to update the picture so its accurate

  2. #2
    Norm's Avatar
    Norm is online now Moderator
    Join Date
    Jun 2008
    Location
    Eastern Florida
    Posts
    17,611
    Rep Power
    25

    Default

    calculate the sum of this
    summation, of (-1) to the power of n+1?
    Can you explain with an algebraic expression or in english what you are trying to do?

    Do you know what x to the power of y means?
    x*x*x*x*... y times
    Does that look like the place to use a loop?

  3. #3
    SapphireSpark is offline Member
    Join Date
    Oct 2008
    Posts
    38
    Rep Power
    0

    Default

    Yes, I know what x to the power of y means. I just can't figure out how to put it in a loop.

    I updated the picture, its accurate now, that's what I'm trying to do: Find the sum of the sequence that continues until that final part is reached.

  4. #4
    SapphireSpark is offline Member
    Join Date
    Oct 2008
    Posts
    38
    Rep Power
    0

    Default

    So, if the user inputs n to equal 5, the last term in the summation sequence will be *1 times (5 over 32).

    And the program displays the sum of all the terms that come before that last term, plus the last term.

  5. #5
    Fubarable's Avatar
    Fubarable is offline Moderator
    Join Date
    Jun 2008
    Posts
    19,316
    Blog Entries
    1
    Rep Power
    26

    Default

    the poster is a spammer.

  6. #6
    SapphireSpark is offline Member
    Join Date
    Oct 2008
    Posts
    38
    Rep Power
    0

    Default

    hey, I said I was sorry for the previous topic, I really need help on this one. I'm not a spammer, it was the only way I could attach that picture on top.

    Could someone please help me fix the loop statement? Please?

  7. #7
    Eranga's Avatar
    Eranga is offline Moderator
    Join Date
    Jul 2007
    Location
    Colombo, Sri Lanka
    Posts
    11,372
    Blog Entries
    1
    Rep Power
    20

    Default

    Why don't you use the math class for that. It's most easiest as well as the best way to follow.

    Java Code:
    import java.util.Scanner;
    
    public class FindSummation {
    
        public static void main(String[] args) {
            Scanner userScann = new Scanner(System.in);
            System.out.println("Enter number of terms: ");
            int terms = userScann.nextInt();
            double sum = 0;
            
            for(int i = 1; i <= terms; i++) {
                sum += (Math.pow(-1, (i + 1))) * (i / (Math.pow(2, i)));
            }
            
            System.out.println(sum);
        }
    
    }

  8. #8
    Eranga's Avatar
    Eranga is offline Moderator
    Join Date
    Jul 2007
    Location
    Colombo, Sri Lanka
    Posts
    11,372
    Blog Entries
    1
    Rep Power
    20

    Default

    Quote Originally Posted by Fubarable View Post
    the poster is a spammer.
    Don't worry pal. I'll monitoring him, and if so I can ban the user. :)

  9. #9
    SapphireSpark is offline Member
    Join Date
    Oct 2008
    Posts
    38
    Rep Power
    0

    Default

    Thank you, but the only rule is that we are not allowed to use Math.pow. We are limited to if, else, or switch. And the result should be a double value. I've tried making this using those guildelines and examples, but I don't think its right. Can you please check my code?
    Last edited by SapphireSpark; 10-08-2008 at 07:58 AM.

  10. #10
    SapphireSpark is offline Member
    Join Date
    Oct 2008
    Posts
    38
    Rep Power
    0

    Default

    Don't worry pal. I'll monitoring him, and if so I can ban the user.
    Thanks, but that won't be necessary, it was only so I could attach my explanation picture. :) And, btw, I'm a girl, just confused by the class. :)

    Your previous suggestion looks helpful, except for the useage of Math.pow. Is there a more basic translation for that, using simple operators, if possible?
    Last edited by SapphireSpark; 10-08-2008 at 07:46 AM. Reason: Expanding

  11. #11
    Eranga's Avatar
    Eranga is offline Moderator
    Join Date
    Jul 2007
    Location
    Colombo, Sri Lanka
    Posts
    11,372
    Blog Entries
    1
    Rep Power
    20

    Default

    Quote Originally Posted by SapphireSpark View Post
    Thank you, but the only rule is that we are not allowed to use Math.pow. We are limited to if, else, or switch. And the result should be a double value. I've tried making this using those guildelines and examples, but I don't think its right. Can you please check my code?
    There are lots of ways to do it. Did you try any.

  12. #12
    SapphireSpark is offline Member
    Join Date
    Oct 2008
    Posts
    38
    Rep Power
    0

    Default

    Yes, I tried, and I thought maybe something to the nth power was a loop statement, but I have no idea how to make it work. The book doesn't mention variable powers in this chapter, and the examples from class were more like (value*value).

    Also, for the first term, its not really 1, its (1/2), so that's not an integer, right? Will that mess up the result? All the class examples used int, so I'm not sure if I should change it to double, although it sounds right.
    Last edited by SapphireSpark; 10-08-2008 at 08:12 AM.

  13. #13
    Eranga's Avatar
    Eranga is offline Moderator
    Join Date
    Jul 2007
    Location
    Colombo, Sri Lanka
    Posts
    11,372
    Blog Entries
    1
    Rep Power
    20

    Default

    Here is the simple way to do it

    Java Code:
    import java.util.Scanner;
    
    public class FindSummation {
    
        public static void main(String[] args) {
            Scanner userScann = new Scanner(System.in);
            System.out.println("Enter number of terms: ");
            int n = userScann.nextInt();
            double sum = 0, newSum = 0;
            
            double d1 = 1, d2 = 1;
            int temp = 0;
            
            for(int j = 1; j <= n; j++) {     
                sum += (Math.pow(-1, (j + 1))) * (j / (Math.pow(2, j)));
            
            }
            
            for(int i = 1; i <= n; i++) {
                
                // Value of (-1)^(n+1)
                do {
                    d1 *= (-1);
                    temp++;
                }while(temp < (i + 1));
                temp = 0;
            
                // Value of 2^n
                do {
                    d2 *= 2;
                    temp++;
                }while(temp < i);
                temp = 0;
            
                newSum += (d1 * i) / d2; // Summation on each iteration
                
                // Clear all dummy values
                d1 = 1;
                d2 = 1;
            }
            System.out.println(sum + " " + newSum);
        }
    
    }

  14. #14
    SapphireSpark is offline Member
    Join Date
    Oct 2008
    Posts
    38
    Rep Power
    0

    Default

    Thank you very much. So, 'd1' is the numerator, and temp is the one changing it? What is *= doing to it?

  15. #15
    Eranga's Avatar
    Eranga is offline Moderator
    Join Date
    Jul 2007
    Location
    Colombo, Sri Lanka
    Posts
    11,372
    Blog Entries
    1
    Rep Power
    20

    Default

    Actually 'd1' is not numerator, but in mathematically it is. It's calculate the part,

    (-1) to the power (n + 1)
    temp is an iterator which is used to keep track of it.

    *= is shorten form of expressing statements. In Java it's called as compound statement. So following two statements are same.

    Java Code:
    d1 *= (-1);
    
    d1 = d1 * (-1);

  16. #16
    SapphireSpark is offline Member
    Join Date
    Oct 2008
    Posts
    38
    Rep Power
    0

    Default

    Ok, thanks, I remember that. I can also see the application of the do/while statement, and why it works well for this, it controls the loop so it only runs up to n. It is useful, despite how my teacher said its not as useful as the regular while statement.

    That's also a great way to make an exponent, I can definitely use that method later, as well. Thank you so much for the help, my notes are starting to make more sense now, as well. :)

  17. #17
    Norm's Avatar
    Norm is online now Moderator
    Join Date
    Jun 2008
    Location
    Eastern Florida
    Posts
    17,611
    Rep Power
    25

    Default

    Remember for the do{} while() loop, you ALWAYS do it once. If there is a possiblity of 0 entries in an array or a null pointer, you'll have to do a separate test before starting the loop to keep from getting an exception.
    The while(){} loop can be done 0 times.
    Which to use depends on the application.

  18. #18
    emceenugget is offline Senior Member
    Join Date
    Sep 2008
    Posts
    564
    Rep Power
    7

    Default

    You know to get the sign, it's really inefficient to create a loop for something so simple. I'm sure it doesn't really matter for your assignment, but you should just set d1, or whatever the necessary variable is, based on whether n is even or odd. Just an idea.

  19. #19
    Eranga's Avatar
    Eranga is offline Moderator
    Join Date
    Jul 2007
    Location
    Colombo, Sri Lanka
    Posts
    11,372
    Blog Entries
    1
    Rep Power
    20

    Default

    Quote Originally Posted by SapphireSpark View Post
    Ok, thanks, I remember that. I can also see the application of the do/while statement, and why it works well for this, it controls the loop so it only runs up to n. It is useful, despite how my teacher said its not as useful as the regular while statement.

    That's also a great way to make an exponent, I can definitely use that method later, as well. Thank you so much for the help, my notes are starting to make more sense now, as well. :)
    Basically do-while loops guaranteed to execute at least once, and then check the condition. While is not like that, check the condition before execute the body.

  20. #20
    Eranga's Avatar
    Eranga is offline Moderator
    Join Date
    Jul 2007
    Location
    Colombo, Sri Lanka
    Posts
    11,372
    Blog Entries
    1
    Rep Power
    20

    Default

    Quote Originally Posted by emceenugget View Post
    You know to get the sign, it's really inefficient to create a loop for something so simple. I'm sure it doesn't really matter for your assignment, but you should just set d1, or whatever the necessary variable is, based on whether n is even or odd. Just an idea.
    Ya, it's nice. Those kind of thing come in design level. That's the advantage of well plane design for an application.

Similar Threads

  1. Big Fraction 1.00
    By JavaBean in forum Java Software
    Replies: 1
    Last Post: 03-26-2008, 05:24 AM
  2. Trouble getting Julia fraction correct.
    By yllawwally in forum New To Java
    Replies: 0
    Last Post: 12-21-2007, 03:02 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •