Programming Forums > Application Development > Java

emdiesse · Mar 9th, 2008, 6:17 PM

Hello. I am attempting to use recursion to output the sequence of numbers 2, 3, 7, 13, 27, 53, ...

public class TDSeq {
   private static int printSequence(int F, int k) {
      if(k ==1) return 2;
      if(k ==2) return 3;
      F = printSequence(F, k-1)  + (2*printSequence(F, k-2));
      System.out.print(F + ", ");
      return F;    
   }
    
   public static void main(String[] args) {
      int k = 8;
      int F = 3;
      printSequence(F, k);
   }  
}

Now this give a horrible output:
7, 13, 7, 27, 7, 13, 53, 7, 13, 7, 27, 107, 7, 13, 7, 27, 7, 13, 53, 213,

No where near the aim of:

2, 3, 7, 13, 27, 53, ...

Now I really can't get my head around this, however I would really like to learn how to use recursion effectively.

Any one got any help or advice they can offer?

Ooble · Mar 9th, 2008, 9:04 PM

You have two main problems. One is that you print the numbers right after you generate them, not when you've finished the recursion, and the other is that you never print 2 or 3.

Here's some pseudocode (actually Python code, but they're essentially the same thing) that'll do it, though it's not efficient. You'd need to work on some kind of caching for that.

(Toggle Plain Text)

def sequence(k):
    if k == 1:
        return 2
    elif k == 2:
        return 3
    else:
        return sequence(k - 1) + 2 * sequence(k - 2)
    
def main():
    for i in xrange(1, 10):
        print sequence(i)

emdiesse · Mar 10th, 2008, 6:54 AM

Quote:

Originally Posted by Ooble

You have two main problems. One is that you print the numbers right after you generate them, not when you've finished the recursion, and the other is that you never print 2 or 3.

Here's some pseudocode (actually Python code, but they're essentially the same thing) that'll do it, though it's not efficient. You'd need to work on some kind of caching for that.

(Toggle Plain Text)

def sequence(k):
    if k == 1:
        return 2
    elif k == 2:
        return 3
    else:
        return sequence(k - 1) + 2 * sequence(k - 2)
    
def main():
    for i in xrange(1, 10):
        print sequence(i)

Ahah. That makes alot more sense to me now. I don't know why i had an F parametre in the last one, lol. If i didn't want an external loop is there a way to place it inside or is that bad programming?

(Toggle Plain Text)

public class TDSeq {
   private static int F = 0;
   
   private static void printSequence(int i) {
	 for(int k=1;k<=i;k++){            //can i place this in calcSequence?
	    System.out.print(calcSequence(k) + ",");   
	 }
   }
   
   private static int calcSequence(int k) {
      if(k ==1) return 2;
      else if(k ==2){ return 3;}
      else return ( calcSequence(k-1)  + (2*calcSequence(k-2)) );
   }
    
   public static void main(String[] args) {
      int i = 7;
	 printSequence(i);
   }  
}

Ooble · Mar 10th, 2008, 1:25 PM

Thinking about it, it doesn't make much sense. The numbers aren't parsed through in order, so they wouldn't be printed in order. I can't think of a way to do it right now... hopefully it'll come to me soon.

OpenLoop · Mar 10th, 2008, 5:18 PM

try this:

 python Syntax (Toggle Plain Text) 
def myseq(k, run):
    if k >= 500:
        return 0
    print k
    if run == 1:
        return myseq((2 * k) + 1, 0)
    else:
        return myseq((2 * k) - 1, 1)
 
def main():
    myseq(2, 0) 
 
main()
def myseq(k, run):
    if k >= 500:
        return 0
    print k
    if run == 1:
        return myseq((2 * k) + 1, 0)
    else:
        return myseq((2 * k) - 1, 1)
 
def main():
    myseq(2, 0) 
        
main()

Mar 9th, 2008, 6:17 PM	#1
emdiesse Programmer Join Date: Jul 2004 Location: Hampshire Posts: 56 Rep Power: 4	Recursion to compute sequences. Hello. I am attempting to use recursion to output the sequence of numbers 2, 3, 7, 13, 27, 53, ... (Toggle Plain Text) public class TDSeq { private static int printSequence(int F, int k) { if(k ==1) return 2; if(k ==2) return 3; F = printSequence(F, k-1) + (2printSequence(F, k-2)); System.out.print(F + ", "); return F; } public static void main(String[] args) { int k = 8; int F = 3; printSequence(F, k); } } public class TDSeq { private static int printSequence(int F, int k) { if(k ==1) return 2; if(k ==2) return 3; F = printSequence(F, k-1) + (2printSequence(F, k-2)); System.out.print(F + ", "); return F; } public static void main(String[] args) { int k = 8; int F = 3; printSequence(F, k); } } Now this give a horrible output: 7, 13, 7, 27, 7, 13, 53, 7, 13, 7, 27, 107, 7, 13, 7, 27, 7, 13, 53, 213, No where near the aim of: 2, 3, 7, 13, 27, 53, ... Now I really can't get my head around this, however I would really like to learn how to use recursion effectively. Any one got any help or advice they can offer?

Mar 9th, 2008, 9:04 PM	#2
Ooble I eat cake for breakfast. Join Date: Jul 2004 Location: In my box. Posts: 4,434 Rep Power: 8	Re: Recursion to compute sequences. You have two main problems. One is that you print the numbers right after you generate them, not when you've finished the recursion, and the other is that you never print 2 or 3. Here's some pseudocode (actually Python code, but they're essentially the same thing) that'll do it, though it's not efficient. You'd need to work on some kind of caching for that. (Toggle Plain Text) def sequence(k): if k == 1: return 2 elif k == 2: return 3 else: return sequence(k - 1) + 2 * sequence(k - 2) def main(): for i in xrange(1, 10): print sequence(i) def sequence(k): if k == 1: return 2 elif k == 2: return 3 else: return sequence(k - 1) + 2 * sequence(k - 2) def main(): for i in xrange(1, 10): print sequence(i) __________________ Me :: You :: Them

Mar 10th, 2008, 1:25 PM	#4
Ooble I eat cake for breakfast. Join Date: Jul 2004 Location: In my box. Posts: 4,434 Rep Power: 8	Re: Recursion to compute sequences. Thinking about it, it doesn't make much sense. The numbers aren't parsed through in order, so they wouldn't be printed in order. I can't think of a way to do it right now... hopefully it'll come to me soon. __________________ Me :: You :: Them

Mar 10th, 2008, 5:18 PM	#5
OpenLoop Expert Programmer Join Date: May 2005 Location: East Lansing, MI Posts: 663 Rep Power: 4	Re: Recursion to compute sequences. try this: python Syntax (Toggle Plain Text) def myseq(k, run): if k >= 500: return 0 print k if run == 1: return myseq((2 * k) + 1, 0) else: return myseq((2 * k) - 1, 1) def main(): myseq(2, 0) main() def myseq(k, run): if k >= 500: return 0 print k if run == 1: return myseq((2 * k) + 1, 0) else: return myseq((2 * k) - 1, 1) def main(): myseq(2, 0) main()

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Programming Forums > Application Development > Java

Recursion to compute sequences.