Programming Forums > Application Development > Java

pr0gm3r · Oct 11th, 2005, 4:21 AM

Hi guys,

I need your expertice to debug my recursion method...

Here is the problem:
I am trying to figure out how come my code is not working properly. The design and algorithm are correct but it seem that the recursion method did not fully return the whole value that passed.

I have two different recursion method but both of them are not returning the rest of the value .

below is the code:
1st recursion method

(Toggle Plain Text)

    public static int reverseDigit(int num)
    {   
        int reverseNum = 0;        
       
       if(num > 0) 
       {          
           reverseNum = reverseNum *10 + num % 10;
           return reverseDigit(num/10);       
       }        
       else
           return reverseNum;
    }

2nd Recursion method

(Toggle Plain Text)

    public static int reverseDigit(int num)
    {   
        int reverseNum = 0;        
       
       if(num < 0) 
       {          
           return reverseNum;
       }        
       
        else{
           reverseNum = reverseNum *10 + num % 10;
           return reverseDigit(num/10);       
        }
    }

Thank you for your help in advance.

stevengs · Oct 11th, 2005, 5:02 AM

Quote:

Originally Posted by pr0gm3r

I have two different recursion method but both of them are not returning the rest of the value.

Please expand on this. What values are you giving them, and what are they returning?

pr0gm3r · Oct 11th, 2005, 9:51 AM

my input is an integer such as: 78491

and it should return 19487
but the method just return: 0

btw this is my homework... hehehehe..
On the recursive method I can only pass one parameter and no global variable...

maybe the reverseNum is always set to 0 because I delcared it at the first level of the recursive called...

hehehe... very starnge to me... Thank you for your help and when I find the solutions I'll post it my answer again in this forums ^__^

GBU

DaWei · Oct 11th, 2005, 12:33 PM

Firstly, you just have some serious logical errors. There's nothing there you can't find if you but invest in a pencil, some paper, and the time for a couple of iterations.

Now, as to your approach, in general. I don't know why you're using recursion, at it seems inefficient for this particular task, but let's say that's beside the point. You are determining the least significant digit, then spiralling into the recursion to do it again. When you're fully nested, your least significant digit is the most significant of the original number. When you back out, multiplying by 10, you are simply reconstructing the number in its original form. Without any real time spent in analysis of the BEST approach, I would say the solution, with the GIVEN approach, is to carry in the MSD, not the LSD, then reconstruct the number.

This is C syntax, but I presume you can convert it easily enough.

(Toggle Plain Text)

int reverseDigits (int num, int msd)
{
    int newNumber = num / 10;
    int newMsd = num % 10;
    msd = msd * 10 + newMsd;
    if (newNumber)
    {
        msd = reverseDigits (newNumber, msd);
    }
    return msd;
}

Originally called thusly: number = reverseDigits (number, 0);

Oct 11th, 2005, 4:21 AM	#1
pr0gm3r Hobbyist Programmer Join Date: Dec 2004 Location: CA Posts: 102 Rep Power: 4	Debug recursion method() Hi guys, I need your expertice to debug my recursion method... Here is the problem: I am trying to figure out how come my code is not working properly. The design and algorithm are correct but it seem that the recursion method did not fully return the whole value that passed. I have two different recursion method but both of them are not returning the rest of the value . below is the code: 1st recursion method (Toggle Plain Text) public static int reverseDigit(int num) { int reverseNum = 0; if(num > 0) { reverseNum = reverseNum 10 + num % 10; return reverseDigit(num/10); } else return reverseNum; } public static int reverseDigit(int num) { int reverseNum = 0; if(num > 0) { reverseNum = reverseNum 10 + num % 10; return reverseDigit(num/10); } else return reverseNum; } 2nd Recursion method (Toggle Plain Text) public static int reverseDigit(int num) { int reverseNum = 0; if(num < 0) { return reverseNum; } else{ reverseNum = reverseNum 10 + num % 10; return reverseDigit(num/10); } } public static int reverseDigit(int num) { int reverseNum = 0; if(num < 0) { return reverseNum; } else{ reverseNum = reverseNum 10 + num % 10; return reverseDigit(num/10); } } Thank you for your help in advance. __________________ -- pr0gm3r

Oct 11th, 2005, 9:51 AM	#3
pr0gm3r Hobbyist Programmer Join Date: Dec 2004 Location: CA Posts: 102 Rep Power: 4	my input is an integer such as: 78491 and it should return 19487 but the method just return: 0 btw this is my homework... hehehehe.. On the recursive method I can only pass one parameter and no global variable... maybe the reverseNum is always set to 0 because I delcared it at the first level of the recursive called... hehehe... very starnge to me... Thank you for your help and when I find the solutions I'll post it my answer again in this forums ^__^ GBU __________________ -- pr0gm3r Last edited by pr0gm3r; Oct 11th, 2005 at 10:01 AM.

Oct 11th, 2005, 12:33 PM	#4
DaWei Resident Grouch Join Date: Jun 2005 Posts: 6,453 Rep Power: 10	Firstly, you just have some serious logical errors. There's nothing there you can't find if you but invest in a pencil, some paper, and the time for a couple of iterations. Now, as to your approach, in general. I don't know why you're using recursion, at it seems inefficient for this particular task, but let's say that's beside the point. You are determining the least significant digit, then spiralling into the recursion to do it again. When you're fully nested, your least significant digit is the most significant of the original number. When you back out, multiplying by 10, you are simply reconstructing the number in its original form. Without any real time spent in analysis of the BEST approach, I would say the solution, with the GIVEN approach, is to carry in the MSD, not the LSD, then reconstruct the number. This is C syntax, but I presume you can convert it easily enough. (Toggle Plain Text) int reverseDigits (int num, int msd) { int newNumber = num / 10; int newMsd = num % 10; msd = msd * 10 + newMsd; if (newNumber) { msd = reverseDigits (newNumber, msd); } return msd; } int reverseDigits (int num, int msd) { int newNumber = num / 10; int newMsd = num % 10; msd = msd * 10 + newMsd; if (newNumber) { msd = reverseDigits (newNumber, msd); } return msd; } Originally called thusly: number = reverseDigits (number, 0); __________________ Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code. Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

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Programming Forums > Application Development > Java

Debug recursion method()