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Feb 28th, 2007

help with recursion

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can someone tell me what is wrong with this code? We are supposed to use recursion to see if the two arrays are equal.

C++ Syntax (Toggle Plain Text)
int Equal(int a[], int b[], int left, int size)
{
 if (a == b)
   {
    return Equal(a, b, left + 1, size);
   }
 else
   return false;
}
 
int Equal2(int a[], int c[], int left, int size)
{
  if (a == c)
    {
     return Equal2(a, c, left + 1, size);
    }
  else
    return false;
 
}
int Equal(int a[], int b[], int left, int size)
{
 if (a == b)
   {
    return Equal(a, b, left + 1, size);
   }
 else
   return false;
}

int Equal2(int a[], int c[], int left, int size)
{
  if (a == c)
    {
     return Equal2(a, c, left + 1, size);
    }
  else
    return false;

}

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ammochck21

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Feb 28th, 2007

Re: help with recursion

Try running through your code on paper, as though you were the computer. You'll see that you're not using the left variable for anything, except the +1 before recursing. And you'll recurse infinitely unless a and b are different. Which they almost always will be, unless you're comparing an array to itself. You need to index the arrays (using left) and check for bounding errors to make sure you don't go over the length of the arrays.

Infarction

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Posting Virtuoso

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Feb 28th, 2007

Re: help with recursion

Why are you using 2 separate functions to perform the recursion?

Your comparison is likely to be what's causing the problem -- you can't just compare the starting memory addresses for 2 arrays to check if the contents are equal, you have to individually check each element.

A few other points:

You need to make sure that the index value is not larger than the size of the array (I'm guessing that left is your index value). If it is, return immediately.
The method described only compares arrays of equal size. Hopefully this is what your teacher wants.
Recursion like this example isn't really going to be that useful. The problem here could have been solved with a loop, but regardless, you still have to do your assignments.

John A

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Feb 28th, 2007

Re: help with recursion

I kind of see what you are saying- and when i ran through it, i did it right- which obviously means that i wrote the program wrong. I dont really understand what you mean by i need to index the arrays using left. I also added a while loop saying while its not at the end of the array but then i got a weird number for Equal 2.

ammochck21

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Feb 28th, 2007

Re: help with recursion

Yes i would rather use a for loop for this too but we arent supposed to. the arrays are all of equal size and the first and third arrays are identical and the second one is one number off. I guess i dont understand how to transform a for loop into a recursive function.

ammochck21

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Feb 28th, 2007

Re: help with recursion

>I dont really understand what you mean by i need to index the arrays using left.
In other words:

C++ Syntax (Toggle Plain Text)
if (a[ left ] == b[ left ]) {
   // element matches
}
else {
   // element doesn't math; return false
}
if (a[ left ] == b[ left ]) {
   // element matches
}
else {
   // element doesn't math; return false
}

Arrgh, those stupid [ left ] tags keep parsing out!

John A

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Feb 28th, 2007

Re: help with recursion

i actually had a(left) == b(left) but when i originally posted it they went away sorry

Last edited by ammochck21; Feb 28th, 2007 at 1:33 am.

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Re: help with recursion

Click to Expand / Collapse Quote originally posted by ammochck21 ...

i actually had a(left) == b(left) but when i originally posted it they went away sorry

Well, then it should just be a matter of implementing bounds-checking, and then your program should work, unless I'm totally missing something here...

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Feb 28th, 2007

Re: help with recursion

I actually kind of figured out what was wrong but i cant seem to figure out how to implement it.

C++ Syntax (Toggle Plain Text)
int Equal(int a[], int b[], int left, int size)
{
  if (left <= size)
   {
    if (a(left) == b(left))
     {
       return  Equal(a, b, left + 1, size);    //need to have a return true here or somewhere
     }
    else
      return false;
   }
}
int Equal(int a[], int b[], int left, int size)
{
  if (left <= size)
   {
    if (a(left) == b(left))
     {
       return  Equal(a, b, left + 1, size);    //need to have a return true here or somewhere
     }
    else
      return false;
   }
}

Last edited by ammochck21; Feb 28th, 2007 at 2:05 am.

ammochck21

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Feb 28th, 2007

Re: help with recursion

You need to do your bounds-checking. If the last elements are equal, you return true, will which will transfer through all the calls of the function.

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