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Posted on 1998-04-30
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I am looking for algorithms which will add and subtract two dynamically allocated linked lists. These lists may be of different sizes. Do I need to add zeros to the front of the shorter list? If so how do I tell when the shorter list needs to be added to? Do I compare the lists first? Please help. Thanks
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Question by:brianhill
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Expert Comment

ID: 1169038
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Expert Comment

ID: 1169039
Can you elaborate on what you mean by adding and subtracting?
i.e.  Is this a list of integers and you need the elements of the lists added together?  Or do you need to add or subtract the nodes of the list, resulting in an expansion or reduction of the list?
Also, are you using STL?

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Expert Comment

ID: 1169040
Hmmmm.  My first answer (now deleted) assumed that when you said "add two linked list" you mean add the items in one list to another list so that the list contained both sets of items.  but now I'm guessing that that is wrong.  what you are asking about is arithmetically adding numbers stored in each element of a linked list?  Is that right?

If so, my first question to you (yes, the expert is asking questions) is how do you plan to store the result.  There are two main strategies.  The first is that one list is altered to contain the results.  That is, each entry will contain the original value plusss the associated value fromt he other list.  The second strategy is to create a third list that contains the results.   This leaves the two input lists unchanged.  Which way are you planning to go?  (This matters for the best answer to your question.)
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Author Comment

ID: 1169041
elaboration: I would have two seperate linked lists of integers and each entire list would represent a single integer. Then I would like for these two seperate representations to be either added or subtracted resulting in a third linked list.For example if I had a linked list with three nodes, 9  7  3 as the values of those respective nodes, then the linked list would represent the integer 973.I hope this clarifies my question. Thanks for the help.
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Accepted Solution

nietod earned 100 total points
ID: 1169042
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LVL 22

Expert Comment

ID: 1169043
Just as an aside, you would be much better of using dynamically allocated arrays for this.  It would be faster, more memory efficent and easier to program.  I can provide details if you are interested.

Lets say you use the following structure  to store digits.

struct Digit
{
char Dig;  // should be 0-9 only.  Could use enum.  Should have validity checks.
Digit *NxtDigit;
}

The "numbers" should be stored with linked lists that start with the lleast significant digit and proceed to the most significant.  Thus 123 would be stored as 3->2->1.

Then you could do.

{
Digit *Result = NULL;;
Digit *LastDigit = NULL;
bool Carry = false;

while (Src1 || Src2)
{
int D1;
int D2;
int DR;

if (Src1)
{
D1 = Src1->Dig;
++Src1;
}
else
D1 = 0;

if (Src2)
{
D2 = Src2->Dig;
++Src2;
}
else
D2 = 0;

R = D1 + D2;
if (Carry)
++R;
if (R > 10)
{
R -= 10;
Carry = true;
}
else
Carry = false.

Digit *R = new Digit;
R->Dig = DR;
R->NxtDigit = NULL;
If (LastDigit)
LastDigit->NxtDigit = R;
else
Result = R
LastDigit = R;
}
return Result;
}

That should be close.  Might not be 100% correct.  Try it and let me know.
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Expert Comment

ID: 1169044
Just checking up.  Is this working for you?
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