Duplicates in list

Chris Losinger

i'm going to guess that that's probably a side effect of: a) not setting prev to anything but NULL and b) not initializing 'start' the first time around. and, maybe i don't understand the task, but why are you doing this:

if(p->prev != p->next)

why compare prev and next pointers ? and why compare prev and next and not p itself:

if(p->key != p->next->key)

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sadas232341s

I fixed the prev problem. "if(p->prev != p->next)" is a shot in the dark. Any ideas how to show only uniques :)

Chris Losinger

you're on the right track. but the key thing that's missing is that the list needs to be sorted before you can be guaranteed that duplicates will show up in p and p->next. duplicates, but p, p->next will never be the same: 1,4,3,5,3 this will work: 1,3,3,4,5

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sadas232341s

it's getting messy. How to sort a list??? (Doubly linked)

Luc Pattyn

You do not need to actually sort the list, since that would mean you first compare a lot of them and move around some nodes, then remove some (the duplicates). That is too much effort for what is needed. What you need to do is compare a lot and remove, but never move. In pseudo-code (always solve an algorithmic problem in pseudo-code first):

foreach(Node n1) {
foreach (Node n2 before n1) {
if (n2.value==n1.value) {
remove(n1);
break;
}
}
}

:) PS: and that could be done on a single-linked list as well, as both loops are enumerating from the start and never need to go backward.

Luc Pattyn [Forum Guidelines] [My Articles] Nil Volentibus Arduum

Please use <PRE> tags for code snippets, they preserve indentation, improve readability, and make me actually look at the code.

sadas232341s

for(int i = 1; i < N; i++) for(int j = N - 1; j >= i; j--) if(p->key > p->next->key) { t = p->key; p->key = p->next->key; p->next->key = t; } Here is the sort but it doesn' t work

sadas232341s

i did not understand anything you said. And whats that code C++?

Chris Losinger

if you don't want to sort:

for each node, Cur
for each node, Temp
if Temp!=Cur
if Temp.value==Cur.value, duplicate
Temp=Temp.next
Cur=Cur.next

basically, for each node, you compare its value to every other node. it's inefficient, but it will work. you can also sort your list when you add: 1. walk the list, find the first node with a value greater than the new value. this is 'Cur' 2. change the next pointer of Cur->prev to point to your new node. 3. set your new node's prev to Cur->prev. now you're all linked in the prev direction. 4. change the prev pointer of Cur to point to your new node. 5. set your new node's cur to point to Cur. now you're all linked in the next direction. that way, the list is always sorted, and you can just walk it, looking for duplicates.

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sadas232341s

OK, I decided to sort the list, but can you tell me why is not working well?

Chris Losinger

first problem: you're never moving p

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sadas232341s

how to move it? p++

Chris Losinger

p = p->next; p = p->prev; (forgot you're inserting at the beginning, not at the end of list) if (p==NULL) bail.

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sadas232341s

OK, here is the full code that I' m working on. Can you tell me my mistakes exactly because I can' t understand where to put what. The UniqueList function is important, which now only shows the list and nothing more. The sort does not work...

#include "iostream"

using namespace std;

void Add(int n);
void UniqueList(int N);

struct Elem
{
int key;
Elem *prev;
Elem *next;
} *start;

int main()
{
int num;
int N = 0;

start = NULL;

while(cin >> num)
{
	Add(num);

	N++;
}

cout << endl;

UniqueList(N);

return 0;

}

void Add(int n)
{
Elem *p = start;
start = new Elem;

start->key = n;
start->next = p;
start->prev = NULL;

if(NULL != p) p->prev = start;

}

void UniqueList(int N)
{
Elem *p = start;
int t;

if(NULL != start) cout << "List:" << endl;
else cout << "Empty list!";

for(int i = 1; i < N; i++)
	for(int j = N - 1; j >= i; j--)
	{
		if(p->key > p->next->key)
		{
			t = p->key;
			p->key = p->next->key;
			p->next->key = t;
		}
	}

while(NULL != p)
{
	cout << p->key << endl;

	p = p->next;
}

}

sadas232341s

Just the sort?

Chris Losinger

maybe it's best to rethink this from the beginning. if your goal is to have a list with no duplicates, the easiest thing to do is to simply not add duplicates. so, every time, you go to add a new Elem, walk through your list and see if you already have an Elem with that key. if you do, don't add anything. to do that, you need a function that can walk through the list and look for an Elem with a given key:

bool haveKey(Elem *p, int k)
{
while (p!=NULL)
{
if (p->key=k) return true;
p = p->next;
}
return false;
}

just call that at the top of your Add. if it returns false, don't add anything.

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sadas232341s

No, the assignment is to add duplicates, but when show the list to show it without them. I think the sort thing will do the job but can' t do it properly.

modified on Monday, April 18, 2011 4:57 PM

Chris Losinger · modified on Monday, April 18, 2011 4:57 PM

ok, then i would recommend sorting the nodes on input. it is far more efficient than sorting the whole list, and it's actually fairly straightforward. (i outlined it above) unfortunately, i have to leave for the day. so.. good luck!

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David Crow

TCPMem wrote:

How to sort a list???

Insert the items in their "sorted" position within the list.

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