### noobinho's blog

By noobinho, history, 3 years ago,

I want a list of balanced binary search trees so that I can answer queries for insert, delete and median in each tree. Based on this post I coded the following:

struct Tree{
multiset<int> ms;
multiset<int>::iterator med_it;
int med_idx;
};

int Ceil(int a, int b){
return (a+1)/b;
}

void initializeTree(Tree (&tree), int el){
(tree.ms).insert(el);
(tree.med_it) = (tree.ms).begin();
(tree.med_idx) = 1;
}

void TreeInsert(Tree (&tree), int el){
(tree.ms).insert(el);

if(el < *(tree.med_it)) (tree.med_idx)++;

int idx = Ceil((tree.ms).size(),2);

if(tree.med_idx < idx) (tree.med_it)++;
else if(tree.med_idx > idx) (tree.med_it)--;

tree.med_idx = idx;
}

void TreeDelete(Tree (&tree), int el){
multiset<int>::iterator aux;

int idx = Ceil((int)(tree.ms).size() - 1,2);

if(el == *(tree.med_it)){
aux = (tree.med_it);

if((tree.med_idx) == idx) aux++;
else aux--;

(tree.ms).erase((tree.med_it));
(tree.med_it) = aux;
}
else{
aux = (tree.ms).find(el);
(tree.ms).erase(aux);

if(el < *(tree.med_it)) (tree.med_idx)--;

if((tree.med_idx) < idx) (tree.med_it)++;
else if((tree.med_idx) > idx) (tree.med_it)--;
}

(tree.med_idx) = idx;
}

int TreeMerge(Tree (&tree1), Tree (&tree2)){
multiset<int>::iterator it;

if((tree1.ms).size() >= (tree2.ms).size()){
for(it = (tree2.ms).begin(); it != (tree2.ms).end(); it++) TreeInsert(tree1, *it);
return 1;
}
else{
for(it = (tree1.ms).begin(); it != (tree1.ms).end(); it++) TreeInsert(tree2, *it);
return 2;
}
}


I tested all the functions and they seem to work well. But then I created a list L to be my list and inserted some trees on it to test the functions on the members of the list. The following code shows that:

Tree tree1, tree2;

initializeTree(tree1,9);
initializeTree(tree2,4);

list<Tree> L;

L.push_back(tree1);
L.push_back(tree2);


Then I acessed the members of the list with list::iterator as shown in the code below:

list<Tree>::iterator L_cur = L.begin(), L_next = L.begin();
L_next++;


So I applied the functions to *L_cur and *L_next and the med_it pointer got some bug. For example, when running the code below:

int merge = TreeMerge(*L_cur,*L_next);


When I access the content of the L_cur and L_next pointers all seems to work as expected except for the med_it pointer of the tree in L_cur, which continues pointing to 9 instead of 4.

When I run the same function with two Tree objects it seems to work well, as shown in the code below:

Tree tree1, tree2;

initializeTree(tree1,9);
initializeTree(tree2,4);

int merge = TreeMerge(tree1,tree2);


Why it occurs?

• +8

| Write comment?
 » 3 years ago, # | ← Rev. 2 →   0 I'm not pretty sure (not compiler) but your $Tree$ is just a struct without explicitly defined copy/move constructor. That means your iterator will be copied too during push_back, which uses copy constructor. Order of actions: Create tree1 with ms1 and iterator Create tree2 with ms2 and iterator list.push_back(tree1) -> tree1_copy.med_it points at element in ms1, tree1_copy.ms1_copy is empty list.push_back(tree2) -> tree2_copy.med_it points at element in ms2, tree2_copy.ms2_copy is empty TreeMerge(tree1_copy, tree2_copy) — correct result since no usage of med_it. TreeDelete would affect ms1, not ms1_copy.
•  » » 3 years ago, # ^ | ← Rev. 2 →   0 Thanks for answer. I run it here and tree1_copy.ms1_copy and tree2_copy.ms2_copy don't seem to be empty. As you can see, the problem is only with the med_it pointer. How could I correct it to run as expected?
•  » » » 3 years ago, # ^ |   0 Yes, my mistake: copy of nonempty multiset is nonempty too.There are a lot of options: Simplest as I see is not create any copy of Tree: create empty trees within list and call initializeTree for list elements: list L; L.push_back(Tree()); L.push_back(Tree()); initializeTree(*L.begin(), 9); initializeTree(*L.rbegin(), 4); int merge = TreeMerge(*L.begin(), *L.rbegin()); And you should mark copy/move constructors of Tree as delete. Then compiler permit any move/copy. If you know that tree1 and tree2 would never used further you can define move constructor Tree(Tree&& other) : ms(move(other.ms)), med_it(move(other.med_it)), med_idx(move(other.med_idx)) {} and do: Tree tree1, tree2; initializeTree(tree1, 9); initializeTree(tree2, 4); list L; L.emplace_back(move(tree1)); L.emplace_back(move(tree2)); int merge = TreeMerge(*L.begin(), *L.rbegin());  Third option is define correct copy constructor. Since you already have med_idx, then you can initialize med_it as ms.begin() + med_idx. Tree(const Tree& other) : ms(other.ms) { med_idx = other.med_idx; med_it = next(ms.begin(), med_idx); } then no edit of your code is needed.