Context: I am implementing the Push-Relable Algorithm for MaxFlow in a network and want to keep track of labels of all nodes, for each possible label (2*V-1 many) I want to have a doubly-linked list containing the nodes with that label.
So I have a vector where each entry is a list. Now I need to delete an element from one list and move it into another list in another vector-entry.
In order to do so, I use an vector (wich size is equal to the number of elements) where each entry is an interator, so I always know the position of each element.
Before implementing it on a bigger scale I wanted to try whether it works at all. So I create the two vectors, add one element into a list, store the iterator in the other vector and try to delete that element again.
But the std::vector::erase() method always gets me SegFaults. Did I miss something?
int V=50;
int i=0, v=42;
vector<list<int> > B(2*V-1);
vector<list<int>::iterator> itstorage(V) ;
B[i].push_back(v);
itstorage[v]=B[i].end();
B[i].erase(itstorage[v]);
B[i].end() does not refer to the last item you pushed, it is one past the item you pushed.
What you want is:
std::list<int>::iterator p = B[i].end();
--p;
Alternatively, instead of using push_back, you could use the insert member function which returns an iterator to the newly inserted item.
itstorage[v] = B[i].insert(B[i].end(), v);
Related
I'm implementing LRUCache, where in unordered_map I store an iterator to list. When I move the most "fresh" element to the head, I need to iterator not changed.
I need to swap exactly nodes, not values in nodes. I'm finding the way to do it.
I tried to do it with std::iter_swap, but it's just implemented as std::swap(*it_first, *it_second)
std::list<std::string> list;
list.emplace_back("first");
list.emplace_back("second");
auto it_first = list.begin();
auto it_second = ++list.begin();
std::iter_swap(it_first, it_second);
assert(list.begin() == it_second);
I need to swap two nodes to passed assert.
splice looks like it can do this with something like:
list.splice(it_first, list, it_second);
That says "Splice in it_second from myself (list, the second argument), before the first node in myself". The method guarantees that "the iterators to moved elements remain valid, but now refer into *this, not into other.", which implies the raw nodes themselves are moved.
Splice is a member function that puts part of a linked list into another linked list in constant time.
Why does it need to be a member function? I would expect that I can splice with just iterators into the lists with having a handle on the list itself. Why should the list to be spliced need to be an argument in addition to the start and end iterators?
For testing, I made three lists and mixed up the containers and iterators.
See in the splice below where the containers (empty) don't match the iterators (test0 and test1):
list<int> test0;
list<int> test1;
list<int> empty;
test0.push_back(1);
test0.push_back(2);
test0.push_back(3);
test1.push_back(4);
test1.push_back(5);
test1.push_back(6);
empty.splice(test0.end(), empty, test1.begin(), test1.end());
printf("empty size: %ld\n", empty.size());
printf("test0 size: %ld\n", test0.size());
printf("test1 size: %ld\n", test1.size());
for (const auto& i : test0) {
printf("%d\n", i);
}
Surprisingly, it all worked fine, even the size!
empty size: 0
test0 size: 6
test1 size: 0
1
2
3
4
5
6
I can somewhat understand the iteration working because it just runs until next is null, without regard to the container's front/back pointers. But how did it get the size right? Maybe size is calculated dynamically?
Edit: Based on this explanation of size, size is calculated dynamically for lists, in linear time. So the container is really just a dummy argument. Maybe it's only needed when adding new elements because it has the allocator for making new nodes in the list?
std::list::splice modifies the size of the container. You can't use a container's iterators to modify it's size. You'll notice that there are no free functions in the standard library that can insert new elements into a range using only iterators. At best they can rearrange them.
For example, std::remove shuffles the elements to remove at the end of the container and returns an iterator identifying the range of elements that need to be removed. It can't really remove elements from the range itself.
There are some workarounds, such as by using std::back_inserter, but that works by simulating an unbound range.
I was looking at the std::list::splice implementation the other day.
Typically the iterator abstracts pointers to the list node private implementation. The list nodes contain the _M_prev and _M_next pointers to their neighbor nodes - this is purely implementation dependent. For an empty list, the list contains a sentinal node which serves as both head and tail (again implementation dependent).
So I thought I would try to implement splice using only the list nodes:
void splice(const_iterator pos, list& other,
const_iterator first, const_iterator last)
{
// Hook up last.
last->_M_next = pos->_M_next;
pos->_M_next->_M_prev = last;
// Hook up first.
pos->_M_next = first;
first->_M_prev = pos;
}
I think that looks correct, but I could be wrong.
So based on that implementation, and if size is calculated dynamically, then that would work as you said.
However as François Andrieux pointed out, size being calculated dynamically would be wasteful, and so the container needs to be involved so that the internal size count can be maintained.
Alright. For the sake of other (more simple but not explanatory enough) questions that this might look like, I am not asking if this is possible or impossible (because I found that out already), I am asking if there is a lighter alternative to my question.
What I have is what would be considered a main class, and in that main class, there is a variable that references to a 'World Map' class. In essence, this 'WorldMap' class is a container of other class variables. The main class does all of the looping and updates all of the respective objects that are active. There are times in this loop that I need to delete an object of a vector that is deep inside a recursive set of containers (As shown in the code provided). It would be extremely tedious to repeatedly have to reference the necessary variable as a pointer to another pointer (and so on) to point to the specific object I need, and later erase it (this was the concept I used before switching to C++11) so instead I have a range for loop (also shown in the code). My example code shows the idea that I have in place, where I want to cut down on the tedium as well as make the code a lot more readable.
This is the example code:
struct item{
int stat;
};
struct character{
int otherStat;
std::vector<item> myItems;
};
struct charContainer{
std::map<int, character> myChars;
};
int main(){
//...
charContainer box;
//I want to do something closer to this
for(item targItem: box.myChars[iter].myItems){
//Then I only have to use targItem as the reference
if(targItem.isFinished)
box.myChars[iter].myItems.erase(targItem);
}
//Instead of doing this
for(int a=0;a<box.myChars[iter].myItems.size();a++){
//Then I have to repeatedly use box.myChars[iter].myItems[a]
if(box.myChars[iter].myItems[a].isFinished)
box.myChars[iter].myItems.erase(box.myChars[iter].myItems[a]);
}
}
TLDR: I want to remove the tedium of repeatedly calling the full reference by using the new range for loops shown in C++11.
EDIT: I am not trying to delete the elements all at once. I am asking how I would delete them in the matter of the first loop. I am deleting them when I am done with them externally (via an if statement). How would I delete specific elements, NOT all of them?
If you simply want to clear an std::vector, there is a very simple method you can use:
std::vector<item> v;
// Fill v with elements...
v.clear(); // Removes all elements from v.
In addition to this, I'd like to point out that [1] to erase an element in a vector requires the usage of iterators, and [2] even if your approach was allowed, erasing elements from a vector inside a for loop is a bad idea if you are not careful. Suppose your vector has 5 elements:
std::vector<int> v = { 1, 2, 3, 4, 5 };
Then your loop would have the following effect:
First iteration: a == 0, size() == 5. We remove the first element, then the vector will contain {2, 3, 4, 5}
Second iteration: a == 1, size() == 4. We then remove the second element, then the vector will contain {2,4,5}
Third iteration: a == 2, size() == 3. We remove the third element, and we are left with the final result {2,4}.
Since this does not actually empty the vector, I suppose it is not what you were looking for.
If instead you have some particular condition that you want to apply to remove the elements, it is very easily applied in C++11 in the following way:
std::vector<MyType> v = { /* initialize vector */ };
// The following is a lambda, which is a function you can store in a variable.
// Here we use it to represent the condition that should be used to remove
// elements from the vector v.
auto isToRemove = [](const MyType & value){
return /* true if to remove, false if not */
};
// A vector can remove multiple elements at the same time using its method erase().
// Erase will remove all elements within a specified range. We use this method
// together with another method provided by the standard library: remove_if.
// What it does is it deletes all elements for which a particular predicate
// returns true within a range, and leaves the empty spaces at the end.
v.erase( std::remove_if( std::begin(v), std::end(v), isToRemove ), std::end(v) );
// Done!
I am deleting them when I am done with them externally (via an if statement). How would I delete specific elements, NOT all of them?
In my opinion, you're looking at this the wrong way. Writing loops to delete items from a sequence container is always problematic and not recommended. Strive to stay away from removing items in this fashion.
When you work with containers, you should strategically set up your code so that you place the deleted or "about to be deleted" items in a part of the container that is easily accessed, away from the items in the container that you do not want to delete. At the time you actually do want to remove them, you know where they are and thus can call some function to expel them from the container.
One answer was already given, and that is to use the erase-remove(if) idiom. When you call remove or remove_if, the items that are "bad" are moved to the end of the container. The return value for remove(_if) is the iterator to the start of the items that will be removed. Then you feed this iterator to the vector::erase method to delete these items permanently from the container.
The other solution (but probably less used) is the std::partition algorithm. The std::partition also can move the "bad" items to the end of the container, but unlike remove(_if), the items are still valid (i.e. you can leave them at the end of the container and still use them safely). Then later on, you can remove them as you wish in a separate step since std::partition also returns an iterator.
Why not have a standard iterator iterating over a vector. That way you can delete the element by passing an iterator. Then .erase() will return the next available iterator. And if your next iterator is iterator::end() then your loop will exit.
I need a list of elements that are always sorted. the operation involved is quite simple, for example, if the list is sorted from high to low, i only need three operations in some loop task:
while true do {
list.sort() //sort the list that has hundreds of elements
val = list[0] //get the first/maximum value in the list
list.pop_front() //remove the first/maximum element
...//do some work here
list.push_back(new_elem)//insert a new element
list.sort()
}
however, since I only add one elem at a time, and I have speed concern, I don't want the sorting go through all the elements, e.g., using bubble sorting. So I just wonder if there is a function to insert the element in order? or whether the list::sort() function is smarter enough to use some kind of quick sort when only one element is added/modified?
Or maybe should I use deque for better speed performance if above are all the operations needed?
thanks alot!
As mentioned in the comments, if you aren't locked into std::list then you should try std::set or std::multiset.
The std::list::insert method takes an iterator which specifies where to add the new item. You can use std::lower_bound to find the correct insertion point; it's not optimal without random access iterators but it still only does O(log n) comparisons.
P.S. don't use variable names that collide with built-in classes like list.
lst.sort(std::greater<T>()); //sort the list that has hundreds of elements
while true do {
val = lst.front(); //get the first/maximum value in the list
lst.pop_front(); //remove the first/maximum element
...//do some work here
std::list<T>::iterator it = std::lower_bound(lst.begin(), lst.end(), std::greater<T>());
lst.insert(it, new_elem); //insert a new element
// lst is already sorted
}
I have written a code for solving the following problem: We have a map<double,double> with (relatively) huge number of items. We want to merge the adjacent items in order to reduce the size of the map keeping a certain "loss factor" as low as possible.
To do so, I first populate a list containing adjacent iterators and the associated loss factor (let's say each list element has the following type:
struct myPair {
map<double,double>::iterator curr, next;
double loss;
myPair(map<double,double>::iterator c, map<double,double>::iterator n,
double l): curr(c), next(n), loss(l) {}
};
). This is done as follows:
for (map<double,double>::iterator it1 = myMap.begin(); it1 != --(myMap.end());
it1++) {
map<double,double>::iterator it2 = it1; it2++;
double l = computeLoss(it1,it2);
List.push(myPair(it1,it2,l));
}
Then, I find the list element corresponding to the lowest loss factor, erase the corresponding elements from the map and insert a new element (result of merging curr and next) in the map. Since this also changes the list elements corresponding to the element after next or before curr I update the corresponding entries and also the associated loss factor.
(I don't get into the details of how to implement the above efficiently but basically I am combining a double linked list and a heap).
While the erase operations should not invalidate the remaining iterators for some specific input instances of the program I get the double free or corruption error exactly at the point where I attempt to erase the elements from the map.
I tried to track this and it seems this happens when both first and second entries of the two map elements are very close (more precisely when the firsts of curr and next are very close).
A strange thing is that I put an assert while populating the list to ensure that in all entries curr and next are different and the same assert in the loop of removing elements. The second one fails!
I would appreciate if anyone can help me.
P.S. I am sorry for not being very precise but I wanted to keep the details as low as possible.
UPDATE: This is (a very simplified version of) how I erase the elements from the map:
while (myMap.size() > MAX_SIZE) {
t = list.getMin();
/* compute the merged version ... let's call the result as (a,b) */
myMap.erase(t.curr);
myMap.erase(t.next);
myMap.insert(pair<double,double>(a,b));
/* update the adjacent entries */
}
Stored iterators in myPair stay invalid after container modification. You should avoid such technique. Probably when you look into header file you will find some ready drafts for your task?
As mentioned already by the other people, it turns out that using double as the key of the map is problematic. In particular when the values are computed.
Hence, my solution was to use std::multimap instead of map (and then merge the elements with the same key just after populating the map). With this, for example even if a is very close to both keys of t.curr and t.next or any other element, for sure the insert operation creates a new element such that no existing iterator in the list would point to that.