Why is inserting into a set<vector<string>> so slow? - c++

For a class project we are making a simple compiler / Relational Database. Mine produces the correct answers, but too slowly on large queries. I ran visual studio's performance analysis and my program is spending 80% of it's time inserting my tuples (rows in a table) into a set. The function is part of computing a cross product, so the result has lots and lots of rows, but I need suggestions on a faster way to insert my tuples into the set.
for (set<vector<string>>::iterator it = tuples.begin(); it != tuples.end(); ++it)
{
for (set<vector<string>>::iterator it2 = tuples2.begin(); it2 != tuples2.end(); ++it2)
{
vector<string> f(*it);
f.insert(f.end(), it2->begin(), it2->end());
newTuples.insert(f); //This is the line that takes all the processing time
}
}

You are copying big vector by value for no reason. You should move: newTuples.insert(std::move(f));

A set might be the wrong container. A set is ordered, and keeps only unique elements. There might be many string comparisons happening when you insert a new vector.
Use a list or a vector instead (if you can).
...and avoid needless copying, as SergeyA already pointed out in his answer

We might as well go C++11 (totally untested code)
for (const auto& it : tuples) {
for (const auto& it2 : tuples2) {
auto where = newTuples.emplace(it); // returns where its placed
auto& vect = where.first; // makes the next more readable
vect.insert(vect.end(), it2.begin(), it2.end());
}
}
Note on collisions some strings disappears from the result, is that really what you want?
Your using the vector as key, will that ever be a collision? add
if (!where.second) {
; // collision
}
to check.
This should remove all double work of moving (if the compiler doesn't optimize it away anyway).

Related

Constraining remove_if on only part of a C++ list

I have a C++11 list of complex elements that are defined by a structure node_info. A node_info element, in particular, contains a field time and is inserted into the list in an ordered fashion according to its time field value. That is, the list contains various node_info elements that are time ordered. I want to remove from this list all the nodes that verify some specific condition specified by coincidence_detect, which I am currently implementing as a predicate for a remove_if operation.
Since my list can be very large (order of 100k -- 10M elements), and for the way I am building my list this coincidence_detect condition is only verified by few (thousands) elements closer to the "lower" end of the list -- that is the one that contains elements whose time value is less than some t_xv, I thought that to improve speed of my code I don't need to run remove_if through the whole list, but just restrict it to all those elements in the list whose time < t_xv.
remove_if() though does not seem however to allow the user to control up to which point I can iterate through the list.
My current code.
The list elements:
struct node_info {
char *type = "x";
int ID = -1;
double time = 0.0;
bool spk = true;
};
The predicate/condition for remove_if:
// Remove all events occurring at t_event
class coincident_events {
double t_event; // Event time
bool spk; // Spike condition
public:
coincident_events(double time,bool spk_) : t_event(time), spk(spk_){}
bool operator()(node_info node_event){
return ((node_event.time==t_event)&&(node_event.spk==spk)&&(strcmp(node_event.type,"x")!=0));
}
};
The actual removing from the list:
void remove_from_list(double t_event, bool spk_){
// Remove all events occurring at t_event
coincident_events coincidence(t_event,spk_);
event_heap.remove_if(coincidence);
}
Pseudo main:
int main(){
// My list
std::list<node_info> event_heap;
...
// Populate list with elements with random time values, yet ordered in ascending order
...
remove_from_list(0.5, true);
return 1;
}
It seems that remove_if may not be ideal in this context. Should I consider instead instantiating an iterator and run an explicit for cycle as suggested for example in this post?
It seems that remove_if may not be ideal in this context. Should I consider instead instantiating an iterator and run an explicit for loop?
Yes and yes. Don't fight to use code that is preventing you from reaching your goals. Keep it simple. Loops are nothing to be ashamed of in C++.
First thing, comparing double exactly is not a good idea as you are subject to floating point errors.
You could always search the point up to where you want to do a search using lower_bound (I assume you list is properly sorted).
The you could use free function algorithm std::remove_if followed by std::erase to remove items between the iterator returned by remove_if and the one returned by lower_bound.
However, doing that you would do multiple passes in the data and you would move nodes so it would affect performance.
See also: https://en.cppreference.com/w/cpp/algorithm/remove
So in the end, it is probably preferable to do you own loop on the whole container and for each each check if it need to be removed. If not, then check if you should break out of the loop.
for (auto it = event_heap.begin(); it != event_heap.end(); )
{
if (coincidence(*it))
{
auto itErase = it;
++it;
event_heap.erase(itErase)
}
else if (it->time < t_xv)
{
++it;
}
else
{
break;
}
}
As you can see, code can easily become quite long for something that should be simple. Thus, if you need to do that kind of algorithm often, consider writing you own generic algorithm.
Also, in practice you might not need to do a complete search for the end using the first solution if you process you data in increasing time order.
Finally, you might consider using an std::set instead. It could lead to simpler and more optimized code.
Thanks. I used your comments and came up with this solution, which seemingly increases speed by a factor of 5-to-10.
void remove_from_list(double t_event,bool spk_){
coincident_events coincidence(t_event,spk_);
for(auto it=event_heap.begin();it!=event_heap.end();){
if(t_event>=it->time){
if(coincidence(*it)) {
it = event_heap.erase(it);
}
else
++it;
}
else
break;
}
}
The idea to make erase return it (as already ++it) was suggested by this other post. Note that in this implementation I am actually erasing all list elements up to t_event value (meaning, I pass whatever I want for t_xv).

Iterating over a vector in C++ [duplicate]

Take the following two lines of code:
for (int i = 0; i < some_vector.size(); i++)
{
//do stuff
}
And this:
for (some_iterator = some_vector.begin(); some_iterator != some_vector.end();
some_iterator++)
{
//do stuff
}
I'm told that the second way is preferred. Why exactly is this?
The first form is efficient only if vector.size() is a fast operation. This is true for vectors, but not for lists, for example. Also, what are you planning to do within the body of the loop? If you plan on accessing the elements as in
T elem = some_vector[i];
then you're making the assumption that the container has operator[](std::size_t) defined. Again, this is true for vector but not for other containers.
The use of iterators bring you closer to container independence. You're not making assumptions about random-access ability or fast size() operation, only that the container has iterator capabilities.
You could enhance your code further by using standard algorithms. Depending on what it is you're trying to achieve, you may elect to use std::for_each(), std::transform() and so on. By using a standard algorithm rather than an explicit loop you're avoiding re-inventing the wheel. Your code is likely to be more efficient (given the right algorithm is chosen), correct and reusable.
It's part of the modern C++ indoctrination process. Iterators are the only way to iterate most containers, so you use it even with vectors just to get yourself into the proper mindset. Seriously, that's the only reason I do it - I don't think I've ever replaced a vector with a different kind of container.
Wow, this is still getting downvoted after three weeks. I guess it doesn't pay to be a little tongue-in-cheek.
I think the array index is more readable. It matches the syntax used in other languages, and the syntax used for old-fashioned C arrays. It's also less verbose. Efficiency should be a wash if your compiler is any good, and there are hardly any cases where it matters anyway.
Even so, I still find myself using iterators frequently with vectors. I believe the iterator is an important concept, so I promote it whenever I can.
because you are not tying your code to the particular implementation of the some_vector list. if you use array indices, it has to be some form of array; if you use iterators you can use that code on any list implementation.
Imagine some_vector is implemented with a linked-list. Then requesting an item in the i-th place requires i operations to be done to traverse the list of nodes. Now, if you use iterator, generally speaking, it will make its best effort to be as efficient as possible (in the case of a linked list, it will maintain a pointer to the current node and advance it in each iteration, requiring just a single operation).
So it provides two things:
Abstraction of use: you just want to iterate some elements, you don't care about how to do it
Performance
I'm going to be the devils advocate here, and not recommend iterators. The main reason why, is all the source code I've worked on from Desktop application development to game development have i nor have i needed to use iterators. All the time they have not been required and secondly the hidden assumptions and code mess and debugging nightmares you get with iterators make them a prime example not to use it in any applications that require speed.
Even from a maintence stand point they're a mess. Its not because of them but because of all the aliasing that happen behind the scene. How do i know that you haven't implemented your own virtual vector or array list that does something completely different to the standards. Do i know what type is currently now during runtime? Did you overload a operator I didn't have time to check all your source code. Hell do i even know what version of the STL your using?
The next problem you got with iterators is leaky abstraction, though there are numerous web sites that discuss this in detail with them.
Sorry, I have not and still have not seen any point in iterators. If they abstract the list or vector away from you, when in fact you should know already what vector or list your dealing with if you don't then your just going to be setting yourself up for some great debugging sessions in the future.
You might want to use an iterator if you are going to add/remove items to the vector while you are iterating over it.
some_iterator = some_vector.begin();
while (some_iterator != some_vector.end())
{
if (/* some condition */)
{
some_iterator = some_vector.erase(some_iterator);
// some_iterator now positioned at the element after the deleted element
}
else
{
if (/* some other condition */)
{
some_iterator = some_vector.insert(some_iterator, some_new_value);
// some_iterator now positioned at new element
}
++some_iterator;
}
}
If you were using indices you would have to shuffle items up/down in the array to handle the insertions and deletions.
Separation of Concerns
It's very nice to separate the iteration code from the 'core' concern of the loop. It's almost a design decision.
Indeed, iterating by index ties you to the implementation of the container. Asking the container for a begin and end iterator, enables the loop code for use with other container types.
Also, in the std::for_each way, you TELL the collection what to do, instead of ASKing it something about its internals
The 0x standard is going to introduce closures, which will make this approach much more easy to use - have a look at the expressive power of e.g. Ruby's [1..6].each { |i| print i; }...
Performance
But maybe a much overseen issue is that, using the for_each approach yields an opportunity to have the iteration parallelized - the intel threading blocks can distribute the code block over the number of processors in the system!
Note: after discovering the algorithms library, and especially foreach, I went through two or three months of writing ridiculously small 'helper' operator structs which will drive your fellow developers crazy. After this time, I went back to a pragmatic approach - small loop bodies deserve no foreach no more :)
A must read reference on iterators is the book "Extended STL".
The GoF have a tiny little paragraph in the end of the Iterator pattern, which talks about this brand of iteration; it's called an 'internal iterator'. Have a look here, too.
Because it is more object-oriented. if you are iterating with an index you are assuming:
a) that those objects are ordered
b) that those objects can be obtained by an index
c) that the index increment will hit every item
d) that that index starts at zero
With an iterator, you are saying "give me everything so I can work with it" without knowing what the underlying implementation is. (In Java, there are collections that cannot be accessed through an index)
Also, with an iterator, no need to worry about going out of bounds of the array.
Another nice thing about iterators is that they better allow you to express (and enforce) your const-preference. This example ensures that you will not be altering the vector in the midst of your loop:
for(std::vector<Foo>::const_iterator pos=foos.begin(); pos != foos.end(); ++pos)
{
// Foo & foo = *pos; // this won't compile
const Foo & foo = *pos; // this will compile
}
Aside from all of the other excellent answers... int may not be large enough for your vector. Instead, if you want to use indexing, use the size_type for your container:
for (std::vector<Foo>::size_type i = 0; i < myvector.size(); ++i)
{
Foo& this_foo = myvector[i];
// Do stuff with this_foo
}
I probably should point out you can also call
std::for_each(some_vector.begin(), some_vector.end(), &do_stuff);
STL iterators are mostly there so that the STL algorithms like sort can be container independent.
If you just want to loop over all the entries in a vector just use the index loop style.
It is less typing and easier to parse for most humans. It would be nice if C++ had a simple foreach loop without going overboard with template magic.
for( size_t i = 0; i < some_vector.size(); ++i )
{
T& rT = some_vector[i];
// now do something with rT
}
'
I don't think it makes much difference for a vector. I prefer to use an index myself as I consider it to be more readable and you can do random access like jumping forward 6 items or jumping backwards if needs be.
I also like to make a reference to the item inside the loop like this so there are not a lot of square brackets around the place:
for(size_t i = 0; i < myvector.size(); i++)
{
MyClass &item = myvector[i];
// Do stuff to "item".
}
Using an iterator can be good if you think you might need to replace the vector with a list at some point in the future and it also looks more stylish to the STL freaks but I can't think of any other reason.
The second form represents what you're doing more accurately. In your example, you don't care about the value of i, really - all you want is the next element in the iterator.
After having learned a little more on the subject of this answer, I realize it was a bit of an oversimplification. The difference between this loop:
for (some_iterator = some_vector.begin(); some_iterator != some_vector.end();
some_iterator++)
{
//do stuff
}
And this loop:
for (int i = 0; i < some_vector.size(); i++)
{
//do stuff
}
Is fairly minimal. In fact, the syntax of doing loops this way seems to be growing on me:
while (it != end){
//do stuff
++it;
}
Iterators do unlock some fairly powerful declarative features, and when combined with the STL algorithms library you can do some pretty cool things that are outside the scope of array index administrivia.
Indexing requires an extra mul operation. For example, for vector<int> v, the compiler converts v[i] into &v + sizeof(int) * i.
During iteration you don't need to know number of item to be processed. You just need the item and iterators do such things very good.
No one mentioned yet that one advantage of indices is that they are not become invalid when you append to a contiguous container like std::vector, so you can add items to the container during iteration.
This is also possible with iterators, but you must call reserve(), and therefore need to know how many items you'll append.
If you have access to C++11 features, then you can also use a range-based for loop for iterating over your vector (or any other container) as follows:
for (auto &item : some_vector)
{
//do stuff
}
The benefit of this loop is that you can access elements of the vector directly via the item variable, without running the risk of messing up an index or making a making a mistake when dereferencing an iterator. In addition, the placeholder auto prevents you from having to repeat the type of the container elements,
which brings you even closer to a container-independent solution.
Notes:
If you need the the element index in your loop and the operator[] exists for your container (and is fast enough for you), then better go for your first way.
A range-based for loop cannot be used to add/delete elements into/from a container. If you want to do that, then better stick to the solution given by Brian Matthews.
If you don't want to change the elements in your container, then you should use the keyword const as follows: for (auto const &item : some_vector) { ... }.
Several good points already. I have a few additional comments:
Assuming we are talking about the C++ standard library, "vector" implies a random access container that has the guarantees of C-array (random access, contiguos memory layout etc). If you had said 'some_container', many of the above answers would have been more accurate (container independence etc).
To eliminate any dependencies on compiler optimization, you could move some_vector.size() out of the loop in the indexed code, like so:
const size_t numElems = some_vector.size();
for (size_t i = 0; i
Always pre-increment iterators and treat post-increments as exceptional cases.
for (some_iterator = some_vector.begin(); some_iterator != some_vector.end(); ++some_iterator){ //do stuff }
So assuming and indexable std::vector<> like container, there is no good reason to prefer one over other, sequentially going through the container. If you have to refer to older or newer elemnent indexes frequently, then the indexed version is more appropropriate.
In general, using the iterators is preferred because algorithms make use of them and behavior can be controlled (and implicitly documented) by changing the type of the iterator. Array locations can be used in place of iterators, but the syntactical difference will stick out.
I don't use iterators for the same reason I dislike foreach-statements. When having multiple inner-loops it's hard enough to keep track of global/member variables without having to remember all the local values and iterator-names as well. What I find useful is to use two sets of indices for different occasions:
for(int i=0;i<anims.size();i++)
for(int j=0;j<bones.size();j++)
{
int animIndex = i;
int boneIndex = j;
// in relatively short code I use indices i and j
... animation_matrices[i][j] ...
// in long and complicated code I use indices animIndex and boneIndex
... animation_matrices[animIndex][boneIndex] ...
}
I don't even want to abbreviate things like "animation_matrices[i]" to some random "anim_matrix"-named-iterator for example, because then you can't see clearly from which array this value is originated.
If you like being close to the metal / don't trust their implementation details, don't use iterators.
If you regularly switch out one collection type for another during development, use iterators.
If you find it difficult to remember how to iterate different sorts of collections (maybe you have several types from several different external sources in use), use iterators to unify the means by which you walk over elements. This applies to say switching a linked list with an array list.
Really, that's all there is to it. It's not as if you're going to gain more brevity either way on average, and if brevity really is your goal, you can always fall back on macros.
Even better than "telling the CPU what to do" (imperative) is "telling the libraries what you want" (functional).
So instead of using loops you should learn the algorithms present in stl.
For container independence
I always use array index because many application of mine require something like "display thumbnail image". So I wrote something like this:
some_vector[0].left=0;
some_vector[0].top =0;<br>
for (int i = 1; i < some_vector.size(); i++)
{
some_vector[i].left = some_vector[i-1].width + some_vector[i-1].left;
if(i % 6 ==0)
{
some_vector[i].top = some_vector[i].top.height + some_vector[i].top;
some_vector[i].left = 0;
}
}
Both the implementations are correct, but I would prefer the 'for' loop. As we have decided to use a Vector and not any other container, using indexes would be the best option. Using iterators with Vectors would lose the very benefit of having the objects in continuous memory blocks which help ease in their access.
I felt that none of the answers here explain why I like iterators as a general concept over indexing into containers. Note that most of my experience using iterators doesn't actually come from C++ but from higher-level programming languages like Python.
The iterator interface imposes fewer requirements on consumers of your function, which allows consumers to do more with it.
If all you need is to be able to forward-iterate, the developer isn't limited to using indexable containers - they can use any class implementing operator++(T&), operator*(T) and operator!=(const &T, const &T).
#include <iostream>
template <class InputIterator>
void printAll(InputIterator& begin, InputIterator& end)
{
for (auto current = begin; current != end; ++current) {
std::cout << *current << "\n";
}
}
// elsewhere...
printAll(myVector.begin(), myVector.end());
Your algorithm works for the case you need it - iterating over a vector - but it can also be useful for applications you don't necessarily anticipate:
#include <random>
class RandomIterator
{
private:
std::mt19937 random;
std::uint_fast32_t current;
std::uint_fast32_t floor;
std::uint_fast32_t ceil;
public:
RandomIterator(
std::uint_fast32_t floor = 0,
std::uint_fast32_t ceil = UINT_FAST32_MAX,
std::uint_fast32_t seed = std::mt19937::default_seed
) :
floor(floor),
ceil(ceil)
{
random.seed(seed);
++(*this);
}
RandomIterator& operator++()
{
current = floor + (random() % (ceil - floor));
}
std::uint_fast32_t operator*() const
{
return current;
}
bool operator!=(const RandomIterator &that) const
{
return current != that.current;
}
};
int main()
{
// roll a 1d6 until we get a 6 and print the results
RandomIterator firstRandom(1, 7, std::random_device()());
RandomIterator secondRandom(6, 7);
printAll(firstRandom, secondRandom);
return 0;
}
Attempting to implement a square-brackets operator which does something similar to this iterator would be contrived, while the iterator implementation is relatively simple. The square-brackets operator also makes implications about the capabilities of your class - that you can index to any arbitrary point - which may be difficult or inefficient to implement.
Iterators also lend themselves to decoration. People can write iterators which take an iterator in their constructor and extend its functionality:
template<class InputIterator, typename T>
class FilterIterator
{
private:
InputIterator internalIterator;
public:
FilterIterator(const InputIterator &iterator):
internalIterator(iterator)
{
}
virtual bool condition(T) = 0;
FilterIterator<InputIterator, T>& operator++()
{
do {
++(internalIterator);
} while (!condition(*internalIterator));
return *this;
}
T operator*()
{
// Needed for the first result
if (!condition(*internalIterator))
++(*this);
return *internalIterator;
}
virtual bool operator!=(const FilterIterator& that) const
{
return internalIterator != that.internalIterator;
}
};
template <class InputIterator>
class EvenIterator : public FilterIterator<InputIterator, std::uint_fast32_t>
{
public:
EvenIterator(const InputIterator &internalIterator) :
FilterIterator<InputIterator, std::uint_fast32_t>(internalIterator)
{
}
bool condition(std::uint_fast32_t n)
{
return !(n % 2);
}
};
int main()
{
// Rolls a d20 until a 20 is rolled and discards odd rolls
EvenIterator<RandomIterator> firstRandom(RandomIterator(1, 21, std::random_device()()));
EvenIterator<RandomIterator> secondRandom(RandomIterator(20, 21));
printAll(firstRandom, secondRandom);
return 0;
}
While these toys might seem mundane, it's not difficult to imagine using iterators and iterator decorators to do powerful things with a simple interface - decorating a forward-only iterator of database results with an iterator which constructs a model object from a single result, for example. These patterns enable memory-efficient iteration of infinite sets and, with a filter like the one I wrote above, potentially lazy evaluation of results.
Part of the power of C++ templates is your iterator interface, when applied to the likes of fixed-length C arrays, decays to simple and efficient pointer arithmetic, making it a truly zero-cost abstraction.

how can I get the fastest iteration possible for some calculus intensive code?

Context
I'm using a QLinkedList to store some class I wrote.
The fact is I must iterate a lot over this list.
By a lot I mean the program I write makes infinite calculus (well, you can still stop it manually) and I need to get through that QLinkedList for each iteration.
Problem
The problem is not if I'm iterating to much over this list.
It's that I'm profiling my code and I see that 1/4 of the time is spent on QLinkedList::end() and QLinkedList::begin() functions.
Sample code
My code is the following :
typedef QLinkedList<Particle*> ParticlesList; // Particle is a custom class
ParticlesList* parts = // assign a QLinkedList
for (ParticlesList::const_iterator itp = parts->begin(); itp != parts->end(); ++itp)
{
//make some calculus
}
Like I said, this code is called so often that it spends a lot of time on parts->begin() and parts->end().
Question
So, the question is how can I reduce the time spent on the iteration of this list ?
Possible solutions
Here are some solutions I've thought of, please help me choose the best or propose me another one :)
Use of classic C array : // sorry for this mistake
Particle** parts = // assing it something
for (int n = 0; n < LENGTH; n++)
{
//access by index
//make some calculus
}
This should be quick right ?
Maybe use Java style iterator ?
Maybe use another container ?
Asm ? Just kidding... or maybe ?
Thank you for your future answers !
PS : I have read stackoverflow posts about when to profile so don't worry about that ;)
Edit :
The list is modified
I'm sorry I think I forgot the most important, I'll write the whole function without stripping :
typedef std::vector<Cell*> Neighbours;
typedef QLinkedList<Particle*> ParticlesList;
Neighbours neighbours = m_cell->getNeighbourhood();
Neighbours::const_iterator it;
for (it = neighbours.begin(); it != neighbours.end(); ++it)
{
ParticlesList* parts = (*it)->getParticles();
for (ParticlesList::const_iterator itp = parts->begin(); itp != parts->end(); ++itp)
{
double d = distanceTo(*itp); // computes sqrt(x^2 + y^2)
if(d>=0 && d<=m_maxForceRange)
{
particleIsClose(d, *itp); // just changes
}
}
}
And just to make sure I'm complete, this whole code is called in a loop ^^.
So yes the list is modified and it is in a inner loop. So there's no way to precompute the beginning and end of it.
And moreover, the list needs to be constructed at each big iteration (I mean in the topmost loop) by inserting one by one.
Debug mode
Yes indeed I profiled in Debug mode. And I think the remark was judicious because the code went 2x faster in Release. And the problem with lists disappeared.
Thanks to all for your answers and sorry for this ^^
If you are profiling in debug mode, a lot of compilers disable inlineing. The begin() and end() times being high may not be "real". The method call times would be much higher than the equivalent inline operations.
Something else I noticed in the full code, you're doing a sqrt in the inner loop. They can be fairly expensive depending on the hardware architecture. I would consider replacing the following code:
double d = distanceTo(*itp); // computes sqrt(x^2 + y^2)
if(d >= 0 && d <= m_maxForceRange)
with:
double d = distanceToSquared(*itp); // computes x^2 + y^2
if(d >= 0 && d <= m_maxForceRangeSquared)
I've done this in code where I was doing collison detection and it sometimes makes a noticible improvement. The tests are equivalent and saves a lot of calls to sqrt. As always with optimization, measure to verify if it improves the speed.
Pre-computing the end iterator will help if your compiler isn't smart enough to realise it is const, and is hence computing it each time through the loop. You can do that like below:
const ParticlesList::const_iterator itp_end = parts->end();
for (ParticlesList::const_iterator itp = parts->begin(); itp != itp_end; ++itp)
{
//make some calculus
}
I can't understand why parts->begin(); is taking so long, it should only be used once. However, if this loop is inside another loop, you could do something like this:
const ParticlesList::const_iterator itp_begin = parts->begin();
const ParticlesList::const_iterator itp_end = parts->end();
for (...)
{
for (ParticlesList::const_iterator itp = itp_begin; itp != itp_end; ++itp)
{
//make some calculus
}
}
But I can't imagine this will make too much difference (unless your inner list is really short), but it shouldn't hurt much either.
On a further note, a linked list possibly isn't the fastest data structure for your purposes. Linked lists are most useful when you frequently need to insert items into the middle of the list. If the list is built and then fixed, you're probably better off with a std::vector. A std::vector may also be better even if you occasionally only need to add/remove items from the end (not the beginning or middle). If you have to add/remove from the beginning/end (but not middle) consider a std::deque.
If you absolutely need raw speed you should measure each possible choice you encounter, and keep the fastest.
Sounds like the list remains unchanged while you iterate over it. I'd try by storing the end of the list on a local variable.
typedef QLinkedList<Particle*> ParticlesList; // Particle is a custom class
ParticlesList* parts = // assign a QLinkedList
ParticlesList::const_iterator end = parts->end();
for (ParticlesList::const_iterator itp = parts->begin(); itp != end; ++itp)
{
// make some calculus
}
Qt containers are compatible with STL algorithms like std::for_each.
Try something like this:
std::for_each( parts->begin(), parts->end(), MyParticleCalculus );
where MyParticleCalculus is a functor that contains your calculus.
Qt also has its own foreach, but it's apparently just a macro to hide the iterators, so it probably won't give you any performance benefit.
(Edit: I'm recommending std::for_each per Scott Meyer's recommendation in "Effective STL": "Prefer algorithm calls to hand-written loops.")

assignment vs std::swap and merging and keeping duplicates in seperate object

Say I have two std::set<std::string>s. The first one, old_options, needs to be merged with additional options, contained in new_options. I can't just use std::merge (well, I do, but not only that) because I also check for doubles and warn the user about this accordingly. To this effect, I have
void merge_options( set<string> &old_options, const set<string> &new_options )
{
// find duplicates and create merged_options, a stringset containing the merged options
// handle duplicated the way I want to
// ...
old_options = merged_options;
}
Is it better to use
std::swap( merged_options, old_options );
or the assignment I have?
Is there a better way to filter duplicates and return the merged set than consecutive calls to std::set_intersection and std::set_union to detect dupes and merge the sets? I know it's slower than one traversal and doing both at once, but these sets are small (performance is not critical) and I trust the Standard more than I trust myself.
What's wrong with
void merge_options( set<string> &old_options, const set<string> &new_options )
{
for (set<string>::iterator i = new_options.begin();
i != new_options.end(); ++i)
if (old_options.find(*i) != old_options.end())
warn_duplicate(*i);
else
old_options.insert(*i);
}
This is a simple O(m lg n) algorithm, where m = new_options.size() and n = old_options.size().
Given (as you've stated) that performance isn't critical here, I'd use the assignment and the two-pass algorithm. It's simpler and easier to understand; it's only worth using a "trick" like swap if you really need what it gains.
Writing your own algorithm wouldn't be such a bad thing, but again, unless you have a real use for the benefit it provides, I wouldn't bother.
This is in part an answer to larsmans. There is a remove_copy_if algorithm that encapsulates his for loop into a single function. The following uses a C++0x lambda for the predicate.
void merge_options( set<string> &old_options, const set<string> &new_options )
{
remove_copy_if(
new_options.begin(),
new_options.end(),
inserter(old_options, old_options.end()),
[&](const string &s) {
return (old_options.count(s)) ? warn_duplicate(s), true : false;
}
);
}

Why use iterators instead of array indices?

Take the following two lines of code:
for (int i = 0; i < some_vector.size(); i++)
{
//do stuff
}
And this:
for (some_iterator = some_vector.begin(); some_iterator != some_vector.end();
some_iterator++)
{
//do stuff
}
I'm told that the second way is preferred. Why exactly is this?
The first form is efficient only if vector.size() is a fast operation. This is true for vectors, but not for lists, for example. Also, what are you planning to do within the body of the loop? If you plan on accessing the elements as in
T elem = some_vector[i];
then you're making the assumption that the container has operator[](std::size_t) defined. Again, this is true for vector but not for other containers.
The use of iterators bring you closer to container independence. You're not making assumptions about random-access ability or fast size() operation, only that the container has iterator capabilities.
You could enhance your code further by using standard algorithms. Depending on what it is you're trying to achieve, you may elect to use std::for_each(), std::transform() and so on. By using a standard algorithm rather than an explicit loop you're avoiding re-inventing the wheel. Your code is likely to be more efficient (given the right algorithm is chosen), correct and reusable.
It's part of the modern C++ indoctrination process. Iterators are the only way to iterate most containers, so you use it even with vectors just to get yourself into the proper mindset. Seriously, that's the only reason I do it - I don't think I've ever replaced a vector with a different kind of container.
Wow, this is still getting downvoted after three weeks. I guess it doesn't pay to be a little tongue-in-cheek.
I think the array index is more readable. It matches the syntax used in other languages, and the syntax used for old-fashioned C arrays. It's also less verbose. Efficiency should be a wash if your compiler is any good, and there are hardly any cases where it matters anyway.
Even so, I still find myself using iterators frequently with vectors. I believe the iterator is an important concept, so I promote it whenever I can.
because you are not tying your code to the particular implementation of the some_vector list. if you use array indices, it has to be some form of array; if you use iterators you can use that code on any list implementation.
Imagine some_vector is implemented with a linked-list. Then requesting an item in the i-th place requires i operations to be done to traverse the list of nodes. Now, if you use iterator, generally speaking, it will make its best effort to be as efficient as possible (in the case of a linked list, it will maintain a pointer to the current node and advance it in each iteration, requiring just a single operation).
So it provides two things:
Abstraction of use: you just want to iterate some elements, you don't care about how to do it
Performance
I'm going to be the devils advocate here, and not recommend iterators. The main reason why, is all the source code I've worked on from Desktop application development to game development have i nor have i needed to use iterators. All the time they have not been required and secondly the hidden assumptions and code mess and debugging nightmares you get with iterators make them a prime example not to use it in any applications that require speed.
Even from a maintence stand point they're a mess. Its not because of them but because of all the aliasing that happen behind the scene. How do i know that you haven't implemented your own virtual vector or array list that does something completely different to the standards. Do i know what type is currently now during runtime? Did you overload a operator I didn't have time to check all your source code. Hell do i even know what version of the STL your using?
The next problem you got with iterators is leaky abstraction, though there are numerous web sites that discuss this in detail with them.
Sorry, I have not and still have not seen any point in iterators. If they abstract the list or vector away from you, when in fact you should know already what vector or list your dealing with if you don't then your just going to be setting yourself up for some great debugging sessions in the future.
You might want to use an iterator if you are going to add/remove items to the vector while you are iterating over it.
some_iterator = some_vector.begin();
while (some_iterator != some_vector.end())
{
if (/* some condition */)
{
some_iterator = some_vector.erase(some_iterator);
// some_iterator now positioned at the element after the deleted element
}
else
{
if (/* some other condition */)
{
some_iterator = some_vector.insert(some_iterator, some_new_value);
// some_iterator now positioned at new element
}
++some_iterator;
}
}
If you were using indices you would have to shuffle items up/down in the array to handle the insertions and deletions.
Separation of Concerns
It's very nice to separate the iteration code from the 'core' concern of the loop. It's almost a design decision.
Indeed, iterating by index ties you to the implementation of the container. Asking the container for a begin and end iterator, enables the loop code for use with other container types.
Also, in the std::for_each way, you TELL the collection what to do, instead of ASKing it something about its internals
The 0x standard is going to introduce closures, which will make this approach much more easy to use - have a look at the expressive power of e.g. Ruby's [1..6].each { |i| print i; }...
Performance
But maybe a much overseen issue is that, using the for_each approach yields an opportunity to have the iteration parallelized - the intel threading blocks can distribute the code block over the number of processors in the system!
Note: after discovering the algorithms library, and especially foreach, I went through two or three months of writing ridiculously small 'helper' operator structs which will drive your fellow developers crazy. After this time, I went back to a pragmatic approach - small loop bodies deserve no foreach no more :)
A must read reference on iterators is the book "Extended STL".
The GoF have a tiny little paragraph in the end of the Iterator pattern, which talks about this brand of iteration; it's called an 'internal iterator'. Have a look here, too.
Because it is more object-oriented. if you are iterating with an index you are assuming:
a) that those objects are ordered
b) that those objects can be obtained by an index
c) that the index increment will hit every item
d) that that index starts at zero
With an iterator, you are saying "give me everything so I can work with it" without knowing what the underlying implementation is. (In Java, there are collections that cannot be accessed through an index)
Also, with an iterator, no need to worry about going out of bounds of the array.
Another nice thing about iterators is that they better allow you to express (and enforce) your const-preference. This example ensures that you will not be altering the vector in the midst of your loop:
for(std::vector<Foo>::const_iterator pos=foos.begin(); pos != foos.end(); ++pos)
{
// Foo & foo = *pos; // this won't compile
const Foo & foo = *pos; // this will compile
}
Aside from all of the other excellent answers... int may not be large enough for your vector. Instead, if you want to use indexing, use the size_type for your container:
for (std::vector<Foo>::size_type i = 0; i < myvector.size(); ++i)
{
Foo& this_foo = myvector[i];
// Do stuff with this_foo
}
I probably should point out you can also call
std::for_each(some_vector.begin(), some_vector.end(), &do_stuff);
STL iterators are mostly there so that the STL algorithms like sort can be container independent.
If you just want to loop over all the entries in a vector just use the index loop style.
It is less typing and easier to parse for most humans. It would be nice if C++ had a simple foreach loop without going overboard with template magic.
for( size_t i = 0; i < some_vector.size(); ++i )
{
T& rT = some_vector[i];
// now do something with rT
}
'
I don't think it makes much difference for a vector. I prefer to use an index myself as I consider it to be more readable and you can do random access like jumping forward 6 items or jumping backwards if needs be.
I also like to make a reference to the item inside the loop like this so there are not a lot of square brackets around the place:
for(size_t i = 0; i < myvector.size(); i++)
{
MyClass &item = myvector[i];
// Do stuff to "item".
}
Using an iterator can be good if you think you might need to replace the vector with a list at some point in the future and it also looks more stylish to the STL freaks but I can't think of any other reason.
The second form represents what you're doing more accurately. In your example, you don't care about the value of i, really - all you want is the next element in the iterator.
After having learned a little more on the subject of this answer, I realize it was a bit of an oversimplification. The difference between this loop:
for (some_iterator = some_vector.begin(); some_iterator != some_vector.end();
some_iterator++)
{
//do stuff
}
And this loop:
for (int i = 0; i < some_vector.size(); i++)
{
//do stuff
}
Is fairly minimal. In fact, the syntax of doing loops this way seems to be growing on me:
while (it != end){
//do stuff
++it;
}
Iterators do unlock some fairly powerful declarative features, and when combined with the STL algorithms library you can do some pretty cool things that are outside the scope of array index administrivia.
Indexing requires an extra mul operation. For example, for vector<int> v, the compiler converts v[i] into &v + sizeof(int) * i.
During iteration you don't need to know number of item to be processed. You just need the item and iterators do such things very good.
No one mentioned yet that one advantage of indices is that they are not become invalid when you append to a contiguous container like std::vector, so you can add items to the container during iteration.
This is also possible with iterators, but you must call reserve(), and therefore need to know how many items you'll append.
If you have access to C++11 features, then you can also use a range-based for loop for iterating over your vector (or any other container) as follows:
for (auto &item : some_vector)
{
//do stuff
}
The benefit of this loop is that you can access elements of the vector directly via the item variable, without running the risk of messing up an index or making a making a mistake when dereferencing an iterator. In addition, the placeholder auto prevents you from having to repeat the type of the container elements,
which brings you even closer to a container-independent solution.
Notes:
If you need the the element index in your loop and the operator[] exists for your container (and is fast enough for you), then better go for your first way.
A range-based for loop cannot be used to add/delete elements into/from a container. If you want to do that, then better stick to the solution given by Brian Matthews.
If you don't want to change the elements in your container, then you should use the keyword const as follows: for (auto const &item : some_vector) { ... }.
Several good points already. I have a few additional comments:
Assuming we are talking about the C++ standard library, "vector" implies a random access container that has the guarantees of C-array (random access, contiguos memory layout etc). If you had said 'some_container', many of the above answers would have been more accurate (container independence etc).
To eliminate any dependencies on compiler optimization, you could move some_vector.size() out of the loop in the indexed code, like so:
const size_t numElems = some_vector.size();
for (size_t i = 0; i
Always pre-increment iterators and treat post-increments as exceptional cases.
for (some_iterator = some_vector.begin(); some_iterator != some_vector.end(); ++some_iterator){ //do stuff }
So assuming and indexable std::vector<> like container, there is no good reason to prefer one over other, sequentially going through the container. If you have to refer to older or newer elemnent indexes frequently, then the indexed version is more appropropriate.
In general, using the iterators is preferred because algorithms make use of them and behavior can be controlled (and implicitly documented) by changing the type of the iterator. Array locations can be used in place of iterators, but the syntactical difference will stick out.
I don't use iterators for the same reason I dislike foreach-statements. When having multiple inner-loops it's hard enough to keep track of global/member variables without having to remember all the local values and iterator-names as well. What I find useful is to use two sets of indices for different occasions:
for(int i=0;i<anims.size();i++)
for(int j=0;j<bones.size();j++)
{
int animIndex = i;
int boneIndex = j;
// in relatively short code I use indices i and j
... animation_matrices[i][j] ...
// in long and complicated code I use indices animIndex and boneIndex
... animation_matrices[animIndex][boneIndex] ...
}
I don't even want to abbreviate things like "animation_matrices[i]" to some random "anim_matrix"-named-iterator for example, because then you can't see clearly from which array this value is originated.
If you like being close to the metal / don't trust their implementation details, don't use iterators.
If you regularly switch out one collection type for another during development, use iterators.
If you find it difficult to remember how to iterate different sorts of collections (maybe you have several types from several different external sources in use), use iterators to unify the means by which you walk over elements. This applies to say switching a linked list with an array list.
Really, that's all there is to it. It's not as if you're going to gain more brevity either way on average, and if brevity really is your goal, you can always fall back on macros.
Even better than "telling the CPU what to do" (imperative) is "telling the libraries what you want" (functional).
So instead of using loops you should learn the algorithms present in stl.
For container independence
I always use array index because many application of mine require something like "display thumbnail image". So I wrote something like this:
some_vector[0].left=0;
some_vector[0].top =0;<br>
for (int i = 1; i < some_vector.size(); i++)
{
some_vector[i].left = some_vector[i-1].width + some_vector[i-1].left;
if(i % 6 ==0)
{
some_vector[i].top = some_vector[i].top.height + some_vector[i].top;
some_vector[i].left = 0;
}
}
Both the implementations are correct, but I would prefer the 'for' loop. As we have decided to use a Vector and not any other container, using indexes would be the best option. Using iterators with Vectors would lose the very benefit of having the objects in continuous memory blocks which help ease in their access.
I felt that none of the answers here explain why I like iterators as a general concept over indexing into containers. Note that most of my experience using iterators doesn't actually come from C++ but from higher-level programming languages like Python.
The iterator interface imposes fewer requirements on consumers of your function, which allows consumers to do more with it.
If all you need is to be able to forward-iterate, the developer isn't limited to using indexable containers - they can use any class implementing operator++(T&), operator*(T) and operator!=(const &T, const &T).
#include <iostream>
template <class InputIterator>
void printAll(InputIterator& begin, InputIterator& end)
{
for (auto current = begin; current != end; ++current) {
std::cout << *current << "\n";
}
}
// elsewhere...
printAll(myVector.begin(), myVector.end());
Your algorithm works for the case you need it - iterating over a vector - but it can also be useful for applications you don't necessarily anticipate:
#include <random>
class RandomIterator
{
private:
std::mt19937 random;
std::uint_fast32_t current;
std::uint_fast32_t floor;
std::uint_fast32_t ceil;
public:
RandomIterator(
std::uint_fast32_t floor = 0,
std::uint_fast32_t ceil = UINT_FAST32_MAX,
std::uint_fast32_t seed = std::mt19937::default_seed
) :
floor(floor),
ceil(ceil)
{
random.seed(seed);
++(*this);
}
RandomIterator& operator++()
{
current = floor + (random() % (ceil - floor));
}
std::uint_fast32_t operator*() const
{
return current;
}
bool operator!=(const RandomIterator &that) const
{
return current != that.current;
}
};
int main()
{
// roll a 1d6 until we get a 6 and print the results
RandomIterator firstRandom(1, 7, std::random_device()());
RandomIterator secondRandom(6, 7);
printAll(firstRandom, secondRandom);
return 0;
}
Attempting to implement a square-brackets operator which does something similar to this iterator would be contrived, while the iterator implementation is relatively simple. The square-brackets operator also makes implications about the capabilities of your class - that you can index to any arbitrary point - which may be difficult or inefficient to implement.
Iterators also lend themselves to decoration. People can write iterators which take an iterator in their constructor and extend its functionality:
template<class InputIterator, typename T>
class FilterIterator
{
private:
InputIterator internalIterator;
public:
FilterIterator(const InputIterator &iterator):
internalIterator(iterator)
{
}
virtual bool condition(T) = 0;
FilterIterator<InputIterator, T>& operator++()
{
do {
++(internalIterator);
} while (!condition(*internalIterator));
return *this;
}
T operator*()
{
// Needed for the first result
if (!condition(*internalIterator))
++(*this);
return *internalIterator;
}
virtual bool operator!=(const FilterIterator& that) const
{
return internalIterator != that.internalIterator;
}
};
template <class InputIterator>
class EvenIterator : public FilterIterator<InputIterator, std::uint_fast32_t>
{
public:
EvenIterator(const InputIterator &internalIterator) :
FilterIterator<InputIterator, std::uint_fast32_t>(internalIterator)
{
}
bool condition(std::uint_fast32_t n)
{
return !(n % 2);
}
};
int main()
{
// Rolls a d20 until a 20 is rolled and discards odd rolls
EvenIterator<RandomIterator> firstRandom(RandomIterator(1, 21, std::random_device()()));
EvenIterator<RandomIterator> secondRandom(RandomIterator(20, 21));
printAll(firstRandom, secondRandom);
return 0;
}
While these toys might seem mundane, it's not difficult to imagine using iterators and iterator decorators to do powerful things with a simple interface - decorating a forward-only iterator of database results with an iterator which constructs a model object from a single result, for example. These patterns enable memory-efficient iteration of infinite sets and, with a filter like the one I wrote above, potentially lazy evaluation of results.
Part of the power of C++ templates is your iterator interface, when applied to the likes of fixed-length C arrays, decays to simple and efficient pointer arithmetic, making it a truly zero-cost abstraction.