I want a program that creates an undetermined amount of lists. The size of each list is fixed, but I can't determine at compile time how many lists am I going to need.
I understand I cannot create a vector of arrays. I also understand I can use a vector of vectors, but I wonder if this is the best efficient way to do it considering the fact I need to reserve a fixed amount of memory each time I need a new array.
Erm, you can use a vector of arrays, for example,
std::vector<std::array<T, N>> some_vec;
#Nim is right, but I cannot upvote yet :p
Also his solution is C++11.
Another alternative which is to be preferred over std::vector<std::vector<T> > is to have one vector with the dimensions X * Y. Then you can access your elements with v[y * Y + x];
This works with all versions of C++ and should be just as efficient as the std::vector<std::array<T, N> > solution.
Related
I give online coding contests, where the speed of the program is everything. Over the time I have come across 3 ways to use the concept of array in C++. The questions of the contests usually require us to create a dynamic array of the given size. So its just a one time creation of the dynamic array as per the input and we don't resize the array again.
std::vector
Vectors look the most fancy and everyone tends to love them. But few days back one of the question gave me the TIME_LIMIT_EXCEEDED error when doing it with vectors.When I implemented the same logic with normal arrays, the program was submitted successfully.On researching, I found out that using the push_back() function takes a long time as compared to a normal arr[i]=x;
std::array
I don't have much knowledge about its performance. But it looks like a nicer way to handle arrays.
default arrays in C++
I do the dynamic allocation using int *arr=new int[given_size]; and then use the array normally.Passing an array as argument is not as simple as vectors but its not a big deal.
Apart from this there are also times when I have to work with 2D arrays and I am always unsure about what could be the fastest way. vector<vector<int>> is regarded slow in some forums and so is using multidimensional pointers. So I like to use a 1D array with a typedef function to handle its index but it gets complicated when I have to pass a row to a function.
Most of the answers in forums are based on what the OP was trying to do and this gives different answers. What I want to know is which is the best and long term way to use to have maximum speed/efficiency.
push_back takes a long time compared to arr[i]=x;.
Sorry but you are showing your lack of experience with vectors here, because your examples do two different things.
You are comparing something like this code
vector<int> vec; // vector created with size zero
for (...)
vec.push_back(x); // vector size increases
with this code
int arr[N];
for (...)
arr[i] = x;
The difference is that in the first case the vector has size 0 and it's size increases as you add items to it (this takes extra time), but in the second case the array starts out at it's final size. With an array this is how it must be, but with vectors you have a choice. If you know what the final size of the vector is you should code it like this
vector<int> vec(N); // vector created at size N, note use () not []
for (...)
vec[i] = x;
That is the code you should be comparing with the array code for efficiency,
You might also want to research the resize and reserve methods of a vector. Vectors (if nothing else) are much more flexible than arrays.
Suppose T is a type, and I want to make a vector<vector<T>>. I know the eventual size will be m x n, where m and n are runtime constants. (If they were compile-time constants I'd use std::array<std::array<T, n>, m>.) Suppose I have three choices of what to do with my double vector before I continue with my program in earnest:
Option 1
std::vector<std::vector<T>> dbl_vect(m);
for (auto & v : dbl_vect)
v.reserve(n);
Option 2
std::vector<std::vector<T>> dbl_vect;
dbl_vect.reserve(m);
Option 3
std::vector<std::vector<T>> dbl_vect;
Let's suppose I am not worried about iterator & reference invalidation from vector reallocation, so we can remove that from the decision process.
Of course the code that follows these would have to be a little different, since #1 creates the (empty) rows of the dbl_vector, so we have to access the rows rather than pushing more back.
Option #2 seems fairly useless, because it has no idea how much space to reserve for each row.
Option #1 requires me to go through a linear pass of m empty vectors and resize them manually, but it prevents reallocation. If T were pretty big, this would almost certainly be preferable, I believe, because it would prevent copies/moves.
Question: Suppose T = char (or pick your favorite POD type). Under what circumstances should I be indifferent between options 1 and 3, or even prefer #3? Is this mostly due to the relatively small size of a char, or because of the way the compiler will (not) default-initialize a char? If T is larger, maybe user-defined, at what point (in size of the double vector or in size of T) should I start caring?
Here a somewhat similar question is asked, regarding one vector and T=int.
If you know the inner size will be m, one possibility is to make a std::vector<S>, where S is your custom type standing for a std::vector<T>, except it knows how many entries it will have. A similar solution is suggested here (except there m is a compile-time constant).
#3 just default-initializes the vector. There is nothing you gain from this, as the containing vector will have a capacity of zero. Dynamically allocating memory is slow, so to minimize this I would always go with #1 or a variant thereof.
Okay, this may sound like a stupid question, but I haven't read anything from the documentation that says it is not possible. Either that, or I overlooked something again.
By multi-dimensional, I mean like arrays. Is something like
vector<vector<double>>
possible? What are the possible drawbacks, at least when compared to arrays?
It's possible, but note that you need a space between the two >s to remove the ambiguity between the right shift operator, i.e.
vector<vector<double> >
Also, I wouldn't call those vectors arrays, since array has a very well-defined meaning in C++:
double matrix[10][10];
edit: As people pointed out, you don't need a space when using C++11.
It is possible.
One of the possible drawbacks could be that it might result in multiple separate allocations from the free store because each vector makes its own allocations. In contrast, a dynamic array allocation is made only once from contiguous memory which is more cache friendly.
What you're describing is absolutely possible, although if you aren't using a C++11 compiler you need to type it as:
vector<vector<double> >
The space between the two > characters being necessary so that the compiler doesn't think you're using the >> operator, as in:
cin >> x;
Of course, with a vector of vectors, you can add and remove elements, either at the top level where the elements are vectors, or at the second level where the elements are doubles. This can be a blessing, a curse, or both, depending on what you are trying to do - note that if you add a double to one of the second-level vectors, the length of that vector is different from all of the others. Because the second-level vectors can have different lengths, I would recommend against using them as a replacement for 2D arrays if fixed dimensions are what you want.
I have a large array of objects (10s Millions) of class A and I want to add a vector as a member to the class A. This vector is needed just for few percent of objects in the array. I was wondering, would it be a wise choice to add a vector to the class? how much memory will take an empty vector?
Now we know that empty vector are not very “big” (VC2012x64 intellisense show sizeof(std::vector<int>) is 16 byte). If sizeof(A) is much bigger than size-of vector adding a vector member to A could be a good solution for you. But if it is not good, and will add to much memory, and really not many A has the vector, I’d create a second container with the vectors. For example:
#include <unordered_map>
unordered_map<size_t , vector<T>> VectorForA;
where size_t is mean to be the type of the index of the big array of A, and vector<T> the type of the vector you want to add to A. This could be good for a fixed index "big" array. If somehow the A's in the big array dont have fixed positions, making the value of A the key could led to a simpler code(also, only if the values of A dont repeat).
NOTE: I was (I'm) wating to see a full answer from #Andy Prowl or #Tony D with I think will be very usefull
This is my little big question about containers, in particular, arrays.
I am writing a physics code that mainly manipulates a big (> 1 000 000) set of "particles" (with 6 double coordinates each). I am looking for the best way (in term of performance) to implement a class that will contain a container for these data and that will provide manipulation primitives for these data (e.g. instantiation, operator[], etc.).
There are a few restrictions on how this set is used:
its size is read from a configuration file and won't change during execution
it can be viewed as a big two dimensional array of N (e.g. 1 000 000) lines and 6 columns (each one storing the coordinate in one dimension)
the array is manipulated in a big loop, each "particle / line" is accessed and computation takes place with its coordinates, and the results are stored back for this particle, and so on for each particle, and so on for each iteration of the big loop.
no new elements are added or deleted during the execution
First conclusion, as the access on the elements is essentially done by accessing each element one by one with [], I think that I should use a normal dynamic array.
I have explored a few things, and I would like to have your opinion on the one that can give me the best performances.
As I understand there is no advantage to use a dynamically allocated array instead of a std::vector, so things like double** array2d = new ..., loop of new, etc are ruled out.
So is it a good idea to use std::vector<double> ?
If I use a std::vector, should I create a two dimensional array like std::vector<std::vector<double> > my_array that can be indexed like my_array[i][j], or is it a bad idea and it would be better to use std::vector<double> other_array and acces it with other_array[6*i+j].
Maybe this can gives better performance, especially as the number of columns is fixed and known from the beginning.
If you think that this is the best option, would it be possible to wrap this vector in a way that it can be accessed with a index operator defined as other_array[i,j] // same as other_array[6*i+j] without overhead (like function call at each access) ?
Another option, the one that I am using so far is to use Blitz, in particular blitz::Array:
typedef blitz::Array<double,TWO_DIMENSIONS> store_t;
store_t my_store;
Where my elements are accessed like that: my_store(line, column);.
I think there are not much advantage to use Blitz in my case because I am accessing each element one by one and that Blitz would be interesting if I was using operations directly on array (like matrix multiplication) which I am not.
Do you think that Blitz is OK, or is it useless in my case ?
These are the possibilities I have considered so far, but maybe the best one I still another one, so don't hesitate to suggest me other things.
Thanks a lot for your help on this problem !
Edit:
From the very interesting answers and comments bellow a good solution seems to be the following:
Use a structure particle (containing 6 doubles) or a static array of 6 doubles (this avoid the use of two dimensional dynamic arrays)
Use a vector or a deque of this particle structure or array. It is then good to traverse them with iterators, and that will allow to change from one to another later.
In addition I can also use a Blitz::TinyVector<double,6> instead of a structure.
So is it a good idea to use std::vector<double> ?
Usually, a std::vector should be the first choice of container. You could use either std::vector<>::reserve() or std::vector<>::resize() to avoid reallocations while populating the vector. Whether any other container is better can be found by measuring. And only by measuring. But first measure whether anything the container is involved in (populating, accessing elements) is worth optimizing at all.
If I use a std::vector, should I create a two dimensional array like std::vector<std::vector<double> > [...]?
No. IIUC, you are accessing your data per particle, not per row. If that's the case, why not use a std::vector<particle>, where particle is a struct holding six values? And even if I understood incorrectly, you should rather write a two-dimensional wrapper around a one-dimensional container. Then align your data either in rows or columns - what ever is faster with your access patterns.
Do you think that Blitz is OK, or is it useless in my case?
I have no practical knowledge about blitz++ and the areas it is used in. But isn't blitz++ all about expression templates to unroll loop operations and optimizing away temporaries when doing matrix manipulations? ICBWT.
First of all, you don't want to scatter the coordinates of one given particle all over the place, so I would begin by writing a simple struct:
struct Particle { /* coords */ };
Then we can make a simple one dimensional array of these Particles.
I would probably use a deque, because that's the default container, but you may wish to try a vector, it's just that 1.000.000 of particles means about a single chunk of a few MBs. It should hold but it might strain your system if this ever grows, while the deque will allocate several chunks.
WARNING:
As Alexandre C remarked, if you go the deque road, refrain from using operator[] and prefer to use iteration style. If you really need random access and it's performance sensitive, the vector should prove faster.
The first rule when choosing from containers is to use std::vector. Then, only after your code is complete and you can actually measure performance, you can try other containers. But stick to vector first. (And use reserve() from the start)
Then, you shouldn't use an std::vector<std::vector<double> >. You know the size of your data: it's 6 doubles. No need for it to be dynamic. It is constant and fixed. You can define a struct to hold you particle members (the six doubles), or you can simply typedef it: typedef double particle[6]. Then, use a vector of particles: std::vector<particle>.
Furthermore, as your program uses the particle data contained in the vector sequentially, you will take advantage of the modern CPU cache read-ahead feature at its best performance.
You could go several ways. But in your case, don't declare astd::vector<std::vector<double> >. You're allocating a vector (and you copy it around) for every 6 doubles. Thats way too costly.
If you think that this is the best option, would it be possible to wrap this vector in a way that it can be accessed with a index operator defined as other_array[i,j] // same as other_array[6*i+j] without overhead (like function call at each access) ?
(other_array[i,j] won't work too well, as i,j employs the comma operator to evaluate the value of "i", then discards that and evaluates and returns "j", so it's equivalent to other_array[i]).
You will need to use one of:
other_array[i][j]
other_array(i, j) // if other_array implements operator()(int, int),
// but std::vector<> et al don't.
other_array[i].identifier // identifier is a member variable
other_array[i].identifier() // member function getting value
other_array[i].identifier(double) // member function setting value
You may or may not prefer to put get_ and set_ or similar on the last two functions should you find them useful, but from your question I think you won't: functions are prefered in APIs between parts of large systems involving many developers, or when the data items may vary and you want the algorithms working on the data to be independent thereof.
So, a good test: if you find yourself writing code like other_array[i][3] where you've decided "3" is the double with the speed in it, and other_array[i][5] because "5" is the the acceleration, then stop doing that and give them proper identifiers so you can say other_array[i].speed and .acceleration. Then other developers can read and understand it, and you're much less likely to make accidental mistakes. On the other hand, if you are iterating over those 6 elements doing exactly the same things to each, then you probably do want Particle to hold a double[6], or to provide an operator[](int). There's no problem doing both:
struct Particle
{
double x[6];
double& speed() { return x[3]; }
double speed() const { return x[3]; }
double& acceleration() { return x[5]; }
...
};
BTW / the reason that vector<vector<double> > may be too costly is that each set of 6 doubles will be allocated on the heap, and for fast allocation and deallocation many heap implementations use fixed-size buckets, so your small request will be rounded up t the next size: that may be a significant overhead. The outside vector will also need to record a extra pointer to that memory. Further, heap allocation and deallocation is relatively slow - in you're case, you'd only be doing it at startup and shutdown, but there's no particular point in making your program slower for no reason. Even more importantly, the areas on the heap may just around in memory, so your operator[] may have cache-faults pulling in more distinct memory pages than necessary, slowing the entire program. Put another way, vectors store elements contiguously, but the pointed-to-vectors may not be contiguous.