Re: [Boost-users] [multi_array] Useful algorithms (from Re: Multi_array: A "proper" iterator over a N>1 Multi_array?)

As I said in a former message that I am afraid fell through the cracks, sorry if I am repeating myself, I don't think rewriting the whole STL algorithm lib for multi_array is a solution. for_each, accumulate, sort, where do we stop? I recently needed to apply random shuffle, for instance. I think the solution is to define iterators that provide a "flat", element by element view of a multi_array. These iterators would be regular skip iterators that just a have a fancy increment operation that looks at the stride vector to decide by how much to jump when incremented. Let's say that these are returned by begin_element and end_element. Then you could do on multi array ma things like for_each(ma.begin_element(), ma.end_element(), do_something()) or accumulate(ma.begin_element(), ma.end_element(), 0) and so on for all STL algorithms, all the statstical functions that I wrote (GPLed, maybe the beginning of a boost stats lib?, mail me if interested) and more. I've been using data() and data()+num_elements for the same purpose, but that breaks for non contiguos storage (that is, not guaranteed to work for views) I don't think rewriting all the algorithms for multi_array is a viable strategy, nor is in keeping with the idea that data structures and algorithms should be made as ortogonal as reasonably possible. Just my 2c Antonio You can do that right now with data() and data() + num_elements On 5/16/06, Beth Jacobson wrote:

On a recent thread, Bruno Martinez provided a std::for_each-like function for multi_arrays which lets users apply a function to each element in a multi_array or view without having to know anything about the dimensions. I've been playing around with it (on MSVC 7.1) and it works perfectly. It seems like a good utility for anyone using boost.multi_array.

Working from Bruno's code, I wrote a similar std::accumulate-like function, which could also be generally useful. I'm fairly new to template programming, so if someone could look over the code below and see if I've done something dangerous or stupid, it would be much appreciated.

Also, is there some standard Boost way of making this sort of code generally available? It probably isn't at a level where it could be included in the library, but it seems like a shame to just let it languish in the bowels of the mailing list archive.

Anyway, here are both algorithms:

My accumulate-like function: template struct accum_impl { F f; T &val;

accum_impl(T &val, F f) : f(f), val(val) {}

T operator()(MA& ma) { std::for_each(ma.begin(), ma.end(), accum_implMA::const_reference::type, T, F, dim-1>(val, f)); return val; } };

template struct accum_impl { F f; T &val;

accum_impl(T &val, F &f) : f(f), val(val) {}

T operator()(Ref r) { val = f(val, r); return val; } };

template T multi_accum(MA& ma, T init, F f) { accum_impl impl(init, f); return impl(ma); }

And a test: typedef boost::multi_array array_type; typedef array_type::index_range range; typedef array_type::index index; array_type B(boost::extents[4][2][3]);

int values = 0; for(index i = 0; i != 4; ++i) for(index j = 0; j != 2; ++j) for(index k = 0; k != 3; ++k) B[i][j][k] = values++;

array_type::array_view<2>::type myview = B[ boost::indices[range(1,3)][1][range(0,2)] ];

cout << "Sum: " << multi_accum(myview, 0, std::plus<int>()) << endl; cout << "Product: " << multi_accum(myview, 1, std::multiplies<int>()) << endl;

Bruno Martinez's for_each-like function:

template struct for_each_impl { F f; for_each_impl(F f) : f(f) {}

void operator()(MA& ma) { std::for_each(ma.begin(), ma.end(), for_each_impl(f)); } };

template struct for_each_impl { F f; for_each_impl(F f) : f(f) {}

void operator()(Ref r) { f(r); } };

template void multi_for_each(MA& ma, F f) { for_each_impl impl(f); impl(ma); }

And a test:

typedef boost::multi_array array_type; typedef array_type::index index; array_type myarray(boost::extents[3][4][2]); typedef array_type::index_range range; array_type::array_view<3>::type myview = myarray[ boost::indices[range(0,2)][range(1,3)][range(0,4,2)] ]; multi_for_each(myview, _1 = 56);

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