Iterate through a boost::multi_array view - c++

Question

I want to understand how to use the view functionality provided by boost::multi_array. Specifically, I want to be able to iterate within a single loop over all elements of a view that represents a particular submatrix of the initial matrix (not necessarily continuous). It seems that the iterator provided will not do what I want (or anything, it will not compile).
In the following example, I have a 2x6 matrix and I want to get its 2x4 submatrix so, if I try to print it I would expect to get "BoosLion". Indeed this is the case if I iterate for each dimension. But when I try to do the iteration with a single iterator, the program will not compile.
#include <boost/multi_array.hpp>
#include <iostream>
int main()
{
boost::multi_array<char, 2> a{boost::extents[2][6]};
a[0][0] = 'B';
a[0][1] = 'o';
a[0][2] = 'o';
a[0][3] = 's';
a[0][4] = 't';
a[0][5] = '\0';
a[1][0] = 'L';
a[1][1] = 'i';
a[1][2] = 'o';
a[1][3] = 'n';
a[1][4] = 's';
a[1][5] = '\0';
typedef boost::multi_array<char, 2>::array_view<2>::type array_view;
typedef boost::multi_array_types::index_range range;
array_view b = a[boost::indices[range{0,2}][range{0,4}] ];
for (unsigned int i = 0; i < 2; i++ ) {
for (unsigned int j = 0; j < 4; j++ ) {
std::cout << b[i][j] << std::endl;
}
}
// I want to do something like this:
// for (auto itr = b.begin(); itr < b.end(); ++itr) {
// std::cout << *itr << std::endl;
// }
}
Does anyone know how to iterate with only a single loop? I tried searching the documentation but have been unable to find anything relevant. Also, if anyone knows of another library that can do this, let me know, thank you!

Answer 1

Here is one way to do this:
#include <iostream>
#include <boost/multi_array.hpp>
// Functor to iterate over a Boost MultiArray concept instance.
template<typename T, typename F, size_t Dimensions = T::dimensionality>
struct IterateHelper {
void operator()(T& array, const F& f) const {
for (auto element : array)
IterateHelper<decltype(element), F>()(element, f);
}
};
// Functor specialization for the final dimension.
template<typename T, typename F>
struct IterateHelper<T, F, 1> {
void operator()(T& array, const F& f) const {
for (auto& element : array)
f(element);
}
};
// Utility function to apply a function to each element of a Boost
// MultiArray concept instance (which includes views).
template<typename T, typename F>
static void iterate(T& array, const F& f) {
IterateHelper<T, F>()(array, f);
}
int main() {
boost::multi_array<char, 2> a{boost::extents[2][6]};
a[0][0] = 'B';
a[0][1] = 'o';
a[0][2] = 'o';
a[0][3] = 's';
a[0][4] = 't';
a[0][5] = '\0';
a[1][0] = 'L';
a[1][1] = 'i';
a[1][2] = 'o';
a[1][3] = 'n';
a[1][4] = 's';
a[1][5] = '\0';
typedef boost::multi_array<char, 2>::array_view<2>::type array_view;
typedef boost::multi_array_types::index_range range;
array_view b = a[boost::indices[range{0,2}][range{0,4}] ];
// Use the utility to apply a function to each element.
iterate(b, [](char& c) {
std::cout << c << std::endl;
});
return 0;
};
The code above defines a utility function iterate(), to which you pass an object satisfying the Boost MultiArray concept (which includes views) and a function to apply to each element. The utility function works by using a Functor that iterates over each dimension recursively.
CoLiRu

Answer 2

update2
Based on the answer provided by #rhashimoto, I tried to make some generalization. With the following functions
// moving the judgement of dimensionality to the function's return-type
template<class T, class F>
typename std::enable_if<(T::dimensionality==1), void>::type IterateArrayView(T& array, F f) {
for (auto& element : array) {
f(element);
}
}
template<class T, class F>
typename std::enable_if<(T::dimensionality>1), void>::type IterateArrayView(T& array, F f) {
for (auto element : array) {
IterateArrayView<decltype(element), F>(element, f);
}
}
// given f() takes extra arguments
template<class T, class F, class... Args>
typename std::enable_if<(T::dimensionality==1), void>::type IterateArrayView(T& array, F f, Args& ...args) {
for (auto& element : array) {
f(element, args...);
}
}
template<class T, class F, class... Args>
typename std::enable_if<(T::dimensionality>1), void>::type IterateArrayView(T& array, F f, Args& ...args) {
for (auto element : array) {
IterateArrayView<decltype(element), F, Args...>(element, f, args...);
}
}
you can apply a function to each element with extra arguments. For example
int main() {
using array_type = boost::multi_array<int, 3>;
using view_type = array_type::array_view<3>::type;
using range = boost::multi_array_types::index_range;
array_type data;
data.resize(boost::extents[16][4][4]);
view_type view = data[boost::indices[range(0,4)][range()][range()]];
int count = 0;
IterateArrayView(view, [](int &i, int &count) { i = count++;}, count);
std::cout << view[3][3][3] << std::endl; // output 63 (=4^3-1)
return 0;
}
wrong and old solution below
In fact, boost::multi_array_view provide a method called origin() (ref: here):
template <typename T, std::size_t NumDims>
class multi_array_view :
public const_multi_array_view<T,NumDims,T*>
{
// a lot of code ...
element* origin() { return this->base_+this->origin_offset_; }
// a lot of code ...
};
So you can loop through it by
array_view b;
for (auto it = b.origin(); it != b.origin()+b.num_elements(); it++) {
// do something, e.g.
*it = 'a';
}
For boost::multi_array, you can use auto it = b.data() instead.
update1: Sorry, my solution was incorrect. I just found that, although b.origin() gives you the correct iterator to begin with, you are still looping through the multi_array instead of this array_view.