Appending ranges in loop

问题

I would like to concatenate ranges returned by function into one big range.Consider following code:

some_type_i_cant_figure_out bar() {
    typedef std::vector<int>::const_iterator iter;
    std::vector<int> aaa;
    /* fill some data into aaa*/
    some_type_i_cant_figure_out cc;
    for (int i = 0; i < aaa.size(); ++i) {
    std::pair<iter, iter> bbb = foo(aaa, i);
    ccc = boost::join(ccc, bbb);
    }
    return ccc;
}

What I'm trying to achieve:
The aaa vector is huge and foo may return quite big ranges. Of course I can just create copies of all elements in range into new vector of integers and return it. It is inefficient, wasting memory and time. So I would like return one boost::joined_range. In worst case, I can live with vector of ranges, but it would be too simple and not that elegant :) besides the joined_range isnt default constructible (which is a problematic for this example implementation) what would be the return value type? the temp variable (ccc) type and what would be the correct and elegant way to achieve the above?

回答1:

First off, the end result of your code would appear to be similar to just

auto cc(aaa);
boost::stable_sort(cc);

(Assuming, from your sample code, that aaa contains integers in the range [0..size()-1))

If you can afford to simply copy, just use a backinsert iterator:

std::vector<int> cc;
for (size_t i = 0; i < aaa.size(); ++i)
    boost::copy(boost::equal_range(aaa, i), back_inserter(cc));

Otherwise, you can hide the accumulating joins by using any_range:

boost::any_range<int, boost::forward_traversal_tag, int> r;
for (size_t i = 0; i < aaa.size(); ++i)
    r = boost::join(r, boost::equal_range(aaa, i));

Live On Coliru

#include <boost/range/any_range.hpp>
#include <boost/range/join.hpp>
#include <boost/range/algorithm.hpp>
#include <iostream>

int main() {
    std::vector<int> const aaa { 1,1,1,4,5,5,9,42,42,42,42,42,42 };

    boost::any_range<int, boost::forward_traversal_tag, int> r;
    for (size_t i = 0; i < aaa.size(); ++i)
        r = boost::join(r, boost::equal_range(aaa, i));

    boost::copy(r, std::ostream_iterator<int>(std::cout << "result: ", " "));
}

Prints

result: 1 1 1 4 5 5 9

来源：https://stackoverflow.com/questions/27012395/appending-ranges-in-loop