Initialize a container with iterator range of container with different type

Question

Let's say we have std::set<int> and we want to create a std::vector<int> with all values from that set:

std::set<int> set;
std::vector<int> vec( set.begin(), set.end() );

This is simple and elegant. But let's say I have a std::map<std::string,int> and I want to copy all values to std::vector<int>. Unfortunately there is no constructor, that accepts range of iterators and converter function. Why there is no such constructor provided? Is there another simple and elegant way to initialize one container with different type values?

Maxim Egorushkin · Accepted Answer · 2015-09-03 14:09:40Z

8

Use transform iterators:

#include <boost/iterator/transform_iterator.hpp>
#include <vector>
#include <map>

int main() {
    std::map<int, double> m;
    auto f = [](auto&& pair) { return pair.second; };
    std::vector<double>(boost::make_transform_iterator(m.begin(), f),
                        boost::make_transform_iterator(m.end(), f));
}

Alternatively, use boost::adaptors:

#include <boost/range/adaptor/map.hpp>
#include <vector>
#include <map>

int main() {
    std::map<int, double> m;
    auto range = boost::adaptors::values(m);
    std::vector<double>(range.begin(), range.end());
}

Or the same as above:

    auto v = boost::copy_range<std::vector<double> >(boost::adaptors::values(m));

Note that using vector's range constructor is more efficient than a solution involving back_inserter.

edited Sep 3, 2015 at 14:09

answered Sep 3, 2015 at 13:51

Maxim Egorushkin

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2 Comments

Alan Over a year ago

Personally, I prefer the "pipe" operator when using boost adaptors auto range = m | boost::adaptors::values;

Maxim Egorushkin Over a year ago

@Alan I prefer function call operator for function calls, and it is shorter to type (` | ` vs ()).

Piotr Skotnicki · Accepted Answer · 2015-09-03 13:51:31Z

2

With boost::transform_iterator:

#include <functional>
#include <boost/iterator/transform_iterator.hpp>

std::map<std::string, int> m{ {"a", 1}, {"b", 2} };
auto second = std::mem_fn(&std::map<std::string, int>::value_type::second);

std::vector<int> vec(boost::make_transform_iterator(std::begin(m), second)
                   , boost::make_transform_iterator(std::end(m), second));

DEMO

answered Sep 3, 2015 at 13:51

Piotr Skotnicki

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3 Comments

Aaron McDaid Over a year ago

Perhaps the downvoter (not me) didn't yet have time to write a comment and is still writing it

Maxim Egorushkin Over a year ago

Not me, but calling through std::mem_fn would likely not be inlined.

Slava Over a year ago

@AaronMcDaid that's gonna be pretty damn long comment

Youka · Accepted Answer · 2015-09-03 14:12:35Z

1

std::map has another memory model, storing values as std::pair. Unpacking these isn't the job of a constructor. You can create your vector, reserve memory equal to the map size and iterate over map pairs to push your integers to the back of your vector. std::transform slims that.

edited Sep 3, 2015 at 14:12

answered Sep 3, 2015 at 13:50

Youka

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2 Comments

Slava Over a year ago

std::set also has another memory model, that does not stop me from initailizing a vector from range of it.

Youka Over a year ago

Because your set iterators return just the expected interger, a map returns a pair which can't be casted to the expected output.

infoshoc · Accepted Answer · 2015-09-03 13:46:14Z

0

I think the best you can do is using range based cycles or for_each function.

answered Sep 3, 2015 at 13:46

infoshoc

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