C++ algorithm like python's 'groupby'

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再見小時候 2020-12-15 22:20

Are there any C++ transformations which are similar to itertools.groupby()?

Of course I could easily write my own, but I\'d prefer to leverage the idiomatic behavior

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情书的邮戳 (楼主)

2020-12-15 23:07

What is the point of bloating standard C++ library with an algorithm that is one line of code?

for (const auto & foo : foos) foos_by_x[foo.x].push_back(foo);

Also, take a look at std::multimap, it might be just what you need.

UPDATE:

The one-liner I have provided is not well-optimized for the case when your vector is already sorted. A number of map lookups can be reduced if we remember the iterator of previously inserted object, so it the "key" of the next object and do a lookup only when the key is changing. For example:

#include 
#include 
#include 
#include 
#include 

struct foo {
    int         x;
    std::string y;
    float       z;
};

class optimized_inserter {
  public:
    typedef std::map > map_type;

    optimized_inserter(map_type & map) : map(&map), it(map.end()) {}

    void operator()(const foo & obj) {
        typedef map_type::value_type value_type;
        if (it != map->end() && last_x == obj.x) {
            it->second.push_back(obj);
            return;
        }
        last_x = obj.x;
        it = map->insert(value_type(obj.x, std::vector({ obj }))).first;
    }

  private:
    map_type          *map;
    map_type::iterator it;
    int                last_x;
};

int main()
{
    std::vector foos;
    std::map> foos_by_x;

    foos.push_back({ 1, "one", 1.0 });
    foos.push_back({ 3, "third", 2.5 });
    foos.push_back({ 1, "one.. but third", 1.5 });
    foos.push_back({ 2, "second", 1.8 });
    foos.push_back({ 1, "one.. but second", 1.5 });

    std::sort(foos.begin(), foos.end(), [](const foo & lhs, const foo & rhs) {
            return lhs.x < rhs.x;
        });

    std::for_each(foos.begin(), foos.end(), optimized_inserter(foos_by_x));

    for (const auto & p : foos_by_x) {
        std::cout << "--- " << p.first << "---\n";
        for (auto & f : p.second) {
            std::cout << '\t' << f.x << " '" << f.y << "' / " << f.z << '\n';
        }
    }
}

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