c++ - Equivalent of Python's list sort with key / Schwartzian transform -
in python, given list, can sort key function, e.g.:
>>> def get_value(k): ... print "heavy computation for", k ... return {"a": 100, "b": 30, "c": 50, "d": 0}[k] ... >>> items = ['a', 'b', 'c', 'd'] >>> items.sort(key=get_value) heavy computation heavy computation b heavy computation c heavy computation d >>> items ['d', 'b', 'c', 'a'] as see, list sorted not alphanumerically return value of get_value().
is there equivalent in c++? std::sort() allows me provide custom comparator (equivalent of python's items.sort(cmp=...)), not key function. if not, there well-tested, efficient, publicly available implementation of equivalent can drop code?
note python version calls key function once per element, not twice per comparison.
you roll own:
template <typename randomit, typename keyfunc> void sort_by_key(randomit first, randomit last, keyfunc func) { using value = decltype(*first); std::sort(first, last, [=](const valuetype& a, const valuetype& b) { return func(a) < func(b); }); } if keyfunc expensive, you'll have create separate vector values.
we can hack class allow still use std::sort:
template <typename randomiter, typename keyfunc> void sort_by_key(randomiter first, randomiter last, keyfunc func) { using keyt = decltype(func(*first)); using valuet = typename std::remove_reference<decltype(*first)>::type; struct pair { keyt key; randomiter iter; boost::optional<valuet> value; pair(const keyt& key, const randomiter& iter) : key(key), iter(iter) { } pair(pair&& rhs) : key(std::move(rhs.key)) , iter(rhs.iter) , value(std::move(*(rhs.iter))) { } pair& operator=(pair&& rhs) { key = std::move(rhs.key); *iter = std::move(rhs.value ? *rhs.value : *rhs.iter); value = boost::none; return *this; } bool operator<(const pair& rhs) const { return key < rhs.key; } }; std::vector<pair> ordering; ordering.reserve(last - first); (; first != last; ++first) { ordering.emplace_back(func(*first), first); } std::sort(ordering.begin(), ordering.end()); } or, if that's hacky, here's original solution, requires write our own sort
template <typename randomit, typename keyfunc> void sort_by_key_2(randomit first, randomit last, keyfunc func) { using keyt = decltype(func(*first)); std::vector<std::pair<keyt, randomit> > ordering; ordering.reserve(last - first); (; first != last; ++first) { ordering.emplace_back(func(*first), first); } // sort vector ordering - we're going // sort ordering, each swap has iter_swap quicksort_with_benefits(ordering, 0, ordering.size()); } although have reimplement quicksort:
template <typename key, typename iter> void quicksort_with_benefits(std::vector<std::pair<key,iter>>& a, size_t p, size_t q) { if (p < q) { size_t r = partition_with_benefits(a, p, q); quicksort_with_benefits(a, p, r); quicksort_with_benefits(a, r+1, q); } } template <typename key, typename iter> size_t partition_with_benefits(std::vector<std::pair<key,iter>>& a, size_t p, size_t q) { auto key = a[p].first; size_t = p; (size_t j = p+1; j < q; ++j) { if (a[j].first < key) { ++i; std::swap(a[i].first, a[j].first); std::iter_swap(a[i].second, a[j].second); } } if (i != p) { std::swap(a[i].first, a[p].first); std::iter_swap(a[i].second, a[p].second); } return i; } which, given simple example:
int main() { std::vector<int> v = {-2, 10, 4, 12, -1, -25}; std::sort(v.begin(), v.end()); print(v); // -25 -2 -1 4 10 12 sort_by_key_2(v.begin(), v.end(), [](int i) { return i*i; }); print(v); // -1 -2 4 10 12 -25 }