How to use std::partial_sum and output to a std::map?

You can't. Not directly, at least. The issue is that std::map<int, int>::iterator::value_type is std::pair<const int, int>, and that const prevents an object of that type from being assigned to.

--

Take a look at this possible implementation for std::partial_sum:

template<class InputIt, class OutputIt, class BinaryOperation> constexpr // since C++20 OutputIt partial_sum(InputIt first, InputIt last, OutputIt d_first, BinaryOperation op) { if (first == last) return d_first; typename std::iterator_traits<InputIt>::value_type sum = *first; *d_first = sum; while (++first != last) { sum = op(std::move(sum), *first); // std::move since C++20 *++d_first = sum; } return ++d_first; } 

Note that sum gets modified each iteration by assigning the result of op. Because sum.first is const that's not possible; hence the compilation error.

--

What you could do is define an iterator type that wraps std::map::iterator and strips out the const. For example, the following will work:

template <typename Pair> struct RemoveFirstConstHelper { using type = Pair; }; template <typename T1, typename T2> struct RemoveFirstConstHelper<std::pair<const T1, T2>> { using type = std::pair<T1, T2>; }; template <typename MapIterator> class RemoveFirstConstIter { public: using difference_type = std::ptrdiff_t; using value_type = typename RemoveFirstConstHelper<typename MapIterator::value_type>::type; using pointer = value_type*; using reference = value_type; using iterator_category = std::input_iterator_tag; RemoveFirstConstIter(MapIterator it) : it_{it} {} reference operator*() { return *it_; } RemoveFirstConstIter& operator++() { ++it_; return *this; } RemoveFirstConstIter operator++(int) const { RemoveFirstConstIter temp{*this}; ++temp; return temp; } bool operator==(const RemoveFirstConstIter& other) const { return it_ == other.it_; } bool operator!=(const RemoveFirstConstIter& other) const { return !(*this == other); } private: MapIterator it_; }; 

Live Demo

Or you could just write your own partial_sum implementation for maps. That seems like it would be simpler to me.