applying partial_sum to a map container

[cesco]

Hi,

I have a map container where the key_type is a pointer to a class and
the value_type is an intenger. I'd like to apply the STL algorithm
partial_sum to the value_type field of that map but I don't know how to
do it. Probably I have to specify a functor as fourth argument of
partial_sum but I don't know how to implement that.
Can anyone help?

Thanks and regards
Cesco

[Nate Barney]

Re: applying partial_sum to a map container

cesco wrote:

>
I have a map container where the key_type is a pointer to a class

If you use a pointer as a key type to an associateive container, you
need to make sure you provide a comparison that's based on the
referent of the pointer. Comparing pointers based on their actual
values is only meaningful when they are pointers into the same
object/array.

and the value_type is an intenger.

I think you mean data_type is an integer. value_type is actually a
std::pair<const key_type,data_t ype>

I'd like to apply the STL algorithm partial_sum to the value_type field of
that map but I don't know how to do it. Probably I have to specify a functor
as fourth argument of partial_sum but I don't know how to implement that.

I thought functor at first, too, but that won't work. The initial
value of the sum is set to the value at the beginning of
the input range, which, for a map, is a std::pair.

What you can do, though, is write a wrapper iterator that
returns only the second part of each pair:

#include <iterator>

template <typename It>
class map_value_itera tor : public
std::iterator<s td::input_itera tor_tag,
typename std::iterator_t raits<It>::valu e_type::second_ type>
{
public:

map_value_itera tor(It it) : it(it) {}
map_value_itera tor(const map_value_itera tor<It&mvi)
: it(mvi.it) {}

map_value_itera tor &operator++( ) { ++it; return *this; }
map_value_itera tor operator++(int) { It i = it; ++it; return i; }

bool operator==(cons t map_value_itera tor<It&mvi) const
{ return it == mvi.it; }
bool operator!=(cons t map_value_itera tor<It&mvi) const
{ return it != mvi.it; }
bool operator==(cons t It &i) const { return it == i; }
bool operator!=(cons t It &i) const { return it != i; }

const typename std::iterator_t raits<It>::valu e_type::second_ type *
operator->() const { return &it->second; }
const typename std::iterator_t raits<It>::valu e_type::second_ type &
operator*() const { return it->second; }

private:

It it;
};

You could then use it like this:

typedef std::map<int,in tIntMap;
typedef std::vector<int IntVec;

IntMap m;
IntVec v;

m[0] = 1;
m[1] = 2;
m[2] = 4;

map_value_itera tor<IntMap::ite ratorbegin = m.begin();
map_value_itera tor<IntMap::ite ratorend = m.end();

std::partial_su m(begin,end,bac k_inserter(v));