accumulate instead of for-loop

[Alex Vinokur]

Let v be:

vector<pair<string, size_t> > v;

sum = 0;
for (size_t k = 0; k < v.size(); k++)
{
sum += v[k].second;
}


Is it possible to compute sum using std::accumulate (i.e. not using
for-loop)?



Alex Vinokur
email: alex DOT vinokur AT gmail DOT com
http://mathforum.org/library/view/10978.html
http://sourceforge.net/users/alexvn

 

[Boris Glawe]

http://www2.roguewave.com/support/docs/leif/sourcepro/html/stdlibref/accumulate.html


greets Boris

 

[James Kanze]

Let v be:

vector<pair<string, size_t> > v;

sum = 0;
for (size_t k = 0; k < v.size(); k++)
{
sum += v[k].second;
}

Is it possible to compute sum using std::accumulate (i.e. not using
for-loop)?

I'm pretty sure that Boost iterators have something which
supports this, probably right out of the box, but at the very
least transform_iteratror in collaboration with bind will do the
trick.

 

[Jerry Coffin]

Let v be:

vector<pair<string, size_t> > v;

sum = 0;
for (size_t k = 0; k < v.size(); k++)
{
sum += v[k].second;
}

Is it possible to compute sum using std::accumulate (i.e. not using
for-loop)?

Yes, though it's open to question whether it's really any improvement:

typedef std:air<std::string, size_t> p;

struct add_second : std::binary_function<size_t, size_t, p> {
size_t operator()(size_t a, p const &b) { return a+b.second; }
};

size_t sum = std::accumulate(v.begin(), v.end(), 0, add_second());

Using Boost.lambda, I believe you should also be able to do something
like this:

size_t sum=std::accumulate(v.begin(), v.end(), 0, var(_1).second);

 

[Michael DOUBEZ]

Alex Vinokur a écrit :

Let v be:

vector<pair<string, size_t> > v;

sum = 0;
for (size_t k = 0; k < v.size(); k++)
{
sum += v[k].second;
}


Is it possible to compute sum using std::accumulate (i.e. not using
for-loop)?

You can use a functor to project on the second element of the pair:

template<class Pair>
class pair_get2nd
{
typedef Pair argument_type;
typedef typename Pair::second_type result_type;

const result_type& operator()(const argument_type& t)const
{
return t.second;
}
};

And then

typedef vector<pair<string, size_t> > my_vector;
my_vector v;
sum = std::accumulate(v.begin(), v.end(),0,
pair_get2nd<my_vector::value_type>());


Michael