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I have a container filled with pairs. I want to iterate in it using the STL generic algorithms (in my case it would be inner_product, but consider it as a generic problem). The algorithm I am using expects iterators first and last. Can I provide special iterators first and last that will iterate not on the pairs but on the first element of each pair?
I know i can do it manually, providing a hand-made function object that will be a wrapper around the standard container iterator, deferencing it to the first member of the pair intend of the pair itself,but I think there is also a clever one-liner to do it for me. What would it be?
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begin() function is used to return an iterator pointing to the first element of the map container.
An iterator is an object (like a pointer) that points to an element inside the container. We can use iterators to move through the contents of the container. They can be visualized as something similar to a pointer pointing to some location and we can access the content at that particular location using them.
Generally, like this: struct vec{ iterator begin() ; const_iterator begin() const; };
Obtain an iterator to the start of the collection by calling the collection's iterator( ) method. Set up a loop that makes a call to hasNext( ). Have the loop iterate as long as hasNext( ) returns true. Within the loop, obtain each element by calling next( ).
I've looked around and found boost::transform_iterator. I've come up with this code. Surprising how well it works:
#include <map>
#include <algorithm>
#include <iostream>
#include <string>
#include <iterator>
#include <boost/iterator/transform_iterator.hpp>
#include <boost/bind.hpp>
#include <boost/function.hpp>
int main() {
typedef std::map<std::string, int>::value_type value_type;
std::map<std::string, int> a;
a["one"] = 1;
a["two"] = 2;
// returns the second element
boost::function<int(value_type&)> f = boost::bind(&value_type::second, _1);
std::copy(boost::make_transform_iterator(a.begin(), f),
boost::make_transform_iterator(a.end(), f),
std::ostream_iterator<int>(std::cout, " "));
}
It's printing "1 2 " to the standard output.
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