C++11: Range-looping vector from the second element?

问题

I have a std::vector<std::string> v; (initialized). How can I use the range-for loop for accessing all elements except the first one (on index zero). For all elements:

for (const string & s: v)
    process(s);

Instead of the v a range expression can be used. How can I write the range expression to skip the first element (or skip the first n elements)?

I know how to get the effect using v.begin() + 1 and using the classic loop. I am searching for the new, more readable, recommended alternative to do that. Possibly something similar to Python slicing? ...like:

for s in v[1:]:
    process(s)

回答1:

Create a wrapper for which begin() and end() return the correct iterators and then you can use that as the second argument.

#include <iostream>
#include <vector>

template< typename Collection >
class FromNth
{
    Collection& coll_;
    size_t offset_;

public:
    FromNth( Collection& coll, size_t offset )
        : coll_( coll ), offset_( offset )
    {
    }

    // will nicely resolve to const_iterator if necessary
    auto begin() const -> decltype( coll_.begin() ) 
       { return coll_.begin() + offset_; }

    auto end() const -> decltype( coll_.end() )
       { return coll_.end(); }
};

template< typename Collection >
FromNth<Collection> makeFromNth( Collection& collection, size_t offset )
{
     return FromNth<Collection>( collection, offset );
}

template< typename Collection >
auto begin( const FromNth<Collection> & wrapper ) -> decltype( wrapper.begin() )
{   
   return wrapper.begin();
}

template< typename Collection >
auto end( const FromNth<Collection> & wrapper ) -> decltype( wrapper.end() )
{  
   return wrapper.end();
}

int main()
{
   std::vector< int > coll { 2, 3, 5, 7, 11, 13, 17, 19, 23 };

   for( auto x : makeFromNth( coll, 1 ) )
   {
       std::cout << x << '\n';
   }
   return 0;
}

Note that my fromNth "begin" is undefined behaviour if the size of the input is less than the offset. (If it's equal then it's well defined and begin == end). Therefore do a size check first.

Note: if you are using a recent enough version of boost then iterator_range may already provide you such a "collection" that is similar to my "FromNth".

for( auto const& s : boost::make_iterator_range( v.begin() + 1, v.end() ) )
{
    process( s );
}

Note: the code above worked on CodingGround using C++11 GNU 4.8.3. (That site is very slow though). From C++14 you will not need the ->decltype statements (which are needed in C++11 for templates).

Output:

sh-4.3$ g++ -std=c++11 -o main *.cpp
sh-4.3$ main
3
5
7
11
13
17
19
23

回答2:

Until ranges make it into the standard library, you won't get any better than a vanilla for loop in plain C++ :

for(auto i = begin(v) + 1, e = end(v); i !=e; ++i)
    // Do something with *i

来源：https://stackoverflow.com/questions/31936787/c11-range-looping-vector-from-the-second-element

标签

c++11

for-loop

stdvector