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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)
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4
  • One way is to make a new vector with the required elements, and then loop through. Or you can use std::for_each: en.cppreference.com/w/cpp/algorithm/for_each Commented Aug 11, 2015 at 8:31
  • 3
    You can't. The range-based loop isn't the solution for each and every problem. Just use an ordinary loop. Commented Aug 11, 2015 at 8:38
  • As mentioned by @GargAnkit you can make a new vector with required elements. You can do this as part of the loop declaration using the vector constructor. So for (auto&& i : std::vector<std::string>(v.begin()+1,v.end())). Demo here. Not as neat as the python example you showed though. Commented Aug 11, 2015 at 8:39
  • See stackoverflow.com/questions/25652505/… which has nice templates for the general problem. Commented Sep 28, 2017 at 7:28

4 Answers 4

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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
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8 Comments

@CashCow as the question is about bending a range-for into shape, I'm assuming plain C++. I've seen enough Boost to know that nothing is impossible if you throw enough templates at it ;)
@pepr I'll be back when C++ has ranges. *evil laugh*
@Quentin: Well, you should not chew leek as on your icon (unless you are really that blue). It makes you too evilish (as some vegetarians are) ;) By the way, I like your ++i at the example. Many programmers would write i++.
@pepr High-grade leek-chewing blue... uh... things, know how to increment their iterators !
Yes, I am confused :))))
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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
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1 Comment

Thanks for the information! I was expecting that there is some std code like this, already (and I could not find) ;)
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C++20 comes with std::ranges::drop_view to achieve this:

// iterate through elements of v skipping the first for (const auto& s : std::ranges::drop_view{v,1}) { process(s); }
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1

You could use Eric Niebler's Range V3 library, and the tail view, that just creates a view of the original container without the first element.

#include <range/v3/view/tail.hpp> for (const auto& s : ranges::views::tail(v)) { process(s); }
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2 Comments

This is very old question. These days, I would probably prefer the std::ranges::drop_view. Anyway, thank you for the information.
@pepr I do also prefer the std when possible. But, for example, if you're using GitHub Actions nowadays, you'll find that STL ranges are not implemented everywhere. I think they were included in Clang 15, and GitHub is still using Clang 14.

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