Iterating C vector from the end to the beginning

Question

Is it possible to iterate a vector from the end to the begin   for  vector lt my class gt   iterator i   my vector end            i    my vector begin                     Or is that only possible with something like that   for  int i   my vector size   - 1  i  gt   0  --i

User · Answer

I like the backwards iterator at the end of Yakk - Adam Nevraumont's answer, but it seemed complicated for what I needed, so I wrote this:

template <class T>
class backwards {
    T& _obj;
public:
    backwards(T &obj) : _obj(obj) {}
    auto begin() {return _obj.rbegin();}
    auto end() {return _obj.rend();}
};

I'm able to take a normal iterator like this:

for (auto &elem : vec) {
    // ... my useful code
}

and change it to this to iterate in reverse:

for (auto &elem : backwards(vec)) {
    // ... my useful code
}

User · Answer

The well-established  pattern  for reverse-iterating through closed-open ranges looks as follows     Iterate over  begin  end  range in reverse for  iterator   end  iterator--    begin           Process   iterator      or  if you prefer       Iterate over  begin  end  range in reverse for  iterator   end  iterator    begin        --iterator       Process   iterator      This pattern is useful  for example  for reverse-indexing an array using an unsigned index  int array N          Iterate over  0  N  range in reverse for  unsigned i   N  i--    0        array i       lt - process it      People unfamiliar with this pattern often insist on using signed integer types for array indexing specifically because they incorrectly believe that unsigned types are somehow  unusable  for reverse indexing   It can be used for iterating over an array using a  sliding pointer  technique     Iterate over  array  array   N  range in reverse for  int  p   array   N  p--    array         p      lt - process it     or it can be used for reverse-iteration over a vector using an ordinary  not reverse  iterator  for  vector lt my class gt   iterator i   my vector end    i--    my vector begin           i      lt - process it

User · Answer

use this code    print the vector element in reverse order by normal iterator  cout  lt  lt  print the vector element in reverse order by normal iterator    lt  lt endl  vector lt string gt   iterator iter vec end    --iter  while  iter    vec begin          cout  lt  lt   iter   lt  lt            --iter

User · Answer

Use reverse iterators and loop from rbegin   to rend

User · Answer

Here s a super simple implementation that allows use of the for each construct and relies only on C  14 std library   namespace Details           simple storage of a begin and end iterator     template lt class T gt      struct iterator range               T beginning  ending          iterator range T beginning  T ending    beginning beginning   ending ending              T begin   const   return beginning            T end   const   return ending                                                                                                                                                                                usage      for  auto e   backwards collection   template lt class T gt  auto backwards T  amp  collection        using namespace std      return Details  iterator range rbegin collection   rend collection        This works with things that supply an rbegin   and rend    as well as with static arrays   std  vector lt int gt  collection  5  9  15  22    for  auto e   backwards collection          long values       3  6  9  12    for  auto e   backwards values

User · Answer

User rend     rbegin   iterators   for  vector lt myclass gt   reverse iterator it   myvector rbegin    it    myvector rend    it

User · Answer

As I don t want to introduce alien-like new C   syntax  and I simply want to build up on existing primitives  the below snippets seems to work   include  lt vector gt   include  lt iostream gt   int main  int argc char  argv          std  vector lt int gt  arr 1 2 3 4 5       std  vector lt int gt   iterator it          iterate forward     for  it   arr begin    it    arr end    it              std  cout  lt  lt   it  lt  lt   quot   quot              std  cout  lt  lt   quot  n             n quot         if  arr size    gt  0               iterate backward  simple Joe version         it   arr end   - 1          while  it    arr begin                  std  cout  lt  lt   it  lt  lt   quot   quot               it--                    std  cout  lt  lt   it  lt  lt   quot   quot                  iterate backwards  the C   way     std  vector lt int gt   reverse iterator rit      for  rit   arr rbegin    rit    arr rend    rit              std  cout  lt  lt   rit  lt  lt   quot   quot              return 0

User · Answer

If you have C  11 you can make use of auto   for  auto it   my vector rbegin    it    my vector rend      it

User · Answer

The best way is   for  vector lt my class gt   reverse iterator i   my vector rbegin             i    my vector rend      i           rbegin   rend   were especially designed for that purpose   And yes  incrementing a reverse interator moves it backward    Now  in theory  your method  using begin   end    amp  --i  would work  std  vector s iterator being bidirectional  but remember  end   isn t the last element  mdash  it s one beyond the last element  so you d have to decrement first  and you are done when you reach begin    mdash  but you still have to do your processing   vector lt my class gt   iterator i   my vector end    while  i    my vector begin           --i        do stuff          UPDATE  I was apparently too aggressive in re-writing the for   loop into a while   loop   The important part is that the --i is at the beginning

User · Answer

If you can use The Boost Library  there is the Boost Range that provides the reverse range adapter by including   include  lt boost range adaptor reversed hpp gt   Then  in combination with a C  11 s range-for loop  you can just write the following  for  auto amp  elem  boost  adaptors  reverse my vector                  Since this code is briefer than the one using the iterator pair  it may be more readable and less prone to errors as there are fewer details to pay attention to

User · Answer

template lt class It gt  std  reverse iterator lt It gt  reversed  It it       return std  reverse iterator lt It gt  std  forward lt It gt  it        Then   for  auto rit   reversed data end     rit    reversed data begin       rit       std  cout  lt  lt   rit    Alternatively in C  14 just do   for  auto rit   std  rbegin data   rit    std  rend data     rit       std  cout  lt  lt   rit    In C  03 11 most standard containers have a  rbegin   and  rend   method as well   Finally  you can write the range adapter backwards as follows   namespace adl aux     using std  begin  using std  end    template lt class C gt    decltype  begin  std  declval lt C gt        adl begin  C amp  amp  c         return begin std  forward lt C gt  c          template lt class C gt    decltype  end  std  declval lt C gt        adl end  C amp  amp  c         return end std  forward lt C gt  c           template lt class It gt  struct simple range     It b   e     simple range   b    e        It begin   const   return b       It end   const   return e       simple range  It b  It e   b  b   e  e        template lt class OtherRange gt    simple range  OtherRange amp  amp  o        simple range adl aux  adl begin o   adl aux  adl end o              explicit defaults    simple range  simple range const amp  o     default    simple range  simple range  amp  amp  o     default    simple range amp  operator   simple range const amp  o     default    simple range amp  operator   simple range  amp  amp  o     default     template lt class C gt  simple range lt  decltype  reversed  adl aux  adl begin  std  declval lt C amp  gt           gt  backwards  C amp  amp  c       return   reversed  adl aux  adl end c     reversed  adl aux  adl begin c           and now you can do this   for  auto amp  amp  x   backwards ctnr     std  cout  lt  lt  x    which I think is quite pretty

User · Answer

Starting with c  20  you can use a std  ranges  reverse view and a range-based for-loop     include lt ranges gt   include lt vector gt   include lt iostream gt   using namespace std  ranges   std  vector lt int gt  const vec 1  2  3  4  5  6  7  8  9  10    for auto amp  i    views  reverse vec         std  cout  lt  lt  i  lt  lt           Or even   for auto amp  i    vec   views  reverse    Unfortunately  at the time of writing  Jan 2020  no major compiler implements the ranges library  but you can resort to Eric Niebler s ranges-v3    include  lt iostream gt   include  lt vector gt   include  range v3 all hpp   int main          using namespace ranges       std  vector lt int gt  const vec 1  2  3  4  5  6  7  8  9  10       for auto amp  i    views  reverse vec             std  cout  lt  lt  i  lt  lt                  return 0

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