Copy map values to vector in STL

Question

Working my way through Effective STL at the moment  Item 5 suggests that it s usually preferable to use range member functions to their single element counterparts  I currently wish to copy all the values in a map  i e  - I don t need the keys  to a vector   What is the cleanest way to do this

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I thought it should be  std  transform  map begin    map end                        std  back inserter vec                       boost  bind  amp MapT  value type  first  1

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You can t easily use a range here because the iterator you get from a map refers to a std  pair  where the iterators you would use to insert into a vector refers to an object of the type stored in the vector  which is  if you are discarding the key  not a pair  I really don t think it gets much cleaner than the obvious   include  lt map gt   include  lt vector gt   include  lt string gt  using namespace std   int main         typedef map  lt string  int gt  MapType      MapType m        vector  lt int gt  v          populate map somehow      for  MapType  iterator it   m begin    it    m end      it             v push back  it- gt second             which I would probably re-write as a template function if I was going to use it more than once  Something like  template  lt typename M  typename V gt   void MapToVec  const  M  amp  m  V  amp  v         for  typename M  const iterator it   m begin    it    m end      it             v push back  it- gt second

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One way  is to use functor    template  lt class T1  class T2 gt      class CopyMapToVec           public           CopyMapToVec std  vector lt T2 gt  amp  aVec   mVec aVec             bool operator     const std  pair lt T1 T2 gt  amp  mapVal  const                       mVec push back mapVal second               return true                private          std  vector lt T2 gt  amp  mVec           int main         std  map lt std  string  int gt  myMap      myMap  test1     1      myMap  test2     2       std  vector lt int gt   myVector         reserve the memory for vector     myVector reserve myMap size           create the functor     CopyMapToVec lt std  string  int gt  aConverter myVector          call the functor     std  for each myMap begin    myMap end    aConverter

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We should use the transform function from STL algorithm  the last parameter of transform function could be a function object  function pointer or a lambda function that convert item of map to item of vector  This case map have items have type pair that need to convert to item that has int type for vector  Here is my solution that I use lambda function        include  lt algorithm gt     for std  transform  include  lt iterator gt      for back inserted     Map of pair  lt int  string gt  need to convert to vector of string std  map lt int  std  string gt  mapExp      1   first     2   second     3   third     4  fourth          vector of string to store the value type of map std  vector lt std  string gt  vValue      Convert function std  transform mapExp begin    mapExp end    std  back inserter vValue             const std  pair lt int  string gt   amp mapItem                    return mapItem second

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Old question  new answer  With C  11 we have the fancy new for loop   for  const auto  amp s   schemas     names push back s first     where schemas is a std  map and names is an std  vector    This populates the array  names  with keys from the map  schemas   change s first to s second to get an array of values

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If you are using the boost libraries  you can use boost  bind to access the second value of the pair as follows    include  lt string gt   include  lt map gt   include  lt vector gt   include  lt algorithm gt   include  lt boost bind hpp gt   int main        typedef std  map lt std  string  int gt  MapT     typedef std  vector lt int gt  VecT     MapT map     VecT vec      map  one     1     map  two     2     map  three     3     map  four     4     map  five     5      std  transform  map begin    map end                       std  back inserter vec                      boost  bind  amp MapT  value type  second  1         This solution is based on a post from Michael Goldshteyn on the boost mailing list

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Using lambdas one can perform the following         std  map lt std  string int gt  m     std  vector lt int gt  v     v reserve m size        std  for each m begin   m end                       amp v  const std  map lt std  string int gt   value type amp  p                      v push back p second

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Here is what I would do  Also I would use a template function to make the construction of select2nd easier    include  lt map gt   include  lt vector gt   include  lt algorithm gt   include  lt memory gt   include  lt string gt         A class to extract the second part of a pair        template lt typename T gt  struct select2nd       typename T  second type operator   T const amp  value  const      return value second             A utility template function to make the use of select2nd easy     Pass a map and it automatically creates a select2nd that utilizes the    value type  This works nicely as the template functions can deduce the    template parameters based on the function parameters       template lt typename T gt  select2nd lt typename T  value type gt  make select2nd T const amp  m        return select2nd lt typename T  value type gt        int main         std  map lt int std  string gt    m      std  vector lt std  string gt     v                 Please note  You must use std  back inserter                       As transform assumes the second range is as large as the first                      Alternatively you could pre-populate the vector                Use make select2nd   to make the function look nice         Alternatively you could use            select2nd lt std  map lt int std  string gt   value type gt                   std  transform m begin   m end                       std  back inserter v                      make select2nd m

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Surprised nobody has mentioned the most obvious solution  use the std  vector constructor   template lt typename K  typename V gt  std  vector lt std  pair lt K V gt  gt  mapToVector const std  unordered map lt K V gt   amp map        return std  vector lt std  pair lt K V gt  gt  map begin    map end

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You could probably use std  transform for that purpose  I would maybe prefer Neils version though  depending on what is more readable     Example by xtofl  see comments     include  lt map gt   include  lt vector gt   include  lt algorithm gt   include  lt iostream gt   template lt  typename tPair  gt  struct second t       typename tPair  second type operator    const tPair amp  p   const   return p second        template lt  typename tMap  gt   second t lt  typename tMap  value type  gt  second  const tMap amp  m     return second t lt  typename tMap  value type  gt         int main         std  map lt int bool gt  m      m 0  true      m 1  false                std  vector lt bool gt  v      std  transform  m begin    m end    std  back inserter  v    second m         std  transform  m begin    m end    std  ostream iterator lt bool gt   std  cout         second m         Very generic  remember to give him credit if you find it useful

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include  lt algorithm gt     std  transform  include  lt iterator gt      std  back inserter std  transform       your map begin         your map end        std  back inserter your values vector          auto  amp kv   return kv second         Sorry that I didn t add any explanation - I thought that code is so simple that is doesn t require any explanation  So   transform  beginInputRange  endInputRange  outputIterator  unaryOperation    this function calls unaryOperation on every item from inputIterator range  beginInputRange-endInputRange   The value of operation is stored into outputIterator   If we want to operate through whole map - we use map begin   and map end   as our input range  We want to store our map values into vector - so we have to     use back inserter on our vector  back inserter your values vector   The back inserter is special outputIterator that pushes new elements at the end of given  as paremeter  collection  The last parameter is unaryOperation - it takes only one parameter - inputIterator s value  So we can use lambda     auto  amp kv             where  amp kv is just a reference to map item s pair  So if we want to return only values of map s items we can simply return kv second      auto  amp kv    return kv second      I think this explains any doubts

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Why not   template lt typename K  typename V gt  std  vector lt V gt  MapValuesAsVector const std  map lt K  V gt  amp  map       std  vector lt V gt  vec     vec reserve map size        std  for each std  begin map   std  end map             amp vec   const std  map lt K  V gt   value type amp  entry                         vec push back entry second                   return vec      usage   auto vec   MapValuesAsVector anymap

[c++] Copy map values to vector in STL

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