How to retrieve all keys or values from a std map and put them into a vector

Question

This is one of the possible ways I come out   struct RetrieveKey       template  lt typename T gt      typename T  first type operator   T keyValuePair  const               return keyValuePair first            map lt int  int gt  m  vector lt int gt  keys      Retrieve all keys transform m begin    m end    back inserter keys   RetrieveKey         Dump all keys copy keys begin    keys end    ostream iterator lt int gt  cout    n       Of course  we can also retrieve all values from the map by defining another functor RetrieveValues   Is there any other way to achieve this easily   I m always wondering why std  map does not include a member function for us to do so

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The best non-sgi  non-boost STL solution is to extend map  iterator like so   template lt class map type gt  class key iterator   public map type  iterator   public      typedef typename map type  iterator map iterator      typedef typename map iterator  value type  first type key type       key iterator const map iterator amp  other    map type  iterator other            key type amp  operator                   return map type  iterator  operator    first               helpers to create iterators easier  template lt class map type gt  key iterator lt map type gt  key begin map type amp  m        return key iterator lt map type gt  m begin       template lt class map type gt  key iterator lt map type gt  key end map type amp  m        return key iterator lt map type gt  m end         and then use them like so           map lt string int gt  test          test  one     1          test  two     2           vector lt string gt  keys              method one         key iterator lt map lt string int gt   gt  kb test begin             key iterator lt map lt string int gt   gt  ke test end             keys insert keys begin    kb  ke               method two         keys insert keys begin                 key iterator lt map lt string int gt   gt  test begin                  key iterator lt map lt string int gt   gt  test end                  method three  with helpers          keys insert keys begin    key begin test   key end test             string one   keys 0

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DanDan s answer  using C  11 is   using namespace std  vector lt int gt  keys   transform begin map in   end map in   back inserter keys                   decltype map in   value type const amp  pair        return pair first         and using C  14  as noted by  ivan ukr  we can replace decltype map in   value type with auto

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You can use the versatile boost  transform iterator  The transform iterator allows you to transform the iterated values  for example in our case when you want to deal only with the keys  not the values  See http   www boost org doc libs 1 36 0 libs iterator doc transform iterator html example

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c  0x too std  map lt int int gt  mapints  std  vector lt int gt  vints  for auto const amp  imap  mapints      vints push back imap first

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While your solution should work  it can be difficult to read depending on the skill level of your fellow programmers   Additionally  it moves functionality away from the call site   Which can make maintenance a little more difficult  I m not sure if your goal is to get the keys into a vector or print them to cout so I m doing both   You may try something like this  std  map lt int  int gt  m  std  vector lt int gt  key  value  for std  map lt int int gt   iterator it   m begin    it    m end      it      key push back it- gt first     value push back it- gt second     std  cout  lt  lt   quot Key   quot   lt  lt  it- gt first  lt  lt  std  endl      std  cout  lt  lt   quot Value   quot   lt  lt  it- gt second  lt  lt  std  endl       Or even simpler  if you are using Boost  map lt int int gt  m  pair lt int int gt  me     what a map lt int  int gt  is made of vector lt int gt  v  BOOST FOREACH me  m      v push back me first     cout  lt  lt  me first  lt  lt   quot  n quot      Personally  I like the BOOST FOREACH version because there is less typing and it is very explicit about what it is doing

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Slightly similar to one of examples here  simplified from std  map usage perspective   template lt class KEY  class VALUE gt  std  vector lt KEY gt  getKeys const std  map lt KEY  VALUE gt  amp  map        std  vector lt KEY gt  keys map size         for  const auto amp  it   map          keys push back it first       return keys      Use like this   auto keys   getKeys yourMap

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I found the following three lines of code as the easiest way     save keys in vector  vector lt string gt  keys  for  auto  amp  it   m        keys push back it first      It is a shorten version of the first way of this answer

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I m always wondering why std  map does not include a member function for us to do so     Because it can t do it any better than you can do it   If a method s implementation will be no superior to a free function s implementation then in general you should not write a method  you should write a free function   It s also not immediately clear why it s useful anyway

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Here s a nice function template using C  11 magic  working for both std  map  std  unordered map   template lt template  lt typename    gt  class MAP  class KEY  class VALUE gt  std  vector lt KEY gt  keys const MAP lt KEY  VALUE gt  amp  map        std  vector lt KEY gt  result      result reserve map size         for const auto amp  it   map           result emplace back it first             return result      Check it out here  http   ideone com lYBzpL

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Based on  rusty-parks solution  but in c  17  std  map lt int  int gt  items  std  vector lt int gt  itemKeys   for  const auto amp   key       items        itemKeys push back key

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The following functor retrieves the key set of a map   include  lt vector gt   include  lt iterator gt   include  lt algorithm gt   template  lt class  Map gt  std  vector lt typename  Map  key type gt  keyset const  Map amp  map        std  vector lt typename  Map  key type gt  result      result reserve map size         std  transform map cbegin    map cend    std  back inserter result      typename  Map  const reference kvpair            return kvpair first              return result     Bonus  The following functors retrieve the value set of a map   include  lt vector gt   include  lt iterator gt   include  lt algorithm gt   include  lt functional gt   template  lt class  Map gt  std  vector lt typename  Map  mapped type gt  valueset const  Map amp  map        std  vector lt typename  Map  mapped type gt  result      result reserve map size         std  transform map cbegin    map cend    std  back inserter result      typename  Map  const reference kvpair            return kvpair second              return result     template  lt class  Map gt  std  vector lt std  reference wrapper lt typename  Map  mapped type gt  gt  valueset  Map amp  map        std  vector lt std  reference wrapper lt typename  Map  mapped type gt  gt  result      result reserve map size         std  transform map begin    map end    std  back inserter result      typename  Map  reference kvpair            return std  ref kvpair second               return result     Usage  int main         std  map lt int  double gt  map           1  9 0            2  9 9            3  9 99            4  9 999              auto ks   keyset map       auto vs   valueset map       for  auto amp  k   ks  std  cout  lt  lt  k  lt  lt    n       std  cout  lt  lt   quot ------------------ n quot       for  auto amp  v   vs  std  cout  lt  lt  v  lt  lt    n       for  auto amp  v   vs  v    100 0      std  cout  lt  lt   quot ------------------ n quot       for  auto amp  v   vs  std  cout  lt  lt  v  lt  lt    n       std  cout  lt  lt   quot ------------------ n quot       for  auto amp   k  v    map  std  cout  lt  lt  v  lt  lt    n        return 0     Expected output  1 2 3 4 ------------------ 9 9 9 9 99 9 999 ------------------ 109 109 9 109 99 109 999 ------------------ 109 109 9 109 99 109 999

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With atomic map example   include  lt iostream gt   include  lt map gt   include  lt vector gt    include  lt atomic gt   using namespace std   typedef std  atomic lt std  uint32 t gt  atomic uint32 t  typedef std  map lt int  atomic uint32 t gt  atomic map t   int main         atomic map t m       m 4    456      m 2    45678       vector lt int gt  v      for map lt int atomic uint32 t gt   iterator it   m begin    it    m end      it          v push back it- gt second         cout  lt  lt  it- gt first  lt  lt      lt  lt it- gt second lt  lt   n              return 0

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The SGI STL has an extension called select1st  Too bad it s not in standard STL

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Also  if you have Boost  use transform iterator to avoid making a temporary copy of the keys

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Your solution is fine but you can use an iterator to do it   std  map lt int  int gt  m  m insert std  pair lt int  int gt  3  4    m insert std  pair lt int  int gt  5  6    for std  map lt int  int gt   const iterator it   m begin    it    m end    it          int key   it- gt first      int value   it- gt second        Do something

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With the structured binding     destructuring     declaration syntax of C  17  you can do this  which is easier to understand     To get the keys std  map lt int  double gt  map  std  vector lt int gt  keys  keys reserve map size     for const auto amp   key  value    map        keys push back key         To get the values std  map lt int  double gt  map  std  vector lt double gt  values  values reserve map size     for const auto amp   key  value    map        values push back value

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C  0x has given us a further  excellent solution   std  vector lt int gt  keys   std  transform      m Inputs begin        m Inputs end        std  back inserter keys          const std  map lt int int gt   value type  amp pair  return pair first

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Bit of a c  11 take   std  map lt uint32 t  uint32 t gt  items  std  vector lt uint32 t gt  itemKeys  for  auto  amp  kvp   items        itemKeys emplace back kvp first       std  cout  lt  lt  kvp first  lt  lt  std  endl

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There is a boost range adaptor for this purpose   include  lt boost range adaptor map hpp gt   include  lt boost range algorithm copy hpp gt  vector lt int gt  keys  boost  copy m   boost  adaptors  map keys  std  back inserter keys     There is a similar map values range adaptor for extracting the values

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I think the BOOST FOREACH presented above is nice and clean  however  there is another option using BOOST as well    include  lt boost lambda lambda hpp gt   include  lt boost lambda bind hpp gt   std  map lt int  int gt  m  std  vector lt int gt  keys   using namespace boost  lambda   transform       m begin                     m end                     back inserter keys                    bind   amp std  map lt int int gt   value type  first   1                  copy  keys begin    keys end    std  ostream iterator lt int gt  std  cout    n        Personally  I don t think this approach is as clean as the BOOST FOREACH approach in this case  but boost  lambda can be really clean in other cases

[c++] How to retrieve all keys (or values) from a std::map and put them into a vector?

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