How to find if a given key exists in a C std map

Question

I m trying to check if a given key is in a map and somewhat can t do it   typedef map lt string string gt   iterator mi  map lt string  string gt  m  m insert make pair  f     --     pair lt mi mi gt  p   m equal range  f     I m not sure if equal range does what I want cout  lt  lt  p first   I m getting error here   so how can I print what is in p

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m find    m end      not found    If you want to use other API  then find go for m count c  gt 0   if  m count  f   gt 0        cout  lt  lt    is an element of m  n       else        cout  lt  lt    is not an element of m  n

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To check if a particular key in the map exists  use the count member function in one of the following ways   m count key   gt  0 m count key     1 m count key     0   The documentation for map  find says   Another member function  map  count  can be used to just check whether a particular key exists    The documentation for map  count says   Because all elements in a map container are unique  the function can only return 1  if the element is found  or zero  otherwise     To retrieve a value from the map via a key that you know to exist  use map  at   value   m at key    Unlike map  operator    map  at will not create a new key in the map if the specified key does not exist

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C  17 simplified this a bit more with an If statement with initializer  This way you can have your cake and eat it too   if   auto it  m find   key       it    std  end  m               Use  structured binding  to get the key        and value      auto  key  value      it            Grab either the key or value stored in the pair         The key is stored in the  first  variable and        the  value  is stored in the second      auto mkey  it- gt first        auto mvalue  it- gt second            That or just grab the entire pair pointed        to by the iterator      auto pair   it       else          Key was not found

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Comparing the code of std  map  find and std  map  count  I d say the first may yield some performance advantage   const iterator find const key type amp   Keyval  const            find an element in nonmutable sequence that matches  Keyval     const iterator  Where   lower bound  Keyval      Here one looks only for lower bound     return   Where    end               DEBUG LT PRED this- gt  Getcomp                 Keyval  this- gt  Key  Where  Mynode                       end      Where          size type count const key type amp   Keyval  const            count all elements that match  Keyval      Paircc  Ans   equal range  Keyval      Here both lower and upper bounds are to be found  which is presumably slower      size type  Num   0       Distance  Ans first   Ans second   Num       return   Num

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template  lt typename T  typename Key gt  bool key exists const T amp  container  const Key amp  key        return  container find key     std  end container        Of course if you wanted to get fancier you could always template out a function that also took a found function and a not found function  something like this   template  lt typename T  typename Key  typename FoundFunction  typename NotFoundFunction gt  void find and execute const T amp  container  const Key amp  key  FoundFunction found function  NotFoundFunction not found function        auto amp  it   container find key       if  it    std  end container                 found function key  it- gt second             else               not found function key             And use it like this       std  map lt int  int gt  some map      find and execute some map  1             int key  int value   std  cout  lt  lt   key    lt  lt  key  lt  lt    found  value     lt  lt  value  lt  lt  std  endl                int key   std  cout  lt  lt   key    lt  lt  key  lt  lt    not found   lt  lt  std  endl        The downside to this is coming up with a good name   find and execute  is awkward and I can t come up with anything better off the top of my head

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C  20 gives us std  map  contains to do that    include  lt iostream gt   include  lt string gt   include  lt map gt   int main         std  map lt int  std  string gt  example     1   One     2   Two                                           3   Three     42   Don  t Panic             if example contains 42             std  cout  lt  lt   Found n         else           std  cout  lt  lt   Not found n

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map  lt int   char gt   iterator itr      for itr   MyMap begin     itr   MyMap end     itr                  if  itr- gt second     c                         cout lt  lt itr- gt first lt  lt endl

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You can use  find     map lt string string gt   iterator i   m find  f     if  i    m end         Not found      else      Found  i- gt first is f  i- gt second is   --

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I know this question already has some good answers but I think my solution is worth of sharing   It works for both std  map and std  vector lt std  pair lt T  U gt  gt  and is available from C  11   template  lt typename ForwardIterator  typename Key gt  bool contains key ForwardIterator first  ForwardIterator last  Key const key        using ValueType   typename std  iterator traits lt ForwardIterator gt   value type       auto search result   std  find if          first  last            amp key  ValueType const amp  item                return item first    key                        if  search result    last            return false        else           return true

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Use map  find  if   m find  f      m end            not found   else        found

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Be careful in comparing the find result with the the end like for map  m  as all answer have  done above      map  iterator i   m find  f      if  i    m end           else           you should not try and perform any operation such as printing the key or value with iterator i if its equal to m end   else it will lead to segmentation fault

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map lt string  string gt  m    check key exist or not  and return number of occurs 0 1 in map     int num   m count  f      if  num gt 0              found      else            not found       check key exist or not  and return iterator    map lt string string gt   iterator mi   m find  f      if mi    m end              found         do something to mi      else            not found           in your question  the error caused by bad operator lt  lt  overload  because p first is map lt string  string gt   you can not print it out  try this    if p first    p second        cout  lt  lt  p first- gt first  lt  lt       lt  lt  p first- gt second  lt  lt  endl

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If you want to compare pair of map you can use this method   typedef map lt double  double gt  TestMap  TestMap testMap  pair lt map lt double double gt   iterator bool gt  controlMapValues   controlMapValues  testMap insert std  pair lt double double gt  x y    if  controlMapValues second    false         TestMap  iterator it      it   testMap find x        if  it- gt second    y                cout lt  lt  Given value is already exist in Map  lt  lt endl            This is a useful technique

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I think you want map  find  If m find  f   is equal to m end    then the key was not found  Otherwise  find returns an iterator pointing at the element found   The error is because p first is an iterator  which doesn t work for stream insertion  Change your last line to cout  lt  lt   p first - gt first   p is a pair of iterators  p first is an iterator  p first- gt first is the key string   A map can only ever have one element for a given key  so equal range isn t very useful  It s defined for map  because it s defined for all associative containers  but it s a lot more interesting for multimap

[c++] How to find if a given key exists in a C++ std::map

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