How to check that an element is in a std set

Question

How do you check that an element is in a set   Is there a simpler equivalent of the following code   myset find x     myset end

User · Answer

I use  if  my set count that element     Element is present        But it is not as efficient as   if my set find that element   my set end            My version only saves my time in writing the code  I prefer it this way for competitive coding

User · Answer

The typical way to check for existence in many STL containers such as std  map  std  set      is   const bool is in   container find element     container end

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Another way of simply telling if an element exists is to check the count    if  myset count x           x is in the set  count is 1   else         count zero  i e  x not in the set     Most of the times  however  I find myself needing access to the element wherever I check for its existence    So I d have to find the iterator anyway  Then  of course  it s better to simply compare it to end too    set lt  X  gt   iterator it   myset find x   if  it    myset end            do something with  it     C   20  In C  20 set gets a contains function  so the following becomes possible as mentioned at  https   stackoverflow com a 54197839 895245  if  myset contains x          x is in the set   else        no x

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general Syntax            set lt int gt   iterator ii   find set1 begin   set1 end    element to be searched         in below code i am trying to find element 4 in and int set if it is present or not    set lt int gt   iterator ii   find set1 begin   set1 end   4    if ii  set1 end           cout lt  lt  element found       set1 erase ii     in case you want to erase that element from set

User · Answer

If you were going to add a contains function  it might look like this    include  lt algorithm gt   include  lt iterator gt   template lt class TInputIterator  class T gt  inline bool contains TInputIterator first  TInputIterator last  const T amp  value        return std  find first  last  value     last     template lt class TContainer  class T gt  inline bool contains const TContainer amp  container  const T amp  value           This works with more containers but requires std  begin and std  end        from C  0x  which you can get either          1  By using a C  0x compiler or         2  Including the utility functions below      return contains std  begin container   std  end container   value           This works pre-C  0x  and without the utility functions below  but doesn t        work for fixed-length arrays        return contains container begin    container end    value      template lt class T gt  inline bool contains const std  set lt T gt  amp  container  const T amp  value        return container find value     container end        This works with std  set  other STL containers  and even fixed-length arrays   void test         std  set lt int gt  set      set insert 1       set insert 4       assert  contains set  3         int set2       1  2  3        assert contains set2  3        Edit   As pointed out in the comments  I unintentionally used a function new to C  0x  std  begin and std  end   Here is the near-trivial implementation from VS2010   namespace std    template lt class  Container gt  inline     typename  Container  iterator begin  Container amp   Cont           get beginning of sequence     return   Cont begin            template lt class  Container gt  inline     typename  Container  const iterator begin const  Container amp   Cont           get beginning of sequence     return   Cont begin            template lt class  Container gt  inline     typename  Container  iterator end  Container amp   Cont           get end of sequence     return   Cont end            template lt class  Container gt  inline     typename  Container  const iterator end const  Container amp   Cont           get end of sequence     return   Cont end            template lt class  Ty      size t  Size gt  inline      Ty  begin  Ty   amp  Array   Size            get beginning of array     return   amp  Array 0           template lt class  Ty      size t  Size gt  inline      Ty  end  Ty   amp  Array   Size            get end of array     return   amp  Array 0     Size

User · Answer

Just to clarify  the reason why there is no member like contains   in these container types is because it would open you up to writing inefficient code  Such a method would probably just do a this- gt find key     this- gt end   internally  but consider what you do when the key is indeed present  in most cases you ll then want to get the element and do something with it  This means you d have to do a second find    which is inefficient  It s better to use find directly  so you can cache your result  like so   auto it   myContainer find key   if  it    myContainer end             Do something with it  no more lookup needed    else          Key was not present      Of course  if you don t care about efficiency  you can always roll your own  but in that case you probably shouldn t be using C

User · Answer

You can also check whether an element is in set or not while inserting the element  The single element version return a pair  with its member pair  first set to an iterator pointing to either the newly inserted element or to the equivalent element already in the set  The pair  second element in the pair is set to true if a new element was inserted or false if an equivalent element already existed   For example  Suppose the set already has 20 as an element    std  set lt int gt  myset   std  set lt int gt   iterator it   std  pair lt std  set lt int gt   iterator bool gt  ret    ret myset insert 20    if ret second  false            do nothing      else          do something      it ret first   points to element 20 already in set    If the element is newly inserted than pair  first will point to the position of new element in set

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I was able to write a general contains function for std  list and std  vector   template lt typename T gt  bool contains  const list lt T gt  amp  container  const T amp  elt       return find  container begin    container end    elt      container end        template lt typename T gt  bool contains  const vector lt T gt  amp  container  const T amp  elt       return find  container begin    container end    elt      container end           use  if  contains  yourList  itemInList        then do something   This cleans up the syntax a bit   But I could not use template template parameter magic to make this work arbitrary stl containers      NOT WORKING  template lt template lt class gt  class STLContainer  class T gt  bool contains  STLContainer lt T gt  container  T elt       return find  container begin    container end    elt      container end         Any comments about improving the last answer would be nice

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In C  20 we ll finally get std  set  contains method    include  lt iostream gt   include  lt string gt   include  lt set gt   int main         std  set lt std  string gt  example     Do    not    panic                if example contains  panic              std  cout  lt  lt   Found n         else           std  cout  lt  lt   Not found n

User · Answer

Write your own   template lt class T gt  bool checkElementIsInSet const T amp  elem  const std  set lt T gt  amp  container      return container find elem     container end

[c++] How to check that an element is in a std::set?

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