How to find out if an item is present in a std vector

Question

All I want to do is to check whether an element exists in the vector or not  so I can deal with each case   if   item present      do this    else    do that

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Use the STL find function   Keep in mind that there is also a find if function  which you can use if your search is more complex  i e  if you re not just looking for an element  but  for example  want see if there is an element that fulfills a certain condition  for example  a string that starts with  abc    find if would give you an iterator that points to the first such element

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You can try this code    include  lt algorithm gt   include  lt vector gt      You can use class  struct or primitive data type for Item struct Item         Some fields    typedef std  vector lt Item gt  ItemVector  typedef ItemVector  iterator ItemIterator        ItemVector vtItem         init data for vtItem  Item itemToFind         ItemIterator itemItr  itemItr   std  find vtItem begin    vtItem end    itemToFind   if  itemItr    vtItem end             Item found        doThis     else          Item not found        doThat

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template  lt typename T gt  bool IsInVector const T  amp  what  const std  vector lt T gt   amp  vec        return std  find vec begin   vec end   what   vec end

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You can use count too  It will return the number of items present in a vector   int t count vec begin   vec end   item

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I ve personally used templates of late to handle multiple types of containers at once rather than deal only with vectors  I found a similar example online  can t remember where  so credit goes to whoever I ve pilfered this from  This particular pattern seems to handle raw arrays as well   template  lt typename Container  typename T   typename std  decay lt decltype  std  begin std  declval lt Container gt      gt   type gt  bool contains Container  amp  amp  c  T v        return std  find std  begin c   std  end c   v     std  end c

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I use something like this      include  lt algorithm gt    template  lt typename T gt   const bool Contains  std  vector lt T gt  amp  Vec  const T amp  Element          if  std  find Vec begin    Vec end    Element     Vec end            return true       return false     if  Contains vector item      blah else    blah      as that way it s actually clear and readable    Obviously you can reuse the template in multiple places

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Using Newton C   it s easier  self-documented and faster than with std  find because of return a bool directly   bool exists linear  INPUT ITERATOR first  INPUT ITERATOR last  const T amp  value    bool exists binary  INPUT ITERATOR first  INPUT ITERATOR last  const T amp  value     I think it s obvious what the functions do   include  lt newton algorithm algorithm hpp gt   if   newton  exists linear first  last  value       do this    else    do that

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Here s a function that will work for any Container   template  lt class Container gt   const bool contains const Container amp  container  const typename Container  value type amp  element         return std  find container begin    container end    element     container end        Note that you can get away with 1 template parameter because you can extract the value type from the Container  You need the typename because Container  value type is a dependent name

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Bear in mind that  if you re going to be doing a lot of lookups  there are STL containers that are better for that   I don t know what your application is  but associative containers like std  map may be worth considering   std  vector is the container of choice unless you have a reason for another  and lookups by value can be such a reason

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In C  11 you can use any of  For example if it is a vector lt string gt  v  then   if  any of v begin    v end    bind equal to lt string gt      1  item       do this    else    do that      Alternatively  use a lambda   if  any of v begin    v end      amp   const std  string amp  elem    return elem    item         do this    else    do that

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As others have said  use the STL find or find if functions  But if you are searching in very large vectors and this impacts performance  you may want to sort your vector and then use the binary search  lower bound  or upper bound algorithms

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You can use the find function  found in the std namespace  ie std  find  You pass the std  find function the begin and end iterator from the vector you want to search  along with the element you re looking for and compare the resulting iterator to the end of the vector to see if they match or not   std  find vector begin    vector end    item     vector end     You re also able to dereference that iterator and use it as normal  like any other iterator

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You can use std  find from  lt algorithm gt     include  lt vector gt  vector lt int gt  vec     can have other data types instead of int but must same datatype as item  std  find vec begin    vec end    item     vec end     This returns a bool  true if present  false otherwise    With your example    include  lt algorithm gt   include  lt vector gt   if   std  find vec begin    vec end    item     vec end        do this    else    do that

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C  17 and above    can use std  search also  This is also useful for searching sequence of elements    include  lt algorithm gt   include  lt iostream gt   include  lt vector gt   template  lt typename Container gt  bool search vector const Container amp  vec  const Container amp  searchvec        return std  search vec begin    vec end    searchvec begin    searchvec end       vec end       int main          std  vector lt int gt  v    2 4 6 8           THIS WORKS  SEARCHING ONLY ONE ELEMENT       std  vector lt int gt  searchVector1    2        if search vector v searchVector1            std  cout lt  lt  searchVector1 found  lt  lt std  endl       else          std  cout lt  lt  searchVector1 not found  lt  lt std  endl          THIS WORKS  AS THE ELEMENTS ARE SEQUENTIAL       std  vector lt int gt  searchVector2    6 8        if search vector v searchVector2            std  cout lt  lt  searchVector2 found  lt  lt std  endl       else          std  cout lt  lt  searchVector2 not found  lt  lt std  endl          THIS WILL NOT WORK  AS THE ELEMENTS ARE NOT SEQUENTIAL       std  vector lt int gt  searchVector3    8 6        if search vector v searchVector3            std  cout lt  lt  searchVector3 found  lt  lt std  endl       else          std  cout lt  lt  searchVector3 not found  lt  lt std  endl      Also there is flexibility of passing some search algorithms  Refer here   https   en cppreference com w cpp algorithm search

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With boost you can use any of equal    include  lt boost algorithm cxx11 any of hpp gt   bool item present   boost  algorithm  any of equal vector  element

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If you wanna find a string in a vector       struct isEqual       isEqual const std  string amp  s   m s s              bool operator   OIDV  l                return l- gt oid    m s             std  string m s     struct OIDV       string oid    else    VecOidv  iterator itFind find if vecOidv begin   vecOidv end   isEqual szTmp

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Use find from the algorithm header of stl I ve illustrated its use with int type  You can use any type you like as long as you can compare for equality  overload    if you need to for your custom class     include  lt algorithm gt   include  lt vector gt   using namespace std  int main            typedef vector lt int gt  IntContainer      typedef IntContainer  iterator IntIterator       IntContainer vw                     find 5     IntIterator i   find vw begin    vw end    5        if  i    vw end                 found it       else              doesn t exist            return 0

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If your vector is not ordered  use the approach MSN suggested   if std  find vector begin    vector end    item   vector end              Found the item     If your vector is ordered  use binary search method Brian Neal suggested   if binary search vector begin    vector end    item            Found the item     binary search yields O log n  worst-case performance  which is way more efficient than the first approach  In order to use binary search  you may use qsort to sort the vector first to guarantee it is ordered

[c++] How to find out if an item is present in a std::vector?

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