Determine if map contains a value for a key

Question

What is the best way to determine if a STL map contains a value for a given key    include  lt map gt   using namespace std   struct Bar       int i      int main         map lt int  Bar gt  m      Bar b    0       Bar b1    1        m 0    b      m 1    b1         Bar b2   m 2       map lt int  Bar gt   iterator iter   m find 2       Bar b3   iter- gt second       Examining this in a debugger  it looks like iter is just garbage data    If I uncomment out this line   Bar b2   m 2    The debugger shows that b2 is  i   0    I m guessing it means that using an undefined index will return a struct with all empty uninitialized values    Neither of these methods is so great  What I d really like is an interface like this   bool getValue int key  Bar amp  out        if  map contains value for key                out   map key           return true            return false      Does something along these lines exist

User · Answer

If you want to determine whether a key is there in map or not  you can use the find   or count   member function of map  The find function which is used here in example returns the iterator to element or map  end otherwise  In case of count the count returns 1 if found  else it returns zero or otherwise    if phone count key       key found   else    key not found    for int i 0 i lt v size   i         phoneMap  iterator itr phone find v i     I have used a vector in this example to check through map you cal receive a value using at   e g  map at key       if itr  phone end            cout lt  lt v i  lt  lt     lt  lt itr- gt second lt  lt endl      else         cout lt  lt  Not found  lt  lt endl

User · Answer

Does something along these lines exist    No  With the stl map class  you use   find   to search the map  and compare the returned iterator to std  map  end    so  map lt int Bar gt   iterator it   m find  2    Bar b3  if it    m end           element found     b3   it- gt second      Obviously you can write your own getValue   routine if you want  also in C    there is no reason to use out   but I would suspect that once you get the hang of using std  map  find   you won t want to waste your time   Also your code is slightly wrong   m find  2    will search the map for a keyvalue that is  2   IIRC the C   compiler will implicitly convert  2  to an int  which results in the numeric value for the ASCII code for  2  which is not what you want   Since your keytype in this example is int you want to search like this  m find 2

User · Answer

You can create your getValue function with the following code   bool getValue const std  map lt int  Bar gt  amp  input  int key  Bar amp  out       std  map lt int  Bar gt   iterator foundIter   input find key      if  foundIter    input end               out   foundIter- gt second        return true          return false

User · Answer

It already exists with find only not in that exact syntax   if  m find 2     m end              key 2 doesn t exist     If you want to access the value if it exists  you can do   map lt int  Bar gt   iterator iter   m find 2   if  iter    m end              key 2 exists  do something with iter- gt second  the value      With C  0x and auto  the syntax is simpler   auto iter   m find 2   if  iter    m end              key 2 exists  do something with iter- gt second  the value      I recommend you get used to it rather than trying to come up with a new mechanism to simplify it  You might be able to cut down a little bit of code  but consider the cost of doing that  Now you ve introduced a new function that people familiar with C   won t be able to recognize   If you want to implement this anyway in spite of these warnings  then   template  lt class Key  class Value  class Comparator  class Alloc gt  bool getValue const std  map lt Key  Value  Comparator  Alloc gt  amp  my map  int key  Value amp  out        typename std  map lt Key  Value  Comparator  Alloc gt   const iterator it   my map find key       if  it    my map end                   out   it- gt second          return true            return false

User · Answer

I just noticed that with C  20  we will have  bool std  map  contains  const Key amp  key   const    That will return true if map holds an element with key key

User · Answer

amap find returns amap  end when it does not find what you re looking for -- you re supposed to check for that

User · Answer

As long as the map is not a multimap  one of the most elegant ways would be to use the count method  if  m count key          key exists   The count would be 1 if the element is indeed present in the map

User · Answer

Check the return value of find against end   map lt int  Bar gt   iterator it   m find  2    if   m end      it           contains

User · Answer

To succinctly summarize some of the other answers   If you re not using C   20 yet  you can write your own mapContainsKey function   bool mapContainsKey std  map lt int  int gt  amp  map  int key      if  map find key     map end    return false    return true      If you d like to avoid many overloads for map vs unordered map and different key and value types  you can make this a template function   If you re using C   20 or later  there will be a built-in contains function   std  map lt int  int gt  myMap      do stuff with myMap here  int key   123   if  myMap contains key          stuff here

User · Answer

Boost multindex can be used for proper solution  Following solution is not a very best option but might be useful in few cases where user is assigning default value like 0 or NULL at initialization and want to check if value has been modified   Ex   lt  int   string  gt   lt  string   int  gt    lt  string   string  gt    consider  lt  string   string  gt  mymap  1st    first   mymap  second       for  std  map lt string string gt   iterator it mymap begin    it  mymap end      it           if   it- gt second                     continue

[c++] Determine if map contains a value for a key?

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