CODEWITHSUNDEEP
c++liststructunique
Lily
Lily

Reputation: 846

Unique list of structure

I have a list of the following structure 

struct Element
{
   int id;
   int groupID;

  };

I want to know how many unique groups there are

for example 

 list<Element> myElements;
  Element e;
  e.id = 0;
  e.groupID = 2;
  myElements.push_back(e);

  e.id = 1;
  e.groupID = 0;
  myElements.push_back(e);

  e.id = 2;
  e.groupID = 2;
  myElements.push_back(e);

  e.id = 3;
  e.groupID = 1;
  myElements.push_back(e);

There are 4 elements here but only 3 unique group ids 0, 1, 2

I'm trying to look for an efficient way to do that because my list grows in size.

I tried this 

struct groupID_unique {
  bool operator() (Element first, Element second)
    { return (first.groupID != second.groupID); }
     };

    myElements.unique(groupID_unique());

But this returns to me the 2 un-repetitive ids 0,1

Upvotes: 0

Views: 1039

Answers (1)

Arun
Arun

Reputation: 20383

Use a set to store the items (temporarily). A set will store only the unique items. The size of the set is the number of unique items.

Add an equality comparator for the object:

struct Element {
  bool operator==(Element const& rhs) const {
    return id == rhs.id && groupId == rhs.groupId;
  }
};

Use a set.

std::set<Element> elementSet{elementList.begin(), elementList.end()};
size_t const numUniqueElements = elementSet.size();

Note that there is some cost (time and space complexity) in constructing the set. If you want to keep the repetitions for some other purpose, then you can continue with the list otherwise you can switch from the list and set so that you always have the unique count readily available (through .size())

Upvotes: 2

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