Pushing unique data into vector

Question

I have the following data:

FolioA Name1 100 FolioA Name2 110 FolioA Name3 100 FolioB Name1 100 FolioB Name3 106 FolioC Name1 108 FolioC Name2 102 FolioC Name3 110 

I want to only insert unique names(i.e. Name1, Name2 and Name3, each once) into

std::vector<std::string> name; 

as I iterate through the data.

So, I have the following code where I have stored the data in a map called test:

std::map<std::string, std::map<std::string, double> >test; std::map<std::string, std::map<std::string, double > >::iterator it1 = test.begin(), end1 = test.end();     while (it1 !=end1) {         std::map<std::string, double>::iterator it2 = it1->second.begin(), end2=it1->second.end();         **name.push_back(it2->first);**         ++it2;     }     ++it1; } 

But, currently by pushing the data into name the way I am has 3 instances of Name1, 2 of Name2, and 3 of Name3, which is expected from my code. How do I fix it to only have unique names.

Answer 1

Since you want to keep the first instance for a given name, you will have to perform a name lookup at some point. A simple algorithm involving only your vector would be to can check if the the entry already exists using std::find

std::vector<std::string> name;  .... if (std::find(name.begin(), name.end(), someName) == name.end()) {   // someName not in name, add it   name.push_back(someName); } 

But here you are performing a search each time you want to insert an element, and this (by itself) is up to O(N) complexity, giving O(N*N) for the whole algorithm. So you could optimize by using an intermediary container with fast look up, such as an std::set as suggested by @Chad and which has O(logN) complexity for look-up, giving O(N*logN) overall, or a hash container such as C++11's std::unordered_set, which has close to constant time look-up, giving ~O(N) overall complexity.

#include <unordered_set>  std::unordered_set<std::string> name_set; ....  // still need to search, since you want to keep  // the first instance of each name, and not the last. // But unordered_set performs the look-up at insertion, // only inserting if someName not already in the set name_set.insert(someName); 

and then, following @Chad's example,

std::vector<std::string> name(names_set.begin(), name_set.end()); 

If you don't have C++11, hash map alternatives are boost::hash_map and tr1::hash_map.

Answer 2

You asked for sample code, so here's how I would have done it:

std::set<std::string> unique_names;  // ... while (it1 !=end1) {     // ...     // **name.push_back(it2->first);**     unique_names.insert(it2->first); }  std::vector<std::string> name(unique_names.begin(), unique_names.end());