将唯一数据推入向量

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(ie 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());

Answer 3

如果您不关心要输入到数据结构中的哪个实例， std::set将满足您的目的

Answer 4

Maybe you should use another map istead of a vector to have unique names.

std::map < std::string, double > name;

Answer 5

list has the ability to .sort() and then .unique(), which will provide you with .

you can iterate over it with an iterator and initialize it with initializer_list.

that data looks actually more like a struct to me:

#include <iterator>
#include <list>
#include <string>
#include <fstream>

typedef struct NODE_S {
    string name1, name2;
    int n;
} NODE_S NODE;

bool compare_NODE (NODE first, NODE second)
{
    unsigned int i=0;
    if (first.name1 < second.name1) {
        return true;
    } else if (first.name2 < second.name2) {
        return true;
    } else if (first.n < second.n) {
        return true;
    } else { return false;}
}


bool readfile(list<NODE>& ln, string filepath) {
    std::ifstream filein;
    NODE n;
    filein.open(filepath.c_str(), std::iofstream::in);
    if (!filein.good()) {
        filein.close();
        std::cerr << "ERROR: unable to open file \"" << filepath << "\" or file is zero-length." << std::endl;
        return false;
    }
    do {
        filein >> n.name1 >> n.name2 >> n.name3 >> std::skipws;
        ln.push_back(n);
        ln.sort(compare_NODE);
        ln.unique();
        //add node to list

    } while (!filein.good()); //can use .eof here, but if bad disk blocks...
    filein.close();
    return true;
}


int main(int argc, char * argv[], char * envp[]) {
    string filepath="somefile.txt";
    if (!readfile(filepath)) {
        return 1;
    }
    list<NODE>::iterator lni;
    for (lni = ln.begin(); lni != ln.end(); lni++) {
        std::cout<<lni->name1<<' '<<lni->name2<<' '<<lni->n<<std::endl;
    }
    return 0;
}

http://www.cplusplus.com/reference/stl/list/sort/

http://www.cplusplus.com/reference/stl/list/unique/