Check if vector contains object with already entered values

so I'm struggling with these things:

I have method that returns istream input and takes istream input as a parameter, sends values to vector and stores them in it. Now, when I've entered 1 value, I'm trying to make a check if vector already contains that value, here is my code to understand it better:

struct Predmet {
    string naziv;
    string odsjek;
    istream& dodaj_predmet(istream &);
};

struct Student {
    string brojIndeksa;
    string ime;
    string prezime;
    map<std::string, int> ocjene;
    istream& dodaj_studenta(istream &);
};

vector<Student> studenti;
vector<Predmet> predmeti;
Student s;
Predmet p;

istream& Student::dodaj_studenta(istream & input){
    cout << "### Unesite podatke o studentu: " << endl;
    cout << endl;
    cout << "Unesite broj indeksa studenta: " << endl;
    getline(input, brojIndeksa);
    cout << endl;
    cout << "Unesite ime studenta: " << endl;
    getline(input, ime);
    cout << endl;
    cout << "Unesite prezime studenta: " << endl;
    getline(input, prezime);
    cout << endl;
    cout << "***     Uspjesno ste unijeli studenta     ***" << endl;
    return input;
}

So I'm trying to make a check if value entered for structure member brojIndeksa already exists in vector, and I'm not sure how to do it.

Also how can I add values to map, because I need to add pair<string, int>, and not sure how, because I need to check if STRING is a member of predmeti vector?

Thanks.