Question

Tying to list all the courses the teacher teaches in c++ map but only the first one is being shown, not the rest of them. please help.

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#include<iostream>

#include<map>

using namespace std;

int main(){

                map <string,string> instructors;

                //mapping course numbers and instructor names

                instructors.insert(pair<string,string>("Haynes","CS101"));
              
                instructors.insert(pair<string,string>("Haynes","CS102"));
              
                instructors.insert(pair<string,string>("Haynes","CS103"));

                instructors.insert(pair<string,string>("Alvarado","CS201"));
              
                instructors.insert(pair<string,string>("Alvarado","CS202"));
              
                instructors.insert(pair<string,string>("Alvarado","CS203"));

              
                char choice='y';

                //looping until user wishes to quit

                while(choice=='y' || choice=='Y'){

                                cout<<"Enter a instructor name: ";

                                string instructor;

                                cin>>instructor;

                                //searching in maps for the required course number, this will return

                                //an iterator

                                map<string, string>::iterator it1=instructors.find(instructor);

                                if(it1!=instructors.end()){

                                                cout<<"Courses: "<<it1->second<<endl;

                                }

             

                              

                                //prompting again

                                cout<<"\nDo you want to search again? (y/n): ";

                                cin>>choice;

                }

}

Answer 1

The code you have written will serve the purpose as keys are unique in a map. You are trying to insert multiple values having the same keys. The insert function fails to insert the key-value pair if the key to be inserted is already present in the map. Thus, you are only getting the first value only.

instructors.insert(pair<string, string>("Haynes", "CS101")); // Insertion successful instructors.insert(pair<string, string>("Haynes", "CS102")); // Insertion failed as key "Haynes" is already present instructors.insert(pair<string, string>("Haynes", "CS103")); // Insertion failed as key "Haynes" is already present instructors.insert(pair<string, string>("Alvarado", "CS201")); // Insertion successful instructors.insert(pair<string, string>("Alvarado","CS202")); // Insertion failed as key "Alvarado" is already present instructors.insert(pair<string, string>("Alvarado","CS203")); // Insertion failed as key "Alvarado" is already present

To support multiple key-value pairs with the same key, you could either use STL's multimap or create an STL map with pair<string, vector<string>>.

An implementation using STL's multimap:

#include <iostream>
#include <map>

using namespace std;

int main() {
    multimap<string, string> instructors;

    // Mapping course numbers and instructor names
    instructors.insert(pair<string, string>("Haynes", "CS101"));
    instructors.insert(pair<string, string>("Haynes", "CS102"));
    instructors.insert(pair<string, string>("Haynes", "CS103"));
    instructors.insert(pair<string, string>("Alvarado", "CS201"));
    instructors.insert(pair<string, string>("Alvarado", "CS202"));
    instructors.insert(pair<string, string>("Alvarado", "CS203"));

    char choice = 'y';
    // Looping until user wishes to quit
    while (choice == 'y' || choice == 'Y') {
        cout << "Enter a instructor name: ";
        string instructor;
        cin >> instructor;

        // Searching for courses of a particular instructor
        cout << "Courses: ";
        for (auto itr = instructors.begin(); itr != instructors.end(); itr++) {
            if (itr->first == instructor)
                cout << itr->second << " ";
        }

        // Prompting again
        cout << "\nDo you want to search again? (y/n): ";
        cin >> choice;
    }
    return 0;
}

Sample output:

shubh@ACER-NITRO MINGW64 ~/OneDrive/Documents/Chegging $ ./main Enter a instructor name: Haynes Courses: CS101 CS102 CS103 Do you want to search again? (y/n): y Enter a instructor name: Alvarado Courses: CS201 CS202 CS203 Do you want to search again? (y/n): n

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