How to check is there an element in map values

Question

There is a map with map<string, vector<string>> buses; structure, and I have a string, so I need to find out how to check string contains in map vector.

string stop;
cin >> stop;

if (buses.find(stop) != buses.end()) {
    cout << "Yes";
} else {
    cout << "No";
}

When I get a stop string I need to check is there in map's values (not keys, to be more precise in map.second) a string equals to stop string, if yes I need to print out all the keys containing stop element in values. { {"bus#1", {"stop#1", "stop#2", "stop#3"}}, ... }

Do you mean "can I find out if there's a key in the map with a given string? If so `find` should do it. Can you give a complete example? — tadman, Sep 03 '18 at 20:19
Can you please show the definition (and population) of `buses`? — Stephan Lechner, Sep 03 '18 at 20:19
@tadman no, I mean can I find a value in map's values not keys. — Bootuz, Sep 03 '18 at 20:21
Possible duplicate of [Find mapped value of map](https://stackoverflow.com/questions/4263640/find-mapped-value-of-map). — Max Vollmer, Sep 03 '18 at 20:23
As far as I know there's no built-in method to search values, especially not if the values themselves are a nested container. Can you please re-write this code to be more complete, to show not only the actual data you're working with, but the result you expect? — tadman, Sep 03 '18 at 20:24
@MaxVollmer I need to find among all the values of the map a `stop` element — Bootuz, Sep 03 '18 at 20:25
I am sure his map is `{ {"bus#1", {"stop#1", "stop#2", "stop#3"}}, ... }`, so he need to first build a reverse map or make a lookup in a nested loop. — Nikita Kniazev, Sep 03 '18 at 20:29
Sounds like you need to reverse your key-value pairs and use `find`. — Hatted Rooster, Sep 03 '18 at 20:29
@SombreroChicken, oh yeah, I think that's a solution, I'll try this. — Bootuz, Sep 03 '18 at 20:32

score 1 · Answer 1 · answered Sep 03 '18 at 20:33

One way to do this is to loop over all map elements and then use std::find on the value vectors. This is really inefficient though.

Instead, you should use a std::multimap<std::string, std::string> (or std::unordered_multimap if you don't need the sorting) and use the stops as key. Then add all buses that use said stop in that key and use find if you want to get all buses for that stop.

score -1 · Answer 2 · answered Sep 03 '18 at 20:26

I suppose this is what you are trying to do.

#include <map>
#include <iostream>
#include <vector>
#include <string>

using namespace std;

int main()
{
    map<string, vector<string>> buses;

    buses["some stop"] = vector<string>();

    if (buses.find("some stop") != buses.end()) {
        cout << "Yes";
    }
    else {
        cout << "No";
    }
}

Are you sure map is the data structure you want for this task?

Otherwise you may want to check out std::set if you only want to see if the stop is in the data structure.

How to check is there an element in map values

2 Answers2