CODEWITHSUNDEEP
c++dictionarystlsetcomposition
Matt
Matt

Reputation: 147

Insert/read a set into a map

I want to 'manage' flights, using map<string, set<string>>. The string keys represents the flight number while the set<string> values represent the people's name registered onto the flight. In my case, I read data from a simple text file such as:

123 jhonny
132 harry
123 bill
145 herry
132 jarry

in order to find the people in the same flight.

I know that a basic way to insert into a map is

map<string, string> m;
m["hi"] = test;

and using the iterator to read the container.

But how can I insert and read the composition of a set into a map?

I tried with a double iterator, or using the while and a iterator to take data from a file:

string pers, volo;
while (wf >> volo >> pers) {
    m[volo] = pers;
}

but it gives error.

I'm new in STL and I've read some files, guide, and others to learn sets and maps, but I haven't found anything about container composition (such as the one I've described). How can I do that? Perhaps with a double iterator on map and on set?

Thank you.

Upvotes: 2

Views: 78

Answers (1)

TrebledJ
TrebledJ

Reputation: 8987

Just treat your m[volo] like any kind of regular set. Your set will be default-constructed once you access its value using std::map::operator[]. This allows you to directly use any of set's member functions. To add a value to the set, use std::set::insert.

Here's what your code might look like using standard input/output:

string a, b;
while (cin >> a >> b) {
    m[a].insert(b);
    cout << m[a].size() << endl;
}

If you want to output your set, a convenient way to do this would be to define an overload over operator<<. The following defines a template any set.

template<typename T>
std::ostream& operator<<(std::ostream& os, const std::set<T>& s)
{
    for (auto& el : s)
        os << el << ' ';
    return os;
}

This allows you to do the following without any errors.

for (auto it = m.begin(); it != m.end(); ++it)
{
    cout << it->first;   // a string
    cout << ' ';
    cout << it->second;   // a set
    cout << endl;
}

Upvotes: 3

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