how to iterate through a set of sets C++ - c++

Question

Pretty new to C++, only at it a week or so, I want to iterate through a set of nested sets and write each element in the inner set to a line in a file.
Each inner set has 3 elements and I want all three elements on the same line.
I have a set up as follows:
// Define "bigSet" and initiate as empty set "Triplets"
typedef set < set<string> > bigSet;
bigSet Triplets;
I tried something of this sort to go through it but it gives me an error...
// Iterate through and print output
set <string>::iterator it;
for(it = Triplets.begin(); it != Triplets.end(); it++){
cout << *it << endl;
}
Any help is greatly appreciated guys thank you!

Answer 1

I would do it this way:
// Iterate through and print output
set < set <string> >::iterator it_ex; // iterator for the "outer" structure
set <string>::iterator it_in; // iterator for the "inner" structure
for(it_ex = Triplets.begin(); it_ex != Triplets.end(); it_ex++)
{
for(it_in = it_ex->begin(); it_in != it_ex->end(); it_in++)
cout << *it_in << ", ";
cout << endl;
}

Answer 2

Triplets is not a set<string>; it is a set<set<string>>; each item in Triplets is itself a set, than can contain several strings.
The iterator must match the type of the container; with two levels of nested containers, you should iterate twice:
set<set<string>>::iterator it;
set<string>::iterator it2;
for(it = Triplets.begin(); it != Triplets.end(); it++) {
for (it2 = it->begin(); it2 != it->end(); ++it2) {
cout << *it2 << endl;
}
}

Answer 3

Triplets is type set < set<string> > and therefore requires an iterator of type set < set<string> >::iterator or bigSet::iterator. It isn't type set <string>. You could also use const_iterator.
Note that iterating Triplets gives you an iterator to another set, and not a string.
Also consider
for (const auto& i : Triplets)
{
for (const auto& j : i)
{
cout << j << endl;
}
}

Answer 4

You have an error because Triplets.begin() is not of type set<string>::iterator, it's set<set<string>>::iterator.
What you need to do is have two loops: one for iterating over the outer set and one for the inner.
set<set<string>>::iterator it;
for(it = Triplets.begin(); it != Triplets.end(); ++it)
{
set<string>::iterator it2;
for(it2 = it->begin(); it2 != it->end(); ++it2)
{
cout << *it2;
}
cout << endl;
}
If you use increment/decrement operators (++/--) on iterators, it might be better to use the prefix versions (++it) instead of the suffix ones (it++). This is because the suffix ones create a copy of the iterator before it is incremented (and that copy is then returned) but in cases like this, you have no need for it.
Moreover, if you're using C++11, you can use the range-based for loops and auto keyword, which simplify things a lot:
for(const auto &innerSet : Triplets)
{
for(const auto &innerSetElement : innerSet)
{
cout << innerSetElement;
}
cout << endl;
}

Answer 5

First: if they're triplets, are you sure that std::set is the type you
want for the inner values. Perhaps a class would be more
appropriate, in which case, you define an operator<< for the `class,
and your simple loop works perfectly. Something like:
class Triplet
{
std::string x;
std::string y;
std::string z;
public:
// Constructors to enforce that none of the entries are identical...
// Accessors, etc.
friend std::ostream& operator<<( std::ostream& dest, Triplet )
{
dest << x << ", " << y << ", " << z;
return dest;
}
};
And then to output:
for ( Triplet const& elem : Triplets ) {
std::cout << elem << std::endl;
}
Otherwise: you need to define the format you want for the output. In
particular, you'll probably want a separator between the strings in the
line, for example. Which means you probably cannot use a range based
for, at least not for the inner loop. You would need something like:
for ( std::set<std::string> const& triplet : Triplets ) {
for ( auto it = triplet.cbegin(); it != triplet.cend(); ++it ) {
if ( it != triplet.cebegin() ) {
std::cout << ", ";
}
std::cout << *it;
}
std::cout << std::endl;
}
(If the set of triplets is large, you'll definitely want to consider
replacing std::endl with '\n'. But of course, if it is really
large, you probably won't be outputting to std::cout.)