如何遍历嵌套向量？

Question

I have a rather complex ordered internal Grammar representation of my productions consisting of typed std libarary classes:

class Grammar {
    std::tuple<int, NonTerminal, std::vector<std::vector<Symbol>>> productions;
}

Now I would like to create LR0-Items from this representation.

From inner to outer:

• std::vector<Symbol> represents a rule
• std::vector<std::vector<Symbol>> represents an alternative rule of the production.
• NonTerminal, std::vector<std::vector<Symbol>>> represents the lhs of the production
• std::tuple<int, NonTerminal, std::vector<std::vector<Symbol>>> represents the order of the productions.

Question: How do I iterate through the productions? I am new to c++. As far as I know the way is through an ::iterator . But then, how do I address one nested datatype to build an iterator from it?

Thanks

EDIT:

Instead of std::tuple I think it's yet better to have the definition consist of two times a std::pair :

std::pair<int, std::pair<NonTerminal, std::vector<std::vector<Symbol>>> productions;

I try to adopt the answer of user idclev 463035818

for( const &auto production : productions.back) {
  for( const &auto alternatives : production.back) {
    for( const &auto rule : alternatives) {
     // ...
    }
  }
}

Correct ?

Answer 1

To iterate a vector whose elements are of type T you can use a range based for loop:

std::vector<T> vect;
for (const auto& element : vect) {
    // element is a const reference to elements of vect
    // put code here
}

Now, if T is again a vector, you just replace //put code here with your code to iterate that vector element .

---

Nesting containers is not as complicated as you might expect. It does not require special treatment. If you know how to iterate one vector and get a reference to its elements, then you already do know how to iterate those elements in case they are vectors.

---

So lets say you have a std::vector<std::vector<Symbol>>> (I'll skip the tuple, because I suppose you don't want to "iterate" it, but just pick the 3rd element from it).

std::vector<std::vector<Symbol>>> vect_vect;

for (const auto& vect : vect_vect) {
    // here vect is a const reference to std::vector<Symbol>
    for (const auto& symbol : vect) {
       // here symbol is a const reference to a Symbol
       // put your code here
       // eg assuming Symbol has a member get_bar
       auto foo = symbol.get_bar();
    }
}