How to access elements in a list in c++

I am working through problems in Accelerated C++ We have a program which stores strings in vectors and outputs them according to certain rules. The original code fragment is as follows.

typedef vector<string> Rule;
typedef vector<Rule> Rule_collection;
typedef map<string, Rule_collection> Grammar;


// read a grammar from a given input stream
Grammar read_grammar(istream& in)
{
    Grammar ret;
    string line;

    // read the input
    while (getline(in, line)) {

        // `split' the input into words
        vector<string> entry = split(line);开发者_C百科

        if (!entry.empty())
            // use the category to store the associated rule
            ret[entry[0]].push_back(
                Rule(entry.begin() + 1, entry.end()));
    }
    return ret;
}

We are then asked to use lists instead of vectors.

So all the vectors above will now become lists.

The bit I am stuck on is how to deal with the line beginning ret[entry]].push_back etc. Also what to put in after Rule... I have tried using entry.begin() but that doesn't work.

Any assistance would be appreciated.

---

with list you can perform most of the operations you can with vector but you cannot use entry[0] as there is no random-access.

entry.front() will get that element instead.

Also you cannot add 1 to entry.begin() because it is bi-directional and does not support +, but you can assign it to a temporary iterator then ++it.

---

entry.begin ()

returns an iterator to the start of the list, whereas

entry [0]

for vector returns the first element, so I think you want something like the following

*(entry.begin ())

EDIT:

Hopefully I understood what you are asking... Other than that begin and end should return comparable values for both vector and list

---

You will need to replace your code with something like this:

ret[*(entry.begin())].push_back(
            Rule(++(entry.begin()), entry.end()));

---

begin() gives you an iterator to the beginning of the list. You could dereference the iterator, which gives you the first value...

ret[*(entry.begin())].push_back(...

Or you could use the front() function, which returns the element at the front of a list (technically a reference, but should work in this case).

ret[entry.front()].push_back(...

EDIT:

As others have pointed out, this works provided entry is a std::list<string>, as you implied.

---

ret[entry[0]] calls Grammar::operator[] passing it a string: the string contained in entry[0], which is the first string put in such vector by split.

Supposing that split returns a std::list, you should just take the first element of the list:

ret[ entry.front() ] ... // where entry is a std::list<string>

---

This code is pretty generic- it should just work when you change the typedefs, except a couple of minor niggles like std::list does not give random-access iterators, and since you don't show the code I can't speak about split.