How to navigate through a vector using iterators? (C++)

Question

The goal is to access the "nth" element of a vector of strings instead of the [] operator or the "at" method. From what I understand, iterators can be used to navigate through containers, but I've never used iterators before, and what I'm reading is confusing.

If anyone could give me some information on how to achieve this, I would appreciate it. Thank you.

Answer 1

Typically, iterators are used to access elements of a container in linear fashion; however, with "random access iterators", it is possible to access any element in the same fashion as operator[].

To access arbitrary elements in a vector vec, you can use the following:

1st: vec.begin()
2nd: vec.begin()+1
3rd: vec.begin()+2
4th: vec.begin()+3
...
ith: vec.begin()+(i-1)
...
last: vec.begin()+(vec.size()-1)

The following is an example of a typical access pattern using an iterator.

int sum = 0;
for (std::vector<int>::const_iterator it = vec.begin(); it!=vec.end(); ++it) {
    sum += *it;
}

The advantage of using iterator is that you can apply the same pattern with other containers:

int sum = 0;
for (std::list<int>::const_iterator it = lst.begin(); it!=lst.end(); ++it) {
    sum += *it;
}

For this reason, it is really easy to create template code that will work the same regardless of whether the container is a vector, a list, or some other container. Another advantage of iterators is that it doesn't assume the data is resident in memory; for example, one could create a forward iterator that can read data from an input stream, or that simply generates data on the fly (e.g. a range or random number generator).

Answer 2

You need to make use of the begin and end method of the vector class, which return the iterator referring to the first and the last element respectively.

    vector<string> myvector; // a vector of stings.

    // push some stirngs in the vector.
    myvector.push_back("a");
    myvector.push_back("b");
    myvector.push_back("c");
    myvector.push_back("d");


    vector<string>::iterator it; // declare an iterator to a vector of strings
    int n = 3; // nth element to be found.
    int i = 0; // counter.

    // now start at from the beginning
    // and keep iterating over the element till you find
    // nth element...or reach the end of vector.
    for(it=myvector.begin() ; it < myvector.end(); it++,i++ ) {

            // found nth element..print and break.
            if(i == n) {
                    cout<< *it; // prints d.
                    break;
            }
    }

    // other easier ways of doing the same.
    // using operator[]
    cout<<myvector[n]<<endl; // prints d.

    // using the at method
    cout<<myvector.at(n)<<endl; // prints d.

Answer 3

Vector's iterators are random access iterators which means they look and feel like plain pointers.

You can access the nth element by adding n to the iterator returned from the container's begin() method, or you can use operator [].

std::vector<int> vec(10);
std::Vector<int>::iterator it = vec.begin();

int sixth = *(it + 5);
int third = *(2 + it);
int second = it[1];

Alternatively you can use the advance function which works with all kinds of iterators. (You'd have to consider whether you really want to perform "random access" with non-random-access iterators, since that might be an expensive thing to do.)

std::vector<int> vec(10);
std::vector<int>::iterator it = vec.begin();

std::advance(it, 5);
int sixth = *it;