More Vectors

More Vectors#

Let’s look at some more ways we can work with vectors.

Initializing#

We can initialize a vector when we declare it. The following creates a vector with 5 elements, all initialized to 0:

std::vector<double> container(5, 0.0);

Notice that we are using () here instead of {}. As we’ll see later, this means that we are calling a function here to do the initialization (the constructor).

Here we instead initialize a vector by telling it the values of each of the elements:

std::vector<double> container2{10.0, 20.0, 30.0};

Size#

As we saw earlier, we can always get the number of elements in a vector via the size() function:

std::vector<int> int_vec{1, 2, 3, 4, 5};

int nlen = int_vec.size();

Note

size() technically returns a value of type std::size_t, and here we implicitly cast it to an int. We learn more about casting later.

try it…

We saw that we access an element via []. What happens if we access the vector out of bounds?

Bounds, iterators, and pointers#

There are 2 ways to access the beginning and end of a vector:

  • .cbegin(), .begin() : these will give you an iterator that points to the first element. The difference is that accessing with .cbegin() will not allow you to modify the vector (the c is for const). You can increment an iterator loop over the contents of the vector.

  • .cend(), .end() : these will return an iterator that points to one past the last element.

An iterator can be thought of as a special type of pointer—a topic that we will discuss much more later. Iterators have restrictions on their use, depending on the container—this makes them more safe to use.

If we think about an iterator like:

auto it = container.cbegin();

Then we can access the next element in container by incrementing the iterator, it++.

If we want to see the value in container that the iterator is pointing to, then we need to dereference it—this is done with the * operator:

std::cout << "cbegin is " << *it << std::endl;

Here’s an example of looping over an entire vector using iterators:

Listing 19 iterator_loop.cpp#
#include <iostream>
#include <vector>

int main() {

    std::vector<int> container{0, 1, 1, 2, 3, 5, 8, 13, 21};

    for (auto it = container.cbegin(); it < container.cend(); ++it) {
        std::cout << *it << std::endl;
    }

}

Reverse iterators#

We can use std::rbegin() / std::rend() to iterate through a container in reverse.

Listing 20 vector_reverse.cpp#
#include <iostream>
#include <vector>

int main() {

    std::vector<double> v{1.0, 2.0, 4.0, 8.0, 16.0, 32.0};

    for (auto it = v.rbegin(); it < v.rend(); ++it) {
        std::cout << *it << " ";
    }
    std::cout << std::endl;

}
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