Vectors

C++ has the standard library (C++ SL) which adds an immense amount of functionality to the language. Let’s look at std::vector. A vector is a container that can hold data of the same type (e.g., double). It differs from an array that you might have used in other languages in that it is not a fixed size—it can grow as needed to hold more data.

Note

The C++ standard library was inspired by an earlier project called the C++ standard template library or STL. These are distinct, and for modern C++ we refer to the C++ standard library.

Note

There are many other types of containers in the C++ SL that have different properties.

Vectors are very useful to store data and loop over it, etc. Information on the properties of C++ vectors can be found at the CPlusPlus vector page. From that description, we see that vectors:

  • store the data contiguously in memory

  • can grow as needed (but occasionally, this means creating a new vector of larger size and copying data into it—a slow operation)

  • have a lot of functions that can work on them

  • can access any location directly

  • are very efficient at adding data to the end, but not efficient for inserting data in the middle

Your text has an example of creating a vector to store student grades and then computing an average. We’ll write our own version.

First lets see how to create a vector and add some data to it.

Here’s a simple example:

Listing 3 simple_vector.cpp
#include <iostream>
#include <vector>

int main() {
    std::vector<double> container;

    container.push_back(10.0);
    container.push_back(-1.0);
    container.push_back(15.3);
    container.push_back(3.14159);

    std::cout << container[3] << std::endl;
    std::cout << "the vector has " << container.size() << " elements" << std::endl;

}

Notice the following:

  • We specify the data type when creating a vector

  • We use push_back to add data to the end of a vector. Here we are using the . operator to indicate that we are performing the push_back on the vector container that we created.

  • We access elements of a vector using [] with an index, and that indices start at 0.

  • We can use the .size() member function to get the number of elements in the vector.

Looping

Often we will want to loop over the elements contained in a vector, and C++ provides a range-based loop for this purpose. For example to loop over all elements and output them to the terminal, we could do:

for (auto e : container) {
    std::cout << e << std::endl;
}

Here we see a new keyword, auto. Each pass through this loop, e will take on the next value in our vector called container. The body of the loop is contained in the {...}.

C++ requires that we declare the data type of e. In this case, the compiler knows that it will have to be a double, since the vector is a vector of double, so we can use the auto keyword to have the compiler automatically deduce the data type.

try it…

Let’s add a loop to our code and compute the average of the elements in the vector.

There is another more classic type of loop in C++ that takes the form:

for (start; end; increment) {
    // do stuff
}

For example, to loop from 0 to 9, we could do:

for (int i = 0; i < 10; ++i) {
   // do stuff depending on i
}

try it…

Try out the loop above, printing out the value of i each iteration.