# 10.1. Vectors¶

A vector is a set of values where each value is identified by a number (called an index). An `string` is similar to a vector, since it is made up of an indexed set of characters. The nice thing about vectors is that they can be made up of any type of element, including basic types like `int`s and `double`s, and user-defined types like `Point` and `Time`.

Note

All elements of a vector must have the same type.

The `vector` type is defined in the C++ Standard Template Library (STL). In order to use it, you have to include the header file `vector`; again, the details of how to do that depend on your programming environment.

You can create a vector the same way you create other variable types:

```vector<int> count;
vector<double> doubleVector;
```

The type that makes up the vector appears in angle brackets (`<` and `>`). The first line creates a vector of integers named `count`; the second creates a vector of `double`s. Although these statements are legal, they are not very useful because they create vectors that have no elements (their size is zero). It is more common to specify the size of the vector in parentheses:

```vector<int> count (4);
```

The syntax here is a little odd; it looks like a combination of a variable declarations and a function call. In fact, that’s exactly what it is. The function we are invoking is an `vector` constructor. A constructor is a special function that creates new objects and initializes their instance variables. In this case, the constructor takes a single argument, which is the size of the new vector.

The following figure shows how vectors are represented in state diagrams:

The large numbers inside the boxes are the elements of the vector. The small numbers outside the boxes are the indices used to identify each box. When you allocate a new vector, the elements are not initialized. They could contain any values.

There is another constructor for `vector`s that takes two parameters; the second is a “fill value,” the value that will be assigned to each of the elements.

```vector<int> count (4, 0);
```

This statement creates a vector of four elements and initializes all of them to zero.