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
doubles, and user-defined types like
All elements of a vector must have the same type.
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
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 (
>). The first line creates a vector of integers named
second creates a vector of
doubles. 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
vectors 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.