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 ints and doubles, 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 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.

Q-1: How would you create a vector of five words and initialize all of them to empty strings?

• vector<string> words ("", 5);

• Incorrect! Vector parameters are in the wrong order.

• vector<string> words (5);

• Correct! Vector elements are default constructed to empty strings.

• vector<string> words (5, "");

• Correct! We made a vector of strings with 5 elements, initialized to empty strings.

• vector<char> words (5, '');

• Incorrect! words should be a vector of strings.

Q-2: Multiple Response Which of the following could be an element of words?

• 1
• Incorrect! This is an integer, not a string.
• "a"
• Correct!
• 'a'
• Incorrect! This is a character, not a string.
• "word"
• Correct!
• "1"
• Correct!

Q-3: What do you call a function that creates an instance of a new object and initializes its instance variables?

• initializer
• Incorrect! Go back and read to find the answer!
• constructor
• Correct!
• creator
• Incorrect! Go back and read to find the answer!
• instance function
• Incorrect! Go back and read to find the answer!

Q-4: What are the values of number’s elements after this declaration?

vector<int> numbers(6);

• undefined (we don’t know the values)

• Integers are default constructed to a known value.

• 0

• Integers are default constructed to a zero value.

• 6

• 6 is the size we want the vector to be.

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