10.1. Vectors

A vector is a set of values where each value is identified by a number (called an index). A 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. Each of these statements create empty vectors (their size is zero). It is common to specify the size of the vector in parentheses:

vector<int> count (5);

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 a 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 count could be represented in a state diagram:

The numbers inside the shaded boxes are the elements of the vector. The numbers below the boxes are the indices used to identify each box. When you allocate a new vector this way, the elements are “default initialized”. That is, each element is assigned the default value for whatever type of data the vector stores.

There is another constructor for vectors that takes two parameters. The second parameter is a “fill value” – the value that will be assigned to each of the elements.

vector<int> count (5, 3);

This statement creates a vector of five elements and initializes all of them to three.

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 initialized to default values.

• vector<string> words (5, "");

• Partly correct! This works, but there is a simpler way that does not create redunadant strings

• vector<char> words (5, '');

• Incorrect! words should be a vector of strings. A char is not a string.

Q-2: Multiple Response Which of the following could be an element of vector<string> 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)

• These elements do have values

• 0

• Yes, we have 6 default initialized integers.

• 6

• 6 is the size of the vector, not the values.

• 0,1,2,3,4,5

• These are the indices, not the values.

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