Vectors

1. Vectors 8.3

2. Basics • Defined in <vector> library • A vector has a base type, just like an array • You can use any type, including class types • Declarations • vector<int> v; • Default constructor • Elements • Indexed starting with 0 • Can access elements using the square bracket notation • v[i] = 42; • cout << “answer is “ << v[i]; • You CANNOT initialize an element using square bracket notation, you can only change an element that has already been given a value

3. New vector syntax • push_back() • Adds an element in the next available position in the vector • size() • Returns the number of elements in a vector (an unsigned int) • vector<int> v(10) • Vector constructor that takes one argument; will initialize the number of positions given as the argument • The declaration above would initialize the first ten elements of v to 0, thus v.size() would return 10

4. Copying/Assigning Vectors • The assignment operator does an element by element assignment • However, the assignment operator on a vector is only as good (or bad) as the assignment operator on its base type

5. Size vs. Capacity • Size – the number of elements in the vector • Capacity – the number of elements for which it currently has memory allocated • When a vector runs out of capacity and needs room for an additional member, capacity is automatically increased • This increase always allows for more space than is needed • Reallocating capacity in large chunks is more efficient than allocating numerous small chunks • If efficiency is an issue, use the member function reserve(int) to explicitly increase the capacity of a vector

6. Member Functions • push_back() • pop_back() • size() • capacity() • reserve() • resize()

7. Class Exercises • Write a program that asks a user to enter a list of positive integers. The user will use a negative number sentinel to indicate he/she is finished. Echo the user’s input. • Write a sorting function that has an argument for a vector of ints. Use the selection sort algorithm.