1. Vectors
the better arrays

2. Why Vectors
vectors are implemented as a class (a template to be exact)
they serve the same purpose as arrays but using object oriented mechanisms (such as operator overloading) making them more powerful
vectors can
grow and shrink as needed
be compared and assigned as a whole
be passed by reference/value/returned
vector size is always available
still more …
part of the so called Standard Template Library (STL)
once you have learned vectors, you do not want to use (raw) arrays anymore.

3. Declaration include <vector> header file using std::vector;
to declare
vector<type_parameter> vectorName;
where
type_parameter – type/class of vector elements
corresponds to base type of the array
examples
vector<int> items; // declares vector with no elements
vector<double> nums(5); // declares vector with 5 elements
vector<myclass> things(5); // declares a vector of 5 objects of myclass

4. Typical Array Operations
indexing: items[3]
still cannot refer past vector size: items[6]++; // range error
checking size: items.size()
careful with this one: returns value of type size_t
example:
for(size_t i=0; i < items.size(); ++i)
cout << items[i];

5. Atypical Array Operations
adding element: items.push_back(55);
removing element: items.pop_back();
checking if empty: items.empty();
accessing first/last element: items.front(); items.back();
changing size: items.resize(newSize);
emptying: items.erase();
example:
int num=0;
cin >> num;
while(num !=0){
items.push_back(num); }
while(!items.empty()){
cout << items.back();
items.pop_back();

6. With Functions, Other Aggregate Operations
can pass by value/reference, return
void myfunc(vector<int>);
void myfunc(vector<int>&);
vector<int> myfunc(void);
can assign
vector<int> v;
v = items;
can swap: items.swap(v);
can compare: if(v == items) … 6

7. With Objects class intData{ // elementary class definition public:
void set(int n){data=n;}; // simple setter
int get(void) const{return data;}; // simple accessor
private:
int data; }; …
vector<intData> v(5); // declaring a vector of objects
for(int i=0; i<5; i++){ // using vector as array
int num;
cin >> num;
v[i].set(num); // invoking a method on vector element }

8. Iterators vector<type_parameter>::iterator iteratorName;
iterator is a generalization of pointer
not a pointer but usually implemented using pointers
further enhances the power of vector
declaring iterator
vector<type_parameter>::iterator iteratorName;
where
type_parameter – type/class of elements
example
vector<int>::iterator p;

9. Iterators Operations operations
++ (pre- and postfix) and -- to advance to the next (previous) data item
advance certain number of positions forward/backward
== and != operators to test whether two iterators point to the same data item
< > to compare if two elements are before and after in the vector
dereferencing (*) provides data item access
functions
v.begin() returns an iterator pointing to the first element of container v
v.end() returns an iterator pointing past the last element of container v useful to compare for end of iteration
inserting: v.insert(ip, 22); // inserts one element at // iterator ip position
what does this do? v.insert(v.end()-2, 55);
erasing: v.erase(ip); // erases (deletes) one element at // iterator ip position
sorting elements: sort(items.begin(),items.end());
need algorithm header
caution: after altering a vector (inserting/erasing/sorting), iterators are invalidated and cannot be used unless given a value again (reassigned).

10. Iterator Usage Example
vector <int> v; // declare vector …
for (
// declare and initialize iterator
vector<int>::iterator p = v.begin();
p < v.end(); // check if end is not reached
++p // move iterator to next element ){
cout << *p; // manipulate element in loop body }

11. Typedef variable declarations can get long and obscure program meaning
typedef – allows to give names to already defined types and classes
note: no new types declared
benefits: shortens the code, gives more intuitive names, adds portability – can change defined type later if needed
syntax:
typedef existingType newType;
example
typedef vector<int> myVector;
typedef myvector::iterator myVectorIterator;
myVector v;
for (myVectorIterator p=v.begin(); p < v.end(); p++){
cout << *p;
do not overdo – typedefs hide original types: give new types intuitive names, do not use for short/understandable types

12. Questions on Vectors
why use vectors instead of arrays? what header file is needed to use vectors?
how is vector declared? how to declare a vector of five integers?
how can one determine the size of a vector? What is the size of this vector vector<int> myvector(4);
how does one add/remove elements from a vector?
how does one get the first/last element from a vector?
can vectors be assigned to each other? What happens to their size? contents? compared with each other? when are they equal?
can vectors be passed by value? reference? returned?
can vectors contain objects?
what is an iterator? how is it used?
what is typedef? How is it used? What are begin() and end()?