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) made more powerful
vectors can
can grow and shrink as needed
can be compared, assigned as a whole
can be passed by reference/value/returned
vector size is always available
still more …
part of so called Standard Template Library (STL)
once you learned vectors, do not use (raw) arrays

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

4. Typical Array Operations
indexing: items[3]
sill 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();
can assign
vector<int> v;
v = items;
can swap: items.swap(v);
can compare: if(v == items) …

7. With Objects class intData{ // elementary class definition public:
void set(int n){data=n;} // simple setter
int get() 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 iterator is a generalization of pointer
not a pointer but usually implemented using pointers
further enhances the power of vector
declaring iterator
vector<typeParameter>::iterator iteratorName;
where
typeParameter – type/class of elements
example
vector<int>::iterator ip;

9. Iterators Operations arithmetic
++ (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 or after in the vector
subtract one iterator from another: gives distance
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 one element at // iterator ip position
sorting elements: sort(v.begin(),v.end());
need algorithm header

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. Iterator Invalidation
after using iterator for update, iterators pointing to vector are invalidated, have to reassign
invalidated iterator is loose: accessing is run-time error, result unspecified
reason: dynamic storage allocation inside vector
vector<int> v(5);
vector<int>::iterator p;
p = v.begin()+3; // p points to 4th element
v.erase(v.begin()); // erases first element of the vector
// this invaidates p
cout << *p; // p is invalidated, result is unspecified
// run-time error

12. 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

13. 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 the 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 iterator? how is it used?
what are begin() and end()?
how does one add/remove elements in the middle of a vector?
what is iterator invalidation? why does it happen?
what is typedef? how is it used?