iterators

Suppose we have a vector consisting of 4 elements. Suppose this is our computer's memory

******************************

******************************

******************************

******************************

******************************

******************************

* shows that at that spot of the memory some data exists.

When we declare a vector and give it 4 elements in memory it is not going to be in some special order. It may be like this

**********v[0]*******v[1]

**********************

***********************

v[3]*******************

********************v[4]

As we can see even if the creators wanted to avoid creating iterator and at the same time they tried to find  a way of calling the elements , they couldn't succeed because the way the elements are scattered in memory is not certain. 

The iterator itself returns you the position of the element we are looking for. It is not an integer, it is a separate data type. It is made in a special way. In our representation of computer memory suppose iterator is a pair of integers defining the row and the column of our element. the iterator for v[0] will be 1 and 12. (1st row and 12th column)

When we add 1 to our iterator (which is a legal move, adding an integer to iterator) the iterator will automatically jump from the 0th element to the 1st element and the new value will be  1 and 20(1st row and 20th column). 

Iterator is defined this way

Data_type_of_our_iterators_target::iterator name_of_iterator;

example

vector<int>::iterator it;

Example of using iterator

for(it=v.begin(); it!=v.end(); it++)

{

       cout<< * it<<endl; ( we wrote * because the iterator itself is a pointer)

}

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C++ STL vector

Vector is another component of C++ STL (standard template library). Vector is also a dynamic array just like queue but with some differences.

This data structure is different from the data structure stack because vector is often use to keep data not to process and it leave it alone. We define vector like this
vector< data type> name_of_the_vector;
(don't forget to include vector class in the beginning of your code #include <vector> )

Suppose we have vector<int> v;

Above you can find the use of vector's functions.

v.at( int I);

This function calls the element with index I. When we have a static array we use [ ] operator to call for an element , in here we use .at function.
(NOTE [] operator can also be used when we use vector but it is a little bit unsafe.)

v.begin()     v.end()
The functions v.begin() and v.end() return the iterator which shows the beginning of the vector and the end of the vector accordingly. You can learn about iterators here.

v.clear()

This function clears the vector completely. After this function the vector is becoming empty.

v.empty()

This is a boolean function. It returns TRUE if there is at least 1 element in vector or false when the vector is completely empty.

v.erase( vector<int>::iterator iter);

This function deletes an element from the given point of our vector. NOTE this function recieves an iterator as a parameter not an integer.

v.insert( vector<int>::iterator iter,int x);

This function inserts the element x in the position of iter. NOTE this function just like the previous one recieves an iterator as a parameter not an integer.

v.pop_back();

This function pops(removes) the last element of the vector.

 v.push_back(int x);

This function adds an element from the back of the vector .

v.size();

This function returns an integer; the number of elements present in the vector.

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C++ (intermediate) STL queue

One of the most important classes in C++ STL( standard template library) is queue. Queue is a dynamic array. You can add elements from the back of the queue and remove elements from the beginning of the queue. The functions are called push and pop. Push stands for pushing an element from the back of the queue and pop stands for popping (taking out) the element from the beginning of the queue.

Queue is being defined like this

queue< data type> name_of_the_queue;

Here is the list of functions of queue.

push

is written like this q.push(a);

This command will add element a to the end of our queue. 

pop

   is written like this q,pop();

  This function has no parameters. It is just removing the first element of our queue.

front

 is written like this a=q,front();

this function is not a void type function it returns the value of the first element (returns not removes).

empty

is written like this q.empty(); 

This is a boolean function. It returns one of 2 values. It returns "true" if the queue is completely empty(no elements are present in the queue) and it returns "false" if there is at least one element present in the queue.

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