All about queue STL in c++ for competitive programming( initialization, traversal and methods/functions ) :

queue STL

Important Points:

1. It is a type of container adaptor which follows FIFO (First In First Out) rule.
2. Elements are pushed from the back of the container and are extracted from its front.
   
3. It is same as the queue in c but with a huge advantage of availability of direct functions/methods for many operations.
4. Remember to use the header file #include<queue> before using queue STL.
   

How do i remember FIFO ?

• Assume a queue (line) of people in front of a ticket counter.
  
• Now think of the people as elements and the room as the container (ignore the ticket counter).
• A person exits the queue from the front.
• A new person can only enter the queue from the last.
• A person can only exit the queue only if the people in front of it exits the queue first which means FIFO (First in First out) i.e the person who entered the queue first can only exit the queue first.
  

 

deque vs queue:

• In queue insertion takes place from one end and removal takes place at the other end while in deque insertion/removal can be done from any end.
  
• We can use iterators with deque but not with queue.

 

 

queue STL initialization

declaring a queue and using push( ) method to put elements into it from back:  

    queue < int > q ;
    q.push( 10 ) ;
    q.push( 20 ) ;
    q.push( 30 ) ;

 

    Therefore, 

    dq = { 10, 20, 30 }

 

 

 

All queue STL methods/ functions  

queue STL important functions:

1. push( ) or emplace( ) :  used to insert a new element at the end of the queue.

2. size( ) : returns the number of elements in the queue.

3. empty( ) : returns true if the container is empty otherwise, false.

 

4. front( ) : returns the top first/front element of the queue.

5. back( ) : returns the last/back/rear element of the queue. 

6. pop( ) : removes the front/first element of the queue.

 

7. swap( ) : used to exchange the content of two queue containers of same type (size may differ).

 

emplace( ) vs push( ) which to use?

• emplace() avoids unnecessary copy of objects.
• for primitive data types it does not matter which one to use but for objects or bigger objects use emplace() for efficiency.
• unlike push(), no copy or move operation are performed in emplace().

 

 

 all queue STL important functions with example

1. Initializing queue : queue< int > q ;

 

2. Putting elements into it : 

     q.push( 10 ) ;

     q.push( 30 ) ; 

     q.push( 20 ) ;  

     Therefore, 

     q = { 10, 30, 20 } 

 

3. cout << q.size( ) ;               // 3

 

4. cout << q.front( ) ;               //10

 

5. cout << q.back( ) ;               // 20 

 

6. Checking whether queue is empty or not :

    if( q.empty( ) = = false )

    cout << "not empty" ;     //it will be printed

    else

    cout << "empty" ;

 

9. q.pop( ) ;  

     // front element removed i.e 10

 

queue STL traversal method

Let our queue be q :

queue < int > q ;

 

   while ( !q.empty( ) )

    {

          cout << q.front( ) << " "; 

          q.pop( );

    }

       

 

 

 

 

 

 

Hi reader! got any queries or need more explanation for something? Feel free to comment below.