210 lines
4.8 KiB
Markdown
210 lines
4.8 KiB
Markdown
---
|
|
title: queue
|
|
---
|
|
|
|
## Queues
|
|
|
|
`queue` is one of the most used containers in C++. A container is a data structure that stores a collection of objects, some in order, some not. All containers have a different set of functions that allow you to access an object(s) in that collection.
|
|
|
|
`std::queue` is part of the C++ standard library (hence the prefix `std::`) and allows you to store data in First In First Out (FIFO) order. NOTE: **All objects within a queue must be of the same data type**
|
|
|
|
The data type you store within a queue goes within angle brackets next to the queue keyword. For example, if you would like to store a collection of integers the queue would be `std::queue<int> queue_name`
|
|
|
|
### Queue LIFO Explanation
|
|
|
|
`queue` allows us to push/enqueue and pop/dequeue in specific order. **Push** means inserting an object at the front of the queue. **Pop** means pulling out the "oldest" object from end of the queue. So when you push it is at the front and when you pop you extract the oldest element.
|
|
|
|
|
|
|
|
### Queue Operations
|
|
|
|
The queue container supports the following operations:
|
|
- push (enqueue)
|
|
- pop (dequeue)
|
|
- empty
|
|
- size
|
|
- front
|
|
- back
|
|
|
|
#### Push
|
|
|
|
Allows you to inserts a new element at the end of the queue, after its current last element.
|
|
|
|
```cpp
|
|
//Push operation in Queue
|
|
#include <iostream> // std::cout
|
|
#include <queue> // std::queue
|
|
|
|
int main ()
|
|
{
|
|
std::queue<int> q;
|
|
|
|
q.push(1); //Pushing 1 at front of the queue
|
|
q.push(2); //Pushing 2 at front of the queue
|
|
|
|
return 0;
|
|
}
|
|
```
|
|
|
|
#### Front
|
|
|
|
Allows you to access the next element in the queue without removing it.
|
|
The next element is the "oldest" element in the queue.
|
|
|
|
```cpp
|
|
//Front operation in Queue
|
|
#include <iostream> // std::cout
|
|
#include <queue> // std::queue
|
|
|
|
int main ()
|
|
{
|
|
std::queue<int> q;
|
|
|
|
q.push(1); //Pushing 1 at front of the queue
|
|
q.push(2); //Pushing 2 at front of the queue
|
|
|
|
std::cout<<q.front()<<'\n'; //Accessing the front of the queue
|
|
std::cout<<q.front()<<'\n'; //Accessing the front of the queue
|
|
|
|
return 0;
|
|
}
|
|
```
|
|
|
|
Output:
|
|
1
|
|
1
|
|
|
|
#### Pop
|
|
|
|
Allows you to remove the next element in the queue, effectively reducing its size by one.
|
|
The element removed is the "oldest" element.
|
|
|
|
```cpp
|
|
//Pop operation in Queue
|
|
#include <iostream> // std::cout
|
|
#include <queue> // std::queue
|
|
|
|
int main ()
|
|
{
|
|
std::queue<int> q;
|
|
|
|
q.push(1); //Pushing 1 at front of the queue
|
|
q.push(2); //Pushing 2 at front of the queue
|
|
|
|
std::cout<<q.front()<<'\n'; //Accessing the front of the queue
|
|
q.pop(); //Removing the oldest element
|
|
std::cout<<q.front()<<'\n'; //Accessing the front of the queue
|
|
q.pop(); //Removing the oldest element
|
|
|
|
return 0;
|
|
}
|
|
```
|
|
|
|
Output:
|
|
1
|
|
2
|
|
|
|
#### Size
|
|
|
|
Returns the number of elements in the `queue`.
|
|
|
|
```cpp
|
|
//Size operation in Queue
|
|
#include <iostream> // std::cout
|
|
#include <queue> // std::queue
|
|
|
|
int main ()
|
|
{
|
|
std::queue<int> q;
|
|
|
|
q.push(1); //Pushing 1 at front of the queue
|
|
q.push(2); //Pushing 2 at front of the queue
|
|
|
|
std::cout<<q.size()<<'\n'; //Accessing the front of the queue
|
|
q.pop(); //Removing the oldest element
|
|
std::cout<<q.size()<<'\n'; //Accessing the front of the queue
|
|
q.pop(); //Removing the oldest element
|
|
std::cout<<q.size()<<'\n'; //Accessing the front of the queue
|
|
|
|
return 0;
|
|
}
|
|
|
|
```
|
|
|
|
Output:
|
|
2
|
|
1
|
|
0
|
|
|
|
#### Empty
|
|
|
|
Returns whether the `queue` is empty ,i.e. whether your queue size is zero.
|
|
It returns `true` if queue's size 0 else returns `false`
|
|
|
|
```cpp//Empty operation in Queue
|
|
#include <iostream> // std::cout
|
|
#include <queue> // std::stack
|
|
|
|
int main ()
|
|
{
|
|
std::queue<int> q;
|
|
|
|
q.push(1);
|
|
q.push(2);
|
|
|
|
while(q.empty() != true){
|
|
std::cout<<q.front()<<'\n';
|
|
q.pop();
|
|
}
|
|
|
|
std::cout<<"Out of loop"<<'\n';
|
|
return 0;
|
|
}
|
|
```
|
|
|
|
Output:
|
|
1
|
|
2
|
|
Out of loop
|
|
|
|
#### Back
|
|
|
|
Allows you to access the last element in the queue without removing it.
|
|
The next element is the "newest" element in the queue.
|
|
|
|
```cpp
|
|
//Back operation in Queue
|
|
#include <iostream> // std::cout
|
|
#include <queue> // std::queue
|
|
|
|
int main ()
|
|
{
|
|
std::queue<int> q;
|
|
|
|
q.push(1); //Pushing 1 at front of the queue
|
|
q.push(2); //Pushing 2 at front of the queue
|
|
|
|
std::cout<<q.back()<<'\n'; //Accessing the back of the queue
|
|
std::cout<<q.back()<<'\n'; //Accessing the back of the queue
|
|
|
|
return 0;
|
|
}
|
|
```
|
|
|
|
Output:
|
|
2
|
|
2
|
|
|
|
|
|
### For More Resources:
|
|
http://www.cplusplus.com/reference/queue/queue/
|
|
|
|
### Citations:
|
|
Image Courtesy: https://en.wikipedia.org/wiki/FIFO_(computing_and_electronics)
|
|
|
|
|
|
|
|
|
|
|
|
|