# Vector

![](/files/x2zYFj3XEvATJ8I9DJfP)

Vector จะเหมือนกับ Dynamic Arrays ที่มีความสามารถในการปรับขนาดตัวเองโดยอัตโนมัติเมื่อมีการแทรกหรือลบองค์ประกอบ โดยคอนเทนเนอร์จะจัดการพื้นที่เก็บข้อมูลโดยอัตโนมัติ

การประกาศ Vector นั้นสามารถทำได้โดยการใช้คำสั่งดังนี้

```cpp
// vector<data_type> vector_name;
vector<int> Hallway(9);
vector<int> Hallway;
vector<int> Hallway {1,2,3,4,5};
vector<int> Hallway = {1,2,3,4,5};
vector<double> grade;
vector <char> Alpgrade;
vector<string> cars {"Jame","Jack","John"};
```

### ฟังก์ชันที่เกี่ยวข้องกับเวกเตอร์คือ:&#x20;

#### &#x20;   ฟังก์ชันที่ถูกใช้บ่อยที่สุด

* push\_back(): appends an element to the end
* pop\_back() Erases the last element
* size() provides the number of elements
* begin() provides reference to last element
* end() provides reference to end of Vector.

![ref: https://www.slideshare.net/SreejithSree1/stl-25264696](/files/7yVk1UK2XQgdMxifDeXK)

1. Iterators (ตัววนซ้ำ)

   * [begin()](https://www.geeksforgeeks.org/vectorbegin-vectorend-c-stl/) – Returns an iterator pointing to the first element in the vector
   * [end()](https://www.geeksforgeeks.org/vectorbegin-vectorend-c-stl/) – Returns an iterator pointing to the theoretical element that follows the last element in the vector
   * [rbegin()](https://www.geeksforgeeks.org/vector-rbegin-and-rend-function-in-c-stl/) – Returns a reverse iterator pointing to the last element in the vector (reverse beginning). It moves from last to first element
   * [rend()](https://www.geeksforgeeks.org/vector-rbegin-and-rend-function-in-c-stl/) – Returns a reverse iterator pointing to the theoretical element preceding the first element in the vector (considered as reverse end)
   * [cbegin()](https://www.geeksforgeeks.org/vector-cbegin-vector-cend-c-stl/) – Returns a constant iterator pointing to the first element in the vector.
   * [cend()](https://www.geeksforgeeks.org/vector-cbegin-vector-cend-c-stl/) – Returns a constant iterator pointing to the theoretical element that follows the last element in the vector.
   * [crbegin()](https://www.geeksforgeeks.org/vectorcrend-vectorcrbegin-examples/) – Returns a constant reverse iterator pointing to the last element in the vector (reverse beginning). It moves from last to first element
   * [crend()](https://www.geeksforgeeks.org/vectorcrend-vectorcrbegin-examples/) – Returns a constant reverse iterator pointing to the theoretical element preceding the first element in the vector (considered as reverse end)

{% code title="Vector\_iterators.cpp" %}

```cpp
// iterators in vector
#include <iostream>
#include <vector>
  
using namespace std;
  
int main()
{
    vector<int> g1;
  
    for (int i = 1; i <= 5; i++)
        g1.push_back(i);
  
    cout << "Output of begin and end: ";
    for (auto i = g1.begin(); i != g1.end(); ++i)
        cout << *i << " ";
  
    cout << "\nOutput of cbegin and cend: ";
    for (auto i = g1.cbegin(); i != g1.cend(); ++i)
        cout << *i << " ";
  
    cout << "\nOutput of rbegin and rend: ";
    for (auto ir = g1.rbegin(); ir != g1.rend(); ++ir)
        cout << *ir << " ";
  
    cout << "\nOutput of crbegin and crend : ";
    for (auto ir = g1.crbegin(); ir != g1.crend(); ++ir)
        cout << *ir << " ";
  
    return 0;
}
```

{% endcode %}

Output

```
Output of begin and end: 1 2 3 4 5 
Output of cbegin and cend: 1 2 3 4 5 
Output of rbegin and rend: 5 4 3 2 1 
Output of crbegin and crend : 5 4 3 2 1
```

2\. Capacity (ความจุ)

* [size()](https://www.geeksforgeeks.org/vectorempty-vectorsize-c-stl/) – Returns the number of elements in the vector.
* [max\_size()](https://www.geeksforgeeks.org/vector-max_size-function-in-c-stl/) – Returns the maximum number of elements that the vector can hold.
* [capacity()](https://www.geeksforgeeks.org/vector-capacity-function-in-c-stl/) – Returns the size of the storage space currently allocated to the vector expressed as number of elements.
* [resize(n)](https://www.geeksforgeeks.org/vector-resize-c-stl/) – Resizes the container so that it contains ‘n’ elements.
* [empty()](https://www.geeksforgeeks.org/vectorempty-vectorsize-c-stl/) – Returns whether the container is empty.
* [shrink\_to\_fit()](https://www.geeksforgeeks.org/vector-shrink_to_fit-function-in-c-stl/) – Reduces the capacity of the container to fit its size and destroys all elements beyond the capacity.
* [reserve() ](https://www.geeksforgeeks.org/using-stdvectorreserve-whenever-possible/)– Requests that the vector capacity be at least enough to contain n elements.

{% code title="Vector\_capacity.cpp" %}

```cpp
// capacity function in vector
#include <iostream>
#include <vector>
  
using namespace std;
  
int main()
{
    vector<int> g1;
  
    for (int i = 1; i <= 5; i++)
        g1.push_back(i);
  
    cout << "Size : " << g1.size();
    cout << "\nCapacity : " << g1.capacity();
    cout << "\nMax_Size : " << g1.max_size();
  
    // resizes the vector size to 4
    g1.resize(4);
  
    // prints the vector size after resize()
    cout << "\nSize : " << g1.size();
  
    // checks if the vector is empty or not
    if (g1.empty() == false)
        cout << "\nVector is not empty";
    else
        cout << "\nVector is empty";
  
    // Shrinks the vector
    g1.shrink_to_fit();
    cout << "\nVector elements are: ";
    for (auto it = g1.begin(); it != g1.end(); it++)
        cout << *it << " ";
  
    return 0;
}
```

{% endcode %}

Output&#x20;

```
Size : 5
Capacity : 8
Max_Size : 4611686018427387903
Size : 4
Vector is not empty
Vector elements are: 1 2 3 4
```

Reference

{% embed url="<https://www.geeksforgeeks.org/vector-in-cpp-stl>" %}


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