# Vector

```cpp

#include <vector>                                  // include vector  in code

vector<int> arr;                                   // create an empty vector
vector<int> arr(10);                               // create a vector of size 10
vector<int> arr{1,2,3};                            // create vector with values 1,2,3
vector<int> arr(10, 5);                            // create an arr of size 10, all elements initialised to 5    

int n = arr.size();                                // get number of elements in arr 

arr.resize(20, 0);                                 // resize the vector to new final size = 20, each new item set as 0
                                                   // here existing values stay intact as new_size > old_size

// check if vector is empty  
if (arr.empty()){
    cout << "arr is empty";
}
         
arr.push_back(3);                                  // add element at end of vector 
  
arr.insert(arr.begin(), 7);                        // add an element at beginning of vector

arr.insert(arr.end(), brr.begin(), brr.end());     // add another vector at end of a vector  

reverse(arr.begin(), arr.end());                   // reverse vector

arr.clear();                                       // clear all enteries                 

sort(arr.begin(), arr.end());                      // sort vector elements

// sort vector elements in descending order using a comparator function
sort(arr.begin(), arr.end(), [](int lhs, int rhs){
    return (lhs > rhs);     // read it like: lhs should come before rhs if (lhs > rhs)
                            // or read it like: 
                            // In the final sorted array, relation between lhs and rhs is lhs > rhs
});



```


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