Vector in C++

Type_t array[size];

vector myvector;

//code snippet to insert five elements to array 

int arr[5];
for(int i=0;i<5;i++){
    cin>>arr[i];
}

int item;
vector myvector;

for(int i=0;i<5;i++){
    cin>>item;
    myvector.push_back(item);
}

in some loop
cin>>item
myvector.push_back(item) 

for(int i=0;i<5;i++)
cout<
Above was the basic demonstration of vector. Till now we have mainly compared with array and similar features. Now, we will focus on vector as part of STL


Further reading:



Vector of vectors in C++

            Read more →
        


Merge two vectors in C++

            Read more →
        

Vector STL in C++

STL is the strength of CPP. Programmers often prefers CPP to implement data structures for its STL. Vector has several STL functions which are listed below. Before going through those functions, it’s worth mentioning that vector can be seen as container and we can access vector elements with help of iterators. An iterator is a similar thing like pointer. Below is an example where vector is access by iterator instead of indexing (previously explained).

//declare iterator
vector:: iterator it;

for(it=myvector.begin();it!=myvector.end();it++){
cout<<*it<
The STL functions widely used are,
Iterator related
.begin(): points to beginning of the vector(first element)

.end(): points to end of the vector, for understanding after the last element of the vector

.rbegin(): points to last element of the vector(for reverse traversal)

.rend(): points to start of the vector, for understanding before the first element of the vector

All of the above returns the iterator pointing to the above-specified positions. Example is the following:
#include 

using namespace std;

int main(){
    //declare vector
    vector myvector;

    //create the vector
    int n,item;
    cout<<"Input number of elements (initially)";
    cin>>n;
    cout<<"Input elements\n";
    while(n--){

        cin>>item;
        myvector.push_back(item);

    }
    //if you want you can add more addding some code snippet for more input

    //iterating vector elements with help of function .begin() and .end()

    cout<<"starting element: "<<*(myvector.begin())< :: iterator it; //another way is to use auto in the loop used next time
    for(it=myvector.begin();it!=myvector.end();it++){
        cout<<"element: "<<*it<

Input number of elements (initially)5

Input elements

2 4 6 8 10

starting element: 2

iterating in forward direction

element: 2

element: 4

element: 6

element: 8

element: 10

iterating in reverse direction

element: 10

element: 8

element: 6

element: 4

element: 2


Insertion-deletion related
.push_back(item): pushes a new item at the end.

.insert(iterator it , item): inserts item before the specified position it

.pop_back(): removes an element from the back.

.erase(iterator it): removes element from specified position it

.clear(): removes all elements from vector

Example of the above is following:
#include 

using namespace std;

int main(){
    //declare vector
    vector myvector;
    vector::iterator it;
    //create the vector
    int n,item;
    cout<<"Input number of elements (initially)";
    cin>>n;
    cout<<"Input elements\n";

    //1.use of push_back

    while(n--){

        cin>>item;

        myvector.push_back(item);

    }
    //2.use of insert()
    cout<<"Enter value to insert\n";
    cin>>item;
    cout<<"inserting at the begining\n";
    myvector.insert(myvector.begin(),item); //it=myvector.begin()
    cout<<"After insertion:\n";
    for(it=myvector.begin();it!=myvector.end();it++){
        cout<<"element: "<<*it<

Input number of elements (initially)7

Input elements

2 3 4 5 7 8 9

Enter value to insert

1

inserting at the begining

After insertion:

element: 1

element: 2

element: 3

element: 4

element: 5

element: 7

element: 8

element: 9

Popping the last element:

After Popping

element: 1

element: 2

element: 3

element: 4

element: 5

element: 7

element: 8

erasing the first element

New first element is : 2

After clearing

Size of vector: 0


Accessing the elements related
at(int pos): returns reference of element at position pos

front(): returns reference of first element

back(): returns reference of last element
Example of the above is following:
#include 

using namespace std;

int main(){
    //declare vector
    vector myvector;
    vector::iterator it;
    //create the vector
    int n,item;
    cout<<"Input number of elements (initially)\n";
    cin>>n;
    cout<<"Input elements\n";

    while(n--){

        cin>>item;

        myvector.push_back(item);

    }
    //1.use of at()
    int index;
    cout<<"Enter value of valid index\n";
    cin>>index;
    cout<<"value at index "<

Input number of elements (initially)

5

Input elements

3 4 5 6 8

Enter value of valid index

3

value at index 3 is: 6

the front element is: 3

the last element is: 8

Size related
.size(): returns current size of vector

.max_size(): returns maximum size possible for the vector

.empty(): Boolean function, returns whether vector is empty or not
Example of the above is following:
#include 

using namespace std;

int main(){
    //declare vector
    vector myvector;
    vector::iterator it;
    //create the vector
    int n,item;
    cout<<"Input number of elements (initially)\n";
    cin>>n;
    cout<<"Input elements\n";

    while(n--){

        cin>>item;

        myvector.push_back(item);

    }
    //1.use of size()

    cout<<"size of the vector is "<

Input number of elements (initially)

6

Input elements

3 4 5 6 7 8

size of the vector is 6

maximum size of the vector can be 4611686018427387903

size of vector: 6

size of vector: 5

size of vector: 4

size of vector: 3

size of vector: 2

size of vector: 1

Vector is empty now

Some other useful functions
sort()
sorting is basic algorithm we know. Vector can be easily sorted by

the following function.
//in ascending order
sort(myvector.begin(),myvector.end())
//in descending order
sort(myvector.begin(),myvector.end(),greater());

find min/max element in vector
You can use following std function to find max and min elements in the vector.
// Find max
*(std::max_element(myvector.begin(),myvector.end()))
// Find min
*(std::min_element(myvector.begin(),myvector.end()))

Example of the above is following:
#include 

using namespace std;

int main(){
    //declare vector
    vector myvector;
    vector::iterator it;
    //create the vector
    int n,item;
    cout<<"Input number of elements (initially)\n";
    cin>>n;
    cout<<"Input elements\n";

    while(n--){

        cin>>item;

        myvector.push_back(item);

    }
    //1.sorting in ascending order

    cout<<"sorting in ascending order\n";

    sort(myvector.begin(),myvector.end());
    cout<<"After sorting\n";
    for(auto it=myvector.begin();it!=myvector.end();it++){
        cout<<*it<<" ";
    }
    cout<());
    cout<<"After sorting in descending order\n";
    for(auto it=myvector.begin();it!=myvector.end();it++){
        cout<<*it<<" ";
    }
    cout<

Input number of elements (initially)

5

Input elements

4 2 3 6 7

sorting in ascending order

After sorting

2 3 4 6 7

sorting in descending order

After sorting in descending order

7 6 4 3 2

The maximum element is: 7

The minimum element is: 2


That’s all for vector in C++. Definitely vector in very strong linear data struct and very easy to use. In our next section we would learn about vector of vectors which can be thought of 2D array in C/C++.