Vector in C++ STL
Vectors are the same as dynamic arrays with the
ability to resize itself automatically when an ele-
ment is inserted or deleted, with their storage be-
ing handled automatically by the container. Vector
elements are placed in contiguous storage so that
they can be accessed and traversed using itera-
tors. In vectors, data is inserted at the end. Insert-
ing at the end takes differential time, as sometimes
the array may need to be extended. Removing the
last element takes only constant time because no
resizing happens. Inserting and erasing at the be-
ginning or in the middle is linear in time.
What is std::vector in C++?
std::vector in C++ is the class template that con-
tains the vector container and its member func-
tions. It is defined inside the <vector> header file.
The member functions of std::vector class provide
various functionalities to vector containers. Some
commonly used member functions are written be-
low:
Iterators
1. begin() – Returns an iterator pointing to the first
element in the vector
2. end() – Returns an iterator pointing to the theo-
retical element that follows the last element in
the vector
3. rbegin() – Returns a reverse iterator pointing to
the last element in the vector (reverse begin-
ning). It moves from last to first element
4. rend() – Returns a reverse iterator pointing to
the theoretical element preceding the first ele-
ment in the vector (considered as reverse end)
5. cbegin() – Returns a constant iterator pointing
to the first element in the vector.
6. cend() – Returns a constant iterator pointing to
the theoretical element that follows the last el-
ement in the vector.
7. crbegin() – Returns a constant reverse iterator
pointing to the last element in the vector (re-
verse beginning). It moves from last to first ele-
ment
8. crend() – Returns a constant reverse iterator
pointing to the theoretical element preceding
the first element in the vector (considered as
reverse end)
•
// C++ program to illustrate the
// 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;
}
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
Capacity
1. size() – Returns the number of elements in the
vector.
2. max_size() – Returns the maximum number of
elements that the vector can hold.
3. capacity() – Returns the size of the storage
space currently allocated to the vector ex-
pressed as number of elements.
4. resize(n) – Resizes the container so that it con-
tains ‘n’ elements.
5. empty() – Returns whether the container is
empty.
6. shrink_to_fit() – Reduces the capacity of the
container to fit its size and destroys all ele-
ments beyond the capacity.
7. reserve() – Requests that the vector capacity
be at least enough to contain n elements.
•
// C++ program to illustrate the
// 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;
}
Output:
Size : 5
Capacity : 8
Max_Size : 4611686018427387903
Size : 4
Vector is not empty
Vector elements are: 1 2 3 4
Element access
1. reference operator [g] – Returns a reference to
the element at position ‘g’ in the vector
2. at(g) – Returns a reference to the element at
position ‘g’ in the vector
3. front() – Returns a reference to the first ele-
ment in the vector
4. back() – Returns a reference to the last element
in the vector
5. data() – Returns a direct pointer to the memory
array used internally by the vector to store its
owned elements.
•
// C++ program to illustrate the
// element access in vector
#include <bits/stdc++.h>
using namespace std;
int main()
{
vector<int> g1;
for (int i = 1; i <= 10; i++)
g1.push_back(i * 10);
cout << "\nReference operator [g] : g1[2] = " <<
g1[2];
cout << "\nat : g1.at(4) = " << g1.at(4);
cout << "\nfront() : g1.front() = " << g1.front();
cout << "\nback() : g1.back() = " << g1.back();
// pointer to the first element
int* pos = g1.data();
cout << "\nThe first element is " << *pos;
return 0;
}
Output:
Reference operator [g] : g1[2] = 30
at : g1.at(4) = 50
front() : g1.front() = 10
back() : g1.back() = 100
The first element is 10
Modifiers
1. assign() – It assigns new value to the vector el-
ements by replacing old ones
2. push_back() – It push the elements into a vec-
tor from the back
3. pop_back() – It is used to pop or remove ele-
ments from a vector from the back.
4. insert() – It inserts new elements before the el-
ement at the specified position
5. erase() – It is used to remove elements from a
container from the specified position or range.
6. swap() – It is used to swap the contents of one
vector with another vector of same type. Sizes
may differ.
7. clear() – It is used to remove all the elements of
the vector container
8. emplace() – It extends the container by insert-
ing new element at position
9. emplace_back() – It is used to insert a new ele-
ment into the vector container, the new ele-
ment is added to the end of the vector
// C++ program to illustrate the
// Modifiers in vector
#include <bits/stdc++.h>
#include <vector>
using namespace std;
int main()
{
// Assign vector
vector<int> v;
// fill the vector with 10 five times
v.assign(5, 10);
cout << "The vector elements are: ";
for (int i = 0; i < v.size(); i++)
cout << v[i] << " ";
// inserts 15 to the last position
v.push_back(15);
int n = v.size();
cout << "\nThe last element is: " << v[n - 1];
// removes last element
v.pop_back();
// prints the vector
cout << "\nThe vector elements are: ";
for (int i = 0; i < v.size(); i++)
cout << v[i] << " ";
// inserts 5 at the beginning
v.insert(v.begin(), 5);
cout << "\nThe first element is: " << v[0];
// removes the first element
v.erase(v.begin());
cout << "\nThe first element is: " << v[0];
// inserts at the beginning
v.emplace(v.begin(), 5);
cout << "\nThe first element is: " << v[0];
// Inserts 20 at the end
v.emplace_back(20);
n = v.size();
cout << "\nThe last element is: " << v[n - 1];
// erases the vector
v.clear();
cout << "\nVector size after clear(): " <<
v.size();
// two vector to perform swap
vector<int> v1, v2;
v1.push_back(1);
v1.push_back(2);
v2.push_back(3);
v2.push_back(4);
cout << "\n\nVector 1: ";
for (int i = 0; i < v1.size(); i++)
cout << v1[i] << " ";
cout << "\nVector 2: ";
for (int i = 0; i < v2.size(); i++)
cout << v2[i] << " ";
// Swaps v1 and v2
v1.swap(v2);
cout << "\nAfter Swap \nVector 1: ";
for (int i = 0; i < v1.size(); i++)
cout << v1[i] << " ";
cout << "\nVector 2: ";
for (int i = 0; i < v2.size(); i++)
cout << v2[i] << " ";
}
Output:
The vector elements are: 10 10 10 10 10
The last element is: 15
The vector elements are: 10 10 10 10 10
The first element is: 5
The first element is: 10
The first element is: 5
The last element is: 20
Vector size after erase(): 0
Vector 1: 1 2
Vector 2: 3 4
After Swap
Vector 1: 3 4
Vector 2: 1 2
The time complexity for doing various operations
on vectors is-
• Random access – constant O(1)
• Insertion or removal of elements at the end –
constant O(1)
• Insertion or removal of elements – linear in the
distance to the end of the vector O(N)
• Knowing the size – constant O(1)
• Resizing the vector- Linear O(N)
