Vector in C++

In this article we are going to see about vector in C++.

What is vector?

Those who are similar to C, can think vector as an array. The usage of vector is quite similar to array. But vector has certain advantages over array. Normally, an array is static in nature both in C or C++.
We declare a static array as:

Type_t =any data type like int/float/char etc.
array =array name
size =static size of array

So, typically, static array is restricted by its predefined size. But, vector is dynamic in nature.
In CPP, we have the feature of vector which can be supplementary of array with additional privileges. Vector is not restricted by size, dynamic in nature. We declare a vector like:

Type_t=any data type like int/float/char etc.
myvector =vector name

Array vs Vector in C++

  • Size of array is fixed where vector is resizable. It can shrink, it can grow as per program flow.
  • Array is not automatically de-allocated where vector is automatically de-allocated.
  • Arrays can’t be returned from a function (if not dynamically allotted), can’t be copied to another array. But same operations are valid for vector.

So, from the above comparison, vector is a better option when we are not sure about the size of array.

Functions in vector

Continuing the above comparison, let’s check the functionalities of array implemented in vector. Then we will check additional STL functions for vector.

Inserting elements in the array/vector

Let’s create an array of 5 integer first. Then try to add more integers. If we use array then

Now to insert more element, we need to create new array and copy the previous elements and then start adding. Such a tedious job!!
Using vector,

To add more items, simple take input and push_back

No worry about the growing size of vector
cin>>arr[i] works perfect but never try cin>>myvector[i] (Or you can try try and check once ☺)
arr[i] refers to some memory, myvector[i] doesn’t.

Accessing elements in the array/vector

The accessing process is same for both array and vector which is by indexing. Both uses 0 indexing and for valid indexing returns the indexed element. Access time is O(1). Continuing the above example,

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

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).

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:

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:

Input number of elements (initially)7
Input elements
2 3 4 5 7 8 9
Enter value to insert
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:

Input number of elements (initially)
Input elements
3 4 5 6 8
Enter value of valid index
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:

Input number of elements (initially)
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


sorting is basic algorithm we know. Vector can be easily sorted by
the following function.

find min/max element in vector

You can use following std function to find max and min elements in the vector.

Example of the above is following:

Input number of elements (initially)
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++.

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