C++ Vectors

In C++, vectors are used to store multiple elements dynamically in a single container. They automatically increase or decrease their size when elements are added or removed.

In this chapter, you will learn about C++ vectors, their features, and commonly used vector functions in C++.

What are C++ Vectors?

Vectors are a part of the standard template library (STL). They are dynamic arrays that can automatically resize themselves when the elements are added or removed.

Unlike normal arrays, vectors provide flexibility because there is no need to define a fixed size in advance. Vectors store elements in contiguous memory locations and automatically allocate more memory when required.

Vectors provide several useful member functions such as push_back(), pop_back(), insert(), size(), clear(), and front() to add, remove, access, and manage elements in a vector.

Syntax

It has the following syntax:

In this syntax,

• T represents the type of elements.
• v represents the name of the vector.

Creating a Vector

If we need to create a vector, we have to include the <vector> header.:

Example

Let us take a simple example to illustrate the C++ Vector.

Output:

Vector created

Inserting Elements

In C++, an element may be inserted into a vector via the insert() method, which takes a linear time. If we want to insert the number at the end, the push_back() method can be used. It works faster and takes only constant time.

Syntax

It has the following system:

Example

Let us take a simple example to insert elements in C++.

Output:

T p o i n t

Accessing of Updating Elements

We can access the elements using the [] operator or at() method. The [] method is fast but unsafe because it does not check whether the given index exists in the vector. We can also use the .at() method to access elements. The elements can be updated by assigning a new value using their index.

Syntax

It has the following syntax:

Example

Let us take an example to illustrate how to access and update elements in vectors.

Output:

The Accessing Value at position 4 is: u
The Accessing Value at position 3 is: o
The Updating Value at Position 4 is: y
The Updating Value at Position 2 is: z

Find Vector Size

One of the main common problems of arrays is the need to maintain a separate variable to track their size. Vector can solve this problem by using the size() function, which helps to return the current number of elements in the vector.

Syntax

It has the following Syntax:

Example

Let us take an example to illustrate the size of vectors in C++.

Output:

The Size of Vector is: 10

Traversing a Vector

We can traverse a vector using traditional for loops, range-based for loops, and iterators. We can utilize the loop and determine the size of the vector via the vector size() method to iterate via this range.

Example

Let us take an example to illustrate how to traverse a vector in C++.

Output:

T p o i n t T e c h

Deleting Elements

The vectors allow the removal of the ultimate element or remove the elements from any condition using erase() and pop_back() functions. If we know the value of the element in the vector, we can use the find() function to find the location of this element. After that, we can use the vector pop_back() function to delete the element that takes only constant time.

Syntax

It has the following syntax:

Example

Let us take an example to illustrate how to delete elements from a vector in C++.

Output:

T p o i n

Time Complexity of Vector Operations

Passing Vector to Functions

The vector can be passed by the value (copy), reference (modifies the original), or const reference (read-only).

Syntax

It has the following syntax:

Internal Working of Vectors

In C++, vectors can manage their storage automatically. When we insert an element and there is no capacity left, the vector resizes itself by allocating new memory, copying existing elements to the new memory, and deleting the old one. This mechanism ensures that push_back() runs in amortized constant time.

Key points

• The capacity often exceeds the size.
• Capacity can be checked using capacity().
• If the size is known, use the reserve() to pre-allocate the memory.

Multidimensional Vector

The vector can be nested to create 2D or 3D structures. It is helpful in problems related to involving matrices.

Syntax

It has the following syntax:

Example

Let us take an example to illustrate the multidimensional vector in C++.

Output:

1 2 3
4 5 6
7 8 9

Explanation

This code creates a 2D vector (matrix) with three rows and three columns, initializing it with specific values. After that, it uses nested loops to print each element in row-wise order, which creates a matrix.

C++ Vector Functions

In C++, vector offers several types of functions to perform different types of operations. The table below shows the function's names with their description.

Example

Let's look at the program to demonstrate various vector functions in C++.

Output:

Vector elements: 10 15 17 25 30 40 50
Size: 7
Capacity: 7
Max Size: 1073741823
After deletion: 15 17 25 30 40
Front: 15
Back: 40
At(1): 17
After swap v1: 100 200
Is v1 empty? Yes
After assign: 99 99 99
First element via data(): 99

Vectors Vs. Arrays

In C++, arrays and vectors are used to store collections of similar data types, but they differ in structure, flexibility and memory management.