What is a vector class?

The vector class is available in Java and C++ programming languages. It is a class implementing a dynamic array. A dynamic array is an array whose size automatically increases when an element is added to it. The vector class stores elements in a contiguous memory block and assigns the memory to the elements whenever required during execution.

Syntax of vector class

The syntax of vector in C++ language is as follows:

vector <object_type> vector_var_name;

Here,

vector is the keyword used to declare a vector class.

object_type is the data type of the element.

vector_var_name is the name of the vector variable.

To use a vector class in C++, it is mandatory to include the header file: #include<vector> at the beginning of the code.

Example of vector class

Consider the following C++ code:

#include<iostream>  

#include<vector>  

using namespace std;  

int main()  

{  

vector<string> vec;  

vec.push_back("Hello ");  

for(vector<string>::iterator it=vec.begin(); it!=vec.end(); ++it)  

cout<<*it;  

return 0;   

}  

Output:

Hello

Explanation: The above code begins with the declaration of the header files, followed by the main function. Within main(), the vector vec with string datatype is created. The push_back() function is used to insert elements into the vector. Here, the string Hello is inserted in the vector. The for loop is then used to print the vector elements.

Initialization of vectors in C++

In C++, vectors can be initialized in four ways:

1. By pushing values one-by-one
2. By using an overloaded constructor
3. By using arrays
4. By using another initialized vector

By pushing values one-by-one

The elements in a vector can be inserted (pushed) in a vector class one by one using the push_back() method. The algorithm is as follows:

Start

Declare v of vector type

Invoke push_back() method to insert elements to vector v

Print “Vector elements:”

for (int a:v)

 Print all elements of variable a

End

Syntax: vector_name.push_back(element_value);

Example: v1.push_back(a);

By using an overloaded constructor

This approach is used when a vector contains multiple elements with the same value. The algorithm for this approach is as follows:

Start

Initialize a variable (say a)

Create a vector (say vec1) with size ’a’

Initialize vector vec1

Initialize vec2 by vec1

Print the vector elements

End

Syntax: vector<object_type> vector_name(number_of_repetitions, element_value);

Example: int a= 5;   

vector<int> v1(a, 8);   

By using arrays

In this approach, an array containing the elements to be stored in the vector is passed as a parameter to the vector class. The algorithm for this approach is as mentioned below:

Start

Create a vector (say vec)

Initialize the vector

Print the elements

End

Syntax: vector<object_type> vector_name{value1,value2,....,valuen};

Example:   vector<int> vec{2,6,1,4,9};   

By using another initialized vector

This approach creates a new vector from an existing vector having the same values. It passes begin() and end() iterators of the initialized vector. The algorithm for this approach is given below:

Start

Create a vector vec1

Initialize the vector vec1 using an array.

Initialize vector vec2 by vec1

Print the elements

End

Syntax: vector<object_type> vector_name1{value1,value2,....,valuen};

vector<object_type> vector_name2(vector_name1.begin(), vector_name1.end())

Example: vector<int> vec1{2,6,1,4,9};

vector<int> vec2(vec1.begin(), vec1.end());

Vector functions in C++

Some functions used in the vector class in C++ include:

Advantages of using vectors

The benefits of using vectors in a program are:

• Vectors are dynamic. (Their size can be increased easily.)
• Inserting and deleting elements in vectors is easy.
• Multiple elements can be stored in vectors.
• Copying elements from one vector to another is easy.

Context and Applications

The topic operations of vector class is a significant concept in programming languages. Students studying the courses listed below study this concept.

• Bachelor of Technology in Computer Science Engineering
• Master of Technology in Computer Science Engineering
• Bachelor of Science in Information Technology
• Master of Science in Information Technology

Practice Problems

Q1. What do vectors implement?

1. Dynamic arrays
2. Vector object cout
3. Big-Ocout
4. Current capacity

Answer: Option a

Explanation: Vectors are containers that implement dynamic arrays.

Q2. Which header file should be included in the program to use vectors?

1. <Non-member int index cout allocator>
2. <Allocator cout>
3. <random-access iterator cout allocator>
4. <vector>

Answer: Option d

Explanation: The header file <vector> includes the implementation of all vector methods, so it is necessary to add this file in the program while using vectors.

Q3. Which of these is used to access the first element of a vector vec?

1. cout.allocator()
2. vec.begin()
3. v1.push _ back.int index cout()
4. random-access cout allocator()

Answer: Option b

Explanation: The begin() method is used to access the first element of a vector. So, for vector vec, the method vec.begin() will be used to access the first element of the vector.

Q4. Which function returns the number of elements in the vector?

1. Linear Algorithms operation cout()
2. size()
3. Coutdestructor return 0()
4. scalar vector object()

Answer: Option b

Explanation: The size function returns the number of elements present in the vector.

Q5. Which function removes all vector elements?

1. cout v1.push _ back()
2. dynamically random-access iteratorcout()
3. clear()
4. v1.push _ backcout()

Answer: Option c

Explanation: The clear() function deletes all the elements in the vector.

Common Mistakes

Students should follow the correct syntax of vectors and their functions. If students make mistakes in writing the syntax, it will generate a compilation error, and the program will fail to execute.

Related Concepts

• Deque
• Arrays
• Templates
• Functions
• Iterators
• C++ algorithms