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Std:: vector.

The elements are stored contiguously, which means that elements can be accessed not only through iterators, but also using offsets to regular pointers to elements. This means that a pointer to an element of a vector may be passed to any function that expects a pointer to an element of an array.

The storage of the vector is handled automatically, being expanded as needed. Vectors usually occupy more space than static arrays, because more memory is allocated to handle future growth. This way a vector does not need to reallocate each time an element is inserted, but only when the additional memory is exhausted. The total amount of allocated memory can be queried using capacity() function. Extra memory can be returned to the system via a call to shrink_to_fit() [1] .

Reallocations are usually costly operations in terms of performance. The reserve() function can be used to eliminate reallocations if the number of elements is known beforehand.

The complexity (efficiency) of common operations on vectors is as follows:

• Random access - constant 𝓞(1) .
• Insertion or removal of elements at the end - amortized constant 𝓞(1) .
• Insertion or removal of elements - linear in the distance to the end of the vector 𝓞(n) .

std::vector (for T other than bool ) meets the requirements of Container , AllocatorAwareContainer (since C++11) , SequenceContainer , ContiguousContainer (since C++17) and ReversibleContainer .

• ↑ In libstdc++, shrink_to_fit() is not available in C++98 mode.

[ edit ] Template parameters

[edit]

[ edit ] Specializations

The standard library provides a specialization of std::vector for the type bool , which may be optimized for space efficiency.

[ edit ] Iterator invalidation

[ edit ] member types, [ edit ] member functions, [ edit ] non-member functions, [ edit ] notes, [ edit ] example, [ edit ] defect reports.

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

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In C++, vectors are used to store elements of similar data types. However, unlike arrays , the size of a vector can grow dynamically.

That is, we can change the size of the vector during the execution of a program as per our requirements.

Vectors are part of the C++ Standard Template Library . To use vectors, we need to include the vector header file in our program.

• C++ Vector Declaration

Once we include the header file, here's how we can declare a vector in C++:

The type parameter <T> specifies the type of the vector. It can be any primitive data type such as int , char , float , etc. For example,

Here, num is the name of the vector.

Notice that we have not specified the size of the vector during the declaration. This is because the size of a vector can grow dynamically, so it is not necessary to define it.

• C++ Vector Initialization

There are different ways to initialize a vector in C++.

Here, we are initializing the vector by providing values directly to the vector. Now, both vector1 and vector2 are initialized with values 1 , 2 , 3 , 4 , 5 .

Here, 5 is the size of the vector and 12 is the value.

This code creates an int vector with size 5 and initializes the vector with the value of 12 . So, the vector is equivalent to

Example: C++ Vector Initialization

Here, we have declared and initialized three different vectors using three different initialization methods and displayed their contents.

Basic Vector Operations

The vector class provides various methods to perform different operations on vectors. We will look at some commonly used vector operations in this tutorial:

• Add elements
• Access elements
• Change elements
• Remove elements

1. Add Elements to a Vector

To add a single element into a vector, we use the push_back() function. It inserts an element into the end of the vector. For example,

Here, we have initialized an int vector num with the elements {1, 2, 3, 4, 5} . Notice the statements

Here, the push_back() function adds elements 6 and 7 to the vector.

Note : We can also use the insert() and emplace() functions to add elements to a vector.

2. Access Elements of a Vector

In C++, we use the index number to access the vector elements. Here, we use the at() function to access the element from the specified index. For example,

• num.at(0) - access element at index 0
• num.at(2) - access element at index 2
• num.at(4) - access element at index 4

Note: Like an array, we can also use the square brackets [] to access vector elements. For example,

However, the at() function is preferred over [] because at() throws an exception whenever the vector is out of bound, while [] gives a garbage value.

3. Change Vector Element

We can change an element of the vector using the same at() function. For example,

In the above example, notice the statements,

Here, we have assigned new values to indexes 1 and 4 . So the value at index 1 is changed to 9 and the value at index 4 is changed to 7 .

4. Delete Elements from C++ Vectors

To delete a single element from a vector, we use the pop_back() function. For example,

In the above example, notice the statement,

Here, we have removed the last element ( 7 ) from the vector.

• C++ Vector Functions

In C++, the vector header file provides various functions that can be used to perform different operations on a vector.

• C++ Vector Iterators

Vector iterators are used to point to the memory address of a vector element. In some ways, they act like pointers in C++.

We can create vector iterators with the syntax

For example, if we have 2 vectors of int and double types, then we will need 2 different iterators corresponding to their types:

Initialize Vector Iterators

We can initialize vector iterators using the begin() and end() functions.

1. begin() function

The begin() function returns an iterator that points to the first element of the vector. For example,

2. end() function

The end() function points to the theoretical element that comes after the final element of the vector. For example,

Here, due to the nature of the end() function, we have used the code num.end() - 1 to point to the last element of the num vector i.e. num[2] .

Example: C++ Vector Iterators

In this program, we have declared an int vector iterator iter to use it with the vector num .

Then, we initialized the iterator to the first element of the vector using the begin() function.

Then, we printed the vector element by dereferencing the iterator:

Then, we printed the 3rd element of the vector by changing the value of iter to num.begin() + 2 .

Finally, we printed the last element of the vector using the end() function.

• Example: Iterate Through Vector Using Iterators

Here, we have used a for loop to initialize and iterate the iterator iter from the beginning of the vector to the end of the vector using the begin() and end() functions.

Also Read :

Table of Contents

• Introduction
• Example: C++ Vector
• Add Elements to a Vector
• Access Elements of a Vector
• Change Vector Element
• Delete Elements from C++ Vectors

Sorry about that.

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C++ Tutorial

Extract the first, last, or nth value from a vector

These are useful helpers for extracting a single value from a vector. They are guaranteed to return a meaningful value, even when the input is shorter than expected. You can also provide an optional secondary vector that defines the ordering.

For nth() , a single integer specifying the position. Negative integers index from the end (i.e. -1L will return the last value in the vector).

An optional vector the same size as x used to determine the order.

A default value to use if the position does not exist in x .

If NULL , the default, a missing value is used.

If supplied, this must be a single value, which will be cast to the type of x .

When x is a list , default is allowed to be any value. There are no type or size restrictions in this case.

Should missing values in x be removed before extracting the value?

If x is a list, a single element from that list. Otherwise, a vector the same type as x with size 1.

For most vector types, first(x) , last(x) , and nth(x, n) work like x[[1]] , x[[length(x)] , and x[[n]] , respectively. The primary exception is data frames, where they instead retrieve rows, i.e. x[1, ] , x[nrow(x), ] , and x[n, ] . This is consistent with the tidyverse/vctrs principle which treats data frames as a vector of rows, rather than a vector of columns.

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How to Change an Element by Index in a Vector in C++?

• How to Access an Element in a Vector Using Index in C++?
• How to copy elements of an Array in a Vector in C++
• How to find index of a given element in a Vector in C++
• How to Access Elements in Set by Index in C++?
• How to Find the Index of an Element in an Array in C++?
• How to Access the Last Element in a Vector in C++?
• Find index of an element in a Set in C++
• How to Check if a Vector is Empty in C++?
• How to Find Minimum Element in a Vector in C++?
• How to Add Element at the End of a Vector in C++?
• How to find Index of Element in Vector in R ?
• How to Find All Indexes of an Element in an Array in C++?
• How to Check if a Vector Contains a Given Element in C++?
• How to Create, Access, and Modify Vector Elements in R ?
• How to Convert a Vector to a List Without Losing Element Order in C++?
• How to Access an Element in Set in C++?
• How to Access the First Element of a Vector in C++?
• How to Convert an Array to a Vector in C++?
• How to Remove an Element from Vector in C++?

In C++, vectors are dynamic containers that store the data in a contiguous memory location but unlike arrays , they can resize themselves to store more elements. In this article, we will learn how to change a specific element by its index in a vector in C++.

Change a Specific Element in a Vector by Index

To replace a specific element in a vector using its index, we can use the [] array subscript operator . It is used with the index and the vector name and we directly assign the new value using the assignment operator.

C++ Program to Change a Specific Element in a Vector by Index

Time Complexity : O(1) Auxilary Space: O(1)

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