Vector

A vector is similar to an array that can change its size. It is also known as a resizable or dynamic array due to its resizable nature.

Similar to an array, vectors are zero-indexed, and access to elements is the exact same as array access.

You must include the vector by using

cpp

#include <vector>

However, it is also included in <bits/stdc++.h>

Creation and access is like this:

cpp

vector<int> v;
v.push_back(1);
v.push_back(2);

cout << v[0]; // 1
cout << v[1]; // 2

The first line of our code creating the vector can be more generally written as

vector<type> name;

Setting an element of the vector to a specific value is the same as it is for an array. We access the element at position and modify it. Using the previous example with vector v, we modify the element at the th position to be .

cpp

v[0] = 0;

cout << v[0]; // 0
cout << v[1]; // 2

There are many ways of initializing a vector. Here are some common types:

cpp

vector<int> nums = {1, 2, 3, 4, 5};
vector<int> allOne(5, 1); //5 is the amount of elements, 1 is what the values are set to
vector<int> allZero(5); //5 is amount of elements, all the elements are set to 0
int arr[] = {1, 2, 3, 4, 5};
vector<int> copyArr = {arr, arr + (sizeof(arr) / sizeof(arr[0]))};
vector<int> copyVector = {nums.begin(), nums.end()};

Each of the types listed above are valid, but the first three are most common. Vectors have a variety of other methods, a few significant ones are listed below:

Method	Function
	Adds an element `e` of type `type` to the end of the vector. `type` must end the datatype that the vector contains.
	Removes the last element in the vector.
	Returns an `int`, giving the amount of elements in the vector.
	Returns a `bool`, saying if the vector is empty or not.
	Removes all elements in the range . If the second parameter is left blank, it removes the element at
	Removes all elements of the vector.
	Returns the last element in the vector.
	Returns the iterator to the first element of the vector.
	Returns the iterator to the last element of the vector.
	Inserts an element `e` of type `type` where `type` is the datatype that the vector contains to the given position.
	Points to the last element. Adding 1 points to the element before.
	Points to the imaginary element before the first. Adding 1 points to the element before.

Iterator

As seen in the chart above, a few methods return or accept an iterator. The iterator points to an element in some sort of data structure: in this case it points to an element of a vector.

The type of the iterator for a data structure of type t is t::iterator. A vector<string> has a iterator of type vector<string>::iterator. In practice, auto is used instead of typing the entire name.

To fetch the value the iterator points to, we use the * operator.

cpp

vector<int> nums = {1, 2, 3, 4, 5};
auto first = nums.begin();
cout << *first; // 1

With an iterator, it is possible to traverse through a vector:

cpp

for (auto it = nums.begin(); it != nums.end(); it++) {
	cout << *it << "\n";
}

However, it is also possible to just access elements at an index:

cpp

for (int i = 0; i < nums.size(); i++) {
	cout << nums[i] << "\n";
}

Even easier, you can use an enhanced for loop (also known as the for-each loop):

cpp

for (auto e : nums) {
	cout << e << "\n";
}

Vector ​

Iterator ​

Vector

Iterator