Vectors

Introduction

Vector is a collection similar to array but with no need to specify size.

There are couple of things to remember about it:

Vector

• Element at random index can easily be accessed.
• It's suitable for small sets of data.
• A size can be defined but it's not mandatory to specify it.

In order to use it the following line should be added:

#include <vector>

Syntax

vector<type> vectorName

type specifies the values type - int, double, string, char etc...

Initialization

Empty Vector

vector<int> numbers;

Initial Elements

vector<double> scores { 87.5, 65, 75.8 };

Fixed size

vector<int> vect(5);

5 elements all of which have value of 0. If strings were hold the value of the elements would be empty string - "".

Set Default Value

vector<int> numbers(3, 5);

Creates 3 elements all of which have value of 5.

Adding element to the back

This can be done via the push_back function. Like in the following code:

vector<string> names; names.push_back("Alex"); names.push_back("John");

Accessing specific index

Accessing element at specific index is identical to arrays.

Let's add elements to vector and access the element at index 1.

#include <iostream> #include <vector> using namespace std; int main() { vector<int> numbers; numbers.push_back(5); numbers.push_back(6); numbers.push_back(7); cout << numbers[1]; }

Output:

6

Outside of bounds

Accessing element outside of vector's bounds results in exception.

Let's look at the following example

#include <iostream> #include <vector> using namespace std; int main() { vector<int> numbers{ 15, 20, 35 }; cout << numbers[3]; // causes exception }

numbers has 3 elements (indexes from 0 to 2) but at line 10 the 3rd index is accessed.

Size

Size is the count of elements in collection.

In order to get it the size function is used.

Example:

#include <iostream> #include <vector> using namespace std; int main() { vector<int> numbers {15, 20, 35}; cout << numbers.size(); }

Output:

3

Looping elements

Let's have a vector of ages and print only those who are greater than 20.

#include <iostream> #include <vector> using namespace std; int main() { vector<int> ages{ 18, 32, 22, 45, 19 }; for (int age : ages) { if (age > 20) { cout << age << " "; } } }

Output:

32 22 45