Lists

Introduction

List is a collection similar to array but without specific size.

There are couple of things to remember about it:

List

• Element at random index cannot be accessed directly. To do it an iterator must be used.
• It's suitable for large sets of data.
• A size (elements count) cannot be defined.

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

#include <list>

Also if there is need of access to element at specific index, the following needs to be included:

#include <iterator>

Syntax

list<type> listName

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

Initialization

Empty List

list<int> numbers;

Initial Elements

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

Looping elements

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

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

Output:

32 22 45

Adding element to the back

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

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

Size

Size is the count of elements in collection.

In order to get it the size function is used.

Example:

#include <iostream> #include <list> using namespace std; int main() { list<double> heights{160, 170, 180}; cout << heights.size(); }

Output:

3

Accessing specific index

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

#include <iostream> #include <string> #include <list> // Used to access element at specific index: #include <iterator> using namespace std; int main() { list<string> colors; colors.push_back("blue"); colors.push_back("green"); colors.push_back("pink"); colors.push_back("white"); colors.push_back("violet"); // Using iterator reach element at index 3. auto iterator = colors.begin(); advance(iterator, 3); string colorAtIndex3 = *iterator; cout << colorAtIndex3; }

Output:

white

There are couple of things to notice:

• In order to access element at specific index the iterator library needs to be added (line 6).
• The iterator starts from the 0-th index of collection - line 21.
• Afterwards it's moved to the 3-rd index - line 22.
• The iterator points to a specific element which is accessed at line 24.

Outside of bounds

Iterator which moves outside ot list bounds results in exception.

Let's look at the following:

#include <iostream> #include <string> #include <list> // Used to access element at specific index: #include <iterator> using namespace std; int main() { list<string> colors; colors.push_back("blue"); colors.push_back("green"); colors.push_back("pink"); auto iterator = colors.begin(); advance(iterator, 5); // Results in exception. }

The collection has 3 elements but the iterator moves 5 elements ahead. This causes an exception.