Leaving notes in code is a good practice. Assume you wrote a large script, used it in your research, and then had to use it again but with few improvements/changes. It's not strange if you will forget what does each part of the code is responsible for. In programming, we call such notes comments.

A comment is a line/part of a line in the code that is not compiled. To make the interpreter understand, starting from where the code must not be compiled, use the hash `#` sign. For example, let's leave two comments: one in a separate line, and one right to some calculation:
```
Input:
# Some calculation
11 + 3 # eleven plus three

Output:
[1] 14
```
As you can see, there will be only the result of `11+3` displayed in the console.

> Please note that using comments is not compulsory but is considered good practice for clean and understandable code.

Comments can also be used while testing to make R ignore some lines of code without removing them.

In the first section, you will consider the very basics of R - outputting and performing basic operations. You will finish the section with the variables theory.

In the second section, you will learn the vector class in R, which represents single values too. You will consider the main data types for single values and how to work with multiple values of a kind at once.

Factors (or categorical variables) are widely used in real life. These are blood types, currencies, nationalities, brands, etc. Unlike numerical variables, factors are limited to a finite number of values.