0. Introduction

A program is a set of commands which are designed to be run by a computer.

These commands must be written in a language that the computer can understand.

But languages that computers understand are difficult for humans, because they differ very much from spoken human languages. Writing programs in "machine code" is hard: creating the program is very error-prone and correcting (or improving) it is very time-consuming.

For example, a program which stores a value "2" at the memory location 1, using a simple 80s computer, based on a Z80 8-bit processor, would look like this:

0011 1110 0000 0010 0011 1010 0001 0000

Almost illegible. That’s the reason why the most commonly used languages are similar to human language. These are called "high-level languages." Typically, they are very similar to English language, but they follow stricter rules.