Variables and constants
If I wish to print the results of multiplying a fixed number by another
that adopts values between 0 and 9, the normal way to program
this would be to create a CONSTANT for the first number and a
pair of VARIABLES for the second and for the result of the product.
A variable, in reality, is not more than a name to identify one (or
several) memory positions where the program keeps the different
values of the same entity. A program must DEFINE all the variables
that it will use, before starting to use them, in order to indicate to
the compiler what type they will be, and therefore how much
memory should be allocated to house each of them.