1. Generalities
2. Variable names
3. Types of variables
4. Arrays
5. Scope

1. Generalities
Variables in Aptilis, like in any other languages, are 'boxes' where values can be placed. One of the main advantages is to save some typing. It is easier to use 'pi' than having to type '3.14159265' all the time.
This is how you fill a variable with a value:

    var = 16

And you can use the variable for its value at any time:

    print(16)

    print(var)

You don't have to declare variables, Aptilis reserves some space for them as they're created, i.e. when you first mention them.

2. Variable names
Variable names can be made up of letters and numbers but have to start with a letter. Names can be of any length. (but keep to a manageable wingspan...)
Meaningful variable names help a lot when it comes to maintenance.
Valid names: a, b, ab, a1, rate, rate5, etc...
Incorrect names: 1, 1a, tax&profit, etc...
What applies to variable names also applies to sub names.

Variable names are case sensitive. 'A' is not the same as 'a'.

3. Types of variables
In aptilis, there is not really a problem of variable types. All variable are stored as text, and their numeric value is calculated when necessary. Sometimes it has to be indicated in which context you want an instruction to handle a variable. By default, the context is numeric. If you want to work in a string context, you have to use the dollar sign.
Example: a = "Hello" $
If you had forgotten the dollar sign, Aptilis would have tried to evaluate the string "Hello" as a number and would have filled 'a' with the value '0.0000'.
a = "hello" $
print(a) produces '0.0000'
print(a$) produces 'hello'

In fact, a$ and a are indeed the same variable unlike in some other close languages. Note that the dollar sign is added at the end of any expression as in:
print(left("hello", 2)$)
to get 'he' and not '0.0000'.

4. Arrays
In aptilis, all variables are arrays, even if you don't realize it!
For example to create an array with the first 10 multiples of 3 you can do:

for i=1 to 10
  m3[i] = i * 3
end for

Note that 'var[0]' is another way to write 'var'. That is you need not use the notation [0] to access the first element of an array.
You do not need to declare arrays prior to using them. Arrays grow as needed when you use them.
Arrays are used to retrieve values of 'MULTIPLE' select boxes in HTML. If you have such a box like in:

       <SELECT name=choice MULTIPLE>
       <OPTION>Pizza
       <OPTION>Lasagne
       <OPTION>Pasta
       <OPTION>Chocolate Ice Cream
       </SELECT>

The options selected by the user are returned in the choice[] array. You can check how many there are with the line:

n = getArraySize(choice[])

String values of array elements are assigned and accessed in the following fashion:

a[2] = "Greetings!" $
print(a[2]$)

5. Scope
Variables in Aptilis may be local or global. But as nothing can happen outside of a sub, even global variables need to be assigned from within a sub.
Being global means that you retain your value across subs. A global variable given the value '24' will retain that value in all subs. Local variable cease to exist outside the sub they appear in.
A corollary to that is that you can have a variable with the same name in different subs without interference, but a global variable such as '_a' is the same for every one.
Global variables need an underscore before their name.
_taxPercent is global
revenue is local.