Help sheet 114: 


What is a variable  what is a constant.
The Picaxe has 14 "byte" variables called b0 to b13.
They are places where you can save numbers, like the memory function on a
calculator.
Constants are simple numbers like "1" or "17" or "%01100000".
When you use a constant in a program, its value is always the same.
When you use b0 or b7 in a program the value depends on what was stored
there from an earlier part of the program.
How do I store a value into a variable?
To set b0 to have the value 6 use "let
b0 = 6"
Maximum value you can store in a variable
"Byte" variables (b0  b13) can only hold values in the range 0 to 255.
There are other variables (called "word" variables) that can store values bigger than 255.
What Maths can you do in a picaxe?
You can Add Subtract Multiply or Divide. There are simple "add 1" and
"subtract 1" operators.
"let
b0 = b3 + b4"
 adds b3 and b4 and stores the answer in b0.
"let
b3 = b3 * 7"  multiplies
b3 by 7 and stores the answer in b3.
"let
b1 = b3  b4"
 subtracts b4 from b3 and stores the answer in b1.
"let
b0 = b3 / 3"  divides B3 by 3
and stores the answer in b0.
"inc
b0"  increases the value of b0 by 1.
"dec
b1"  decreases the value of b1 by 1.
How do I add 1 to a variable?
To add 1 to b3 use either ...
 "let
b3 = b3 + 1"
.. or
 "inc
b3 "
Where can I use variables?.
Variables can be used in most places where you would use constants:
for example:
"pause
b0"
"let
pins = b7"
You have to use a variable immediately after a "let"
You can't use variables for port numbers: "high
b0" is not allowed
What happens when you add 1 to 255.
If you try to store a number greater than 255 into a byte variable (b0b13)
the number is reduced automatically by subtracting 256.
(If it's still greater than 255 after subtraction, a further 255 is subtracted)
So if you add 1 to 255 you get 0. if you try to set b0 to 300 you get
(300256 = 44)
What happens when you subtract 1 from 0.
If you try to store a number less than 0 into a byte variable (b0b13) the
number is increased automatically by adding 256.
(If it's still less than 0 after addition of 256, a further 255 is added)
So if you subtract 1 from 0 you get 255.
Suppose you want to double the contents of a variable.
...you add it to itself or multiply it by 2
"let
b3 = b3 +
b3" or "let
b3 = b3 *
2"
Where you can look this up.  see the help pages in programming editor.
Where you can try things out.  use the basic simulator in programming editor.
Where else you can try things out.  use the
'Let' simulator
in these pages.
Content Scl Version 20/04/07