Variables and names

A variable isn't declared — it's just named. Which names are free, and which are already taken

easyYou need: Number, object and text, The variables window

mlog has no line that says “declare a variable”. A variable comes into being the moment its name first appears in the program — whether in a field written to or in a field read from.

Everything else in this lesson follows from that.

Variables belong to the processor

Not to a line, not to a piece of the program, not to an instruction — to the whole processor. Every line sees every variable; mlog has no scopes at all.

That is why names have to be meaningful: fifty lines later, the x from the top of the program is still the same x.

A neighbouring processor is a different set of variables, even if the names match. They share nothing: passing a value needs memory or a link, and those get groups of lessons of their own.

Note

A name that is only read also creates a variable — with the value null. That is exactly why a typo in a name is not an error but a new empty variable, and the only place you see it is the table.

Case matters

stock and Stock are two different variables. mlog has no “case-insensitive” comparison of any kind.

steptick 0

Set0

=

Set1

=

Operation2

=

+

Set3

=

@counter0number

stocknullnull

Stocknullnull

totalnullnull

Press “step” four times. Both variables show up in the table — 100 and 5 — and total becomes 105. The program does not consider this an error and does not warn: mlog has nothing to warn with.

Hence a simple rule: pick one way of writing names and stick to it. Non-Latin letters work in names just as well as Latin ones — the game stores the program as UTF-8 text.

What the editor does to what you typed

A block’s field is not a text editor: the game replaces some characters as you type, so that a name cannot break the way the program is written down.

What you typed	What you get
space, tab, #	_
;	s
"	'

So my stock turns into my_stock by itself. This is not the editor being fussy: the program is stored as text, its lines are separated by newlines, and # starts a comment — a space inside a name would make two variables out of one.

Names that are already taken

A name starting with an at-sign belongs to the game: @copper is an item, @time is time, @thisx is the processor’s coordinate. Such names are read-only: an assignment to them goes nowhere, silently and without consequence.

The fourth line of the example is set @copper 7. It does nothing, and @copper never appears in the variables table at all: constants do not go there.

Three names without an at-sign are taken too: true, false and null. They are constants as well — 1, 0 and emptiness — and writing to them fails in exactly the same way.

Silent failure

There is one exception — @counter. It is the number of the line that will run next, and it is an ordinary variable: you can write to it. Writing to @counter is a jump — the program continues from whatever line number you put there.

Jumps are normally written with a separate instruction — the jump group — and why you would ever write to the counter gets a lesson of its own: “@counter: a jump as a value”.

How many variables there can be

There is no limit: as many names as appear in the program, that is how many there are. What is limited is the program itself — 1000 instructions, and at that limit the buttons that add a line go dark.

Remember

• A variable is created by the first mention of its name, reading included.
• All variables are shared across the processor: mlog has no scopes.
• stock and Stock are different variables.
• A space, # and ; in a name are replaced by the editor itself.
• Names with an at-sign, plus true, false and null, belong to the game: writing does nothing.
• The exception is @counter: writing to it is a jump.

Task

Add a fifth line to the example: set Total 1 — with a capital letter. A new variable appears in the table next to the old total, and the one that was being counted does not change.

Then put a space inside a name: “my stock”. The field turns it into my_stock by itself, before you even move on to the next one.