The variables window

The only place that tells the truth about what the program has computed

easyYou need: Writing it in the game, Number, object and text

mlog has neither error messages nor crashes. A program with a typo does not complain — it silently computes the wrong thing. That makes the second most important button under the program window “Variables”: it shows the contents of the processor’s memory at this moment.

On the course pages that table is always open to the right of the lines: in an example you need to look at it more often than you do in the game.

What is in a row

A row of the table is three cells: name, value, type. The game names the type in its own word and paints it in its own colour:

Type	What it is
number	a number
null	empty: the variable was never written to
string	text in quotes
content	an item, a liquid, a block or a unit type
building	a building — the thing a link gives you
unit	a unit
team	a team
enum	a value from a list, like enabled

The table usually starts with @counter — the number of the line that will run next. Then your variables, in the order they first appeared in the program.

Not everything is visible

Constants do not make it into the table: @copper, @this, @ipt, @links and link names (cell1, switch1) will not show up there — they do not change, so there is nothing to show.

Their list opens with the “Built-in variables” button in the same window; here it lives on the built-in variables page.

A typo is visible at once

The example below is the program from the previous lessons, but with one letter out of place: the second line says stok, not stock. Press “run” and look at the table.

steptick 0

Set0

=

Operation1

=

*

Operation2

=

+

@counter0number

stocknullnull

doublednullnull

stoknullnull

totalnullnull

total is five instead of twenty-five. Why, is visible in the same place:

• stok is an extra row with the value null. For a name that was never assigned, mlog creates a variable of its own and puts null in it. An unfamiliar name is not an error to it, but a new variable.
• doubled is zero. In arithmetic null counts as zero: zero times two is zero.
• total is five. Zero plus five. The program ran honestly, just with the wrong number.

This is the most common way to break a program, and it is cured with your eyes: two nearly identical rows in the table mean you made a typo.

Note

The second common case is a variable that does not change at all. That means the line writing it never runs: there is a jump or an end above it. Where the program goes is shown by @counter, and it is the best answer to “does it even get here”.

Values do not refresh instantly

In the game the table re-reads its values once every fifteen ticks — about four times a second — and a changed number flashes yellow. That is not a delay in the counting but the rate of display: in that time the processor makes hundreds of steps, and you cannot see each one.

While the variables window is open the game is not paused: the program window pauses it and the variables window unpauses it — otherwise the numbers would not change.

In the course examples the table refreshes every frame, and the “step” button lets you look at exactly one instruction — the game cannot do that.

Silent failure

Any edit to the program clears every variable and returns the counter to the first line. So “fix a line and see what has accumulated” will not work: after an edit nothing has accumulated.

Remember

• “Variables” shows the name, value and type of every variable of the processor.
• Constants and links are not shown there — “Built-in variables” is for them.
• null in a row means the variable was never written to; in arithmetic that is zero.
• Two nearly identical rows in the table are a typo in a name.
• In the game the table refreshes four times a second, and an edit clears everything.

Task

Fix the example: open the second line, type stock instead of stok and press “restart”. The extra row disappears from the table, doubled becomes twenty and total twenty-five.

Then break it differently: type stock in the first line with a different typo, say stck. A second nearly identical row appears in the table again — and now you know what that means.

Harder debugging is in the lesson “When it does not work”.