Loops

The same jump, only the arrow points up — and the program starts repeating a piece of itself

mediumYou need: Condition and branching, Arithmetic

A loop in mlog is not a separate instruction and not a special way of writing things. It is the same jump with its arrow pointing up: at a line that has already run.

The example adds up the numbers from zero to four. Press “step” seventeen times — or “run” once, then pause.

steptick 0

Set0

=

Set1

=

Operation2

=

+

Operation3

=

+

Jump -> 24

if <

End5

@counter0number

countnullnull

totalnullnull

total becomes ten: 0 + 1 + 2 + 3 + 4. count stops at five — the value at which the condition stopped holding.

The three parts of a loop

Part	In the example	What for
preparation	set count 0, set total 0	clear whatever the loop will accumulate
body	op add total total count	the reason for all of this
step and check	op add count count 1, jump	move on and decide whether another iteration is needed

The check here is at the end, and that matters: the body runs at least once even if the condition was false from the start. Such a loop assembles itself, and most of the time it is the one you want.

If the body must not run at all, the check goes at the top: a jump forward, past the end of the loop, with an unconditional jump backwards at the end of the body. Two jump lines instead of one.

Preparation is compulsory

A program runs in an endless loop, and the second iteration does not start from a clean slate: the variables hold what was left from the first. So set count 0 is not a formality but part of the loop.

Silent failure

Remove the first line from the example and the first iteration works as before, while every following one gives a seemingly random number: 5, then 6, then 7. The body runs exactly once, because count arrived already above five and the condition is false straight away.

That is the most common way a loop breaks, and it shows in the variables table at once — a counter that does not start over.

An endless loop does not freeze the game

A loop with no way out is no catastrophe. A processor runs exactly as many instructions per tick as it is allowed (@ipt: two on a micro processor, eight on a logic one, twenty-five on a hyper) and gives control back to the game. It will spin for ever, and the game meanwhile goes on as usual.

Something else is bad about it: the program will never reach the other lines. Everything below the loop will never run again — the processor is busy.

Note

If you need to “wait”, do not write a waiting loop: wait exists for that, and it spends ticks properly instead of eating them. That is the lesson “Wait”.

How many ticks a loop takes

Eighteen instructions on a micro processor is nine ticks, about a seventh of a second. Such a loop cannot be seen on “run”, which is why it is walked a step at a time.

The longer the loop, the less often the program returns to the top. That is not a fault but a price: logic in Mindustry has no parallelism, and the only ways to get more done are a faster processor or a shorter loop.

Remember

• A loop is a jump with the arrow pointing up; there is no separate instruction for it.
• The parts of a loop: preparation, body, step and check.
• A check at the end means the body runs at least once; a check at the top needs a second jump.
• Without clearing the variables the second iteration computes the wrong thing.
• An endless loop does not freeze the game, but nothing below it will ever run again.

Task

Change the five in the condition to 10 and run the example again: total becomes 45 — the sum of the numbers from zero to nine.

Then drag the jump’s arrow onto the line op add count count 1 instead of op add total total count. The addition drops out of the loop’s body, and total stays zero for ever, though the loop keeps spinning dutifully up to five.