Arrival: within

A unit will never stand exactly on a point. The question to ask is not «is it there» but «is it close»

mediumYou need: Movement and stopping, Whole numbers and precision

steptick 0

Unit Bind0

type

Unit Control1

typemovexy

Unit Control2

typewithin?xyradiusresult

Sensor3

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 in

@counter0number

arrivednullnull

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ucontrol move 16 4 0 0 0
ucontrol within 16 4 2 arrived 0

While the poly is flying, arrived is zero; once it comes within two tiles of the target, it becomes one. Note x: the unit settled around 15.87, not exactly on sixteen.

Field	What is in it
first	within
second and third	a point in tiles
fourth	the radius, also in tiles
fifth	where to put the answer: 1 or 0

Silent failure

jump … equal x 16 will never fire. A unit’s coordinates are fractional numbers, and there is no hitting a whole one exactly with them. Even if the unit stood perfectly, the engine computes position in fractions, and 15.999998 is not 16.

A distance check is the only correct way to ask “is it there yet”.

within is a question, not an order

within does nothing to the unit: it only answers. So it does not take control — a unit nobody has commanded will not fall under control from a within alone. unbind behaves the same way.

But control that has already been taken is extended by within: if the unit was commanded earlier, every question refreshes the ten-second term just as an order does.

ucontrol move 16 4 0 0 0
ucontrol within 16 4 2 arrived 0
jump 0 equal arrived 0

Here move is repeated together with the check, and that is the right shape of waiting: the order keeps the unit on course rather than merely holding the term open.

What radius to take

Radius	When
1–2 tiles	ordinary “has arrived”
3–5 tiles	switching to the next waypoint, where precision does not matter
transfer range	before itemTake and itemDrop: the unit has to get right up close

Too small a radius and the unit is for ever “almost there”: inertia will not let it stop half a tile from the target. Too large, and the program decides it has arrived while the store is still a flight away.

Note

approach and within are different things. approach tells a unit to stop at a distance, within asks whether it is close. They are often confused because of the similar third field, but the first is an order and the second a question.

Remember

• ucontrol within x y radius answer puts down 1 or 0: whether the unit is close.
• Coordinates cannot be compared directly — they are fractional.
• within does not take control, but it extends control already taken.
• The radius is chosen to fit: 1–2 tiles for arrival, more for waypoints.

Task

Change the radius from 2 to 0.1 and run it: arrived stays zero — a unit will never stand that precisely.

Then set the radius to 20: the one appears at once, before the unit has even moved. The whole map fits inside the check’s radius.