14.3. Units

| Chapter 14: Numbers and Equations |

| 14.3. Units

Suppose we want to talk about how tall people are. We could just create a "number" property, like this:

> A person has a number called height.

But then we would have to write lines like "Isabella has height 68", which nobody would naturally say. What we want is to be able to write "Isabella is 5 foot 8." Perhaps the computer will need to store that measurement as the number 68 in some register or other, but we don't want to know about that.

"5 foot 8" is a complicated notation in a way - it involves both feet and inches - so let's start with a simpler example:

> A weight is a kind of value. 10kg specifies a weight.

This is a little different to the kinds of value seen so far, which were all created like so:

> A colour is a kind of value. The colours are red, green and blue.

We can't mix the two styles: a new kind of value will either be numerical at heart ("10kg") or verbal at heart ("blue").

The effect of "10kg specifies a weight" is to tell Inform that this is the notation for writing a constant "weight". So, for instance,

> The maximum load is a weight that varies. The maximum load is 8000kg.

> if the maximum load is greater than 8000kg, ...

Inform is then careful not to allow weights to be mixed up with other numerical values. For instance, it won't allow "if the maximum load is 400", because 400 is a number, not a weight.

More or less anything we can do with numbers, we can now do with weights. For instance, we can write:

> The Weighbridge is a room. "A sign declares that the maximum load is [maximum load]."

...which will produce the text "A sign declares that the maximum load is 8000kg."

Numerical kinds of value are sometimes called "units", because one of their main uses is to allow us to write quantities using scientific units such as kilograms. But they have other uses too. We have a great deal of freedom in creating notations like "10kg", or "4 foot 10" - the main thing is that new notations must not already mean a value. So "10 specifies a weight" will not be allowed, because 10 specifies a number already.

Sometimes it is unnatural to write negative values, and so Inform will issue a Problem message if this is tried - for instance, Inform would not allow us to write a weight of -4 kg. (This doesn't mean that arithmetic on units is forbidden to get a negative result: we may want to work out the difference between two weights. Inform's Problem message is simply to try to prevent the accidental writing of incorrect values.) If we do want the ability to write negative values in the source text, we signal that in the notation itself:

> -10 kg specifies a weight.

That alerts Inform that both positive and negative values for this unit make sense.

If we set up a spread of multiple notations (see the next section) then this is automatically enabled, because then we're clearly dealing with proper physics, where negative values are common.

245
| Example rBGH The player character's height is selected randomly at the start of play. |
RB

246
| Example Wonderland Hiking Mount Rainier, with attention to which locations are higher and which lower than the present location. |
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Suppose we have a landscape with a great deal of up and down variation, where GO UP and GO DOWN will be significant almost everywhere, and specifying them all individually a tremendous pain: "Wonderland" An altitude is a kind of value. 1000 feet specifies an altitude. A room has an altitude. Definition: a room is low if its altitude is 3000 feet or less. Definition: a room is high if its altitude is 5000 feet or more. Instead of going down: if an adjacent room is lower than the location: let the valley be the lowest adjacent room; let the way be the best route from the location to the valley; say "(that is, [way])[paragraph break]"; try going the way; otherwise: say "You're in a local valley: there's no down from here." Instead of going up: if an adjacent room is higher than the location: let the peak be the highest adjacent room; let the way be the best route from the location to the peak; say "(that is, [way])[paragraph break]"; try going the way; otherwise: say "You're on a local peak." Paradise is a room. Paradise has altitude 5400 feet. "A handsome parking lot, a picnic ground, and the Henry M. Jackson Memorial Visitor Center. The latter offers, for serious climbers, a hot shower; for nature enthusiasts, an interpretive museum; and for car-trippers, a gift shop selling canned slugs. All of which is a largely unsuccessful distraction from the peak of Mt. Rainier beyond." Cougar Rock is southwest of Paradise. The altitude of Cougar Rock is 3180 feet. "Numerous individual campsites and (on the road inventively labeled 'F') a handful of larger campgrounds suitable for church groups and family reunions." Longmire is southwest of Cougar Rock. It has altitude 2760 feet. "A tiny town: it has to offer a few groceries, a post office, and a lodge for people who do not care to camp, all built in a rustic Park Service way." Panorama Point is north of Paradise. It has altitude 6800 feet. Camp Muir is north of Panorama Point. It has altitude 10188 feet. Columbia Crest is northwest of Camp Muir. It has altitude 14410 feet. St Andrews Rock is west of Columbia Crest. It has altitude 10992 feet. Camp Schuman is northeast of Columbia Crest. It has altitude 9510 feet. Since Mount Rainier National Park runs to over 235,000 acres, we will omit the rest of the locations, but it does seem fair to give a little more credit to anyone who makes the summit: Instead of going up in the highest room: say "You're standing at the summit of Mt. Rainier, the highest point in the state of Washington. There is no up." Test me with "up / up / up / down / down / up / up".

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