Re: Storing units in the database

> 20 feet = 20*0.3 meters
> Now, when applied to the database, how should we store 20 feet? User
> defined types provide us some (quite unsophisticated) solution. But
> do we really have to program a class every time we want to do trivial
> conversions?

Below script models 20 ft, 20 sec, conversion from ft to meter, etc in xrdb.
Application code (OO or not) should perform conversion based on data in db.

// Create type distance, time, qty and unit
// and make them items of main directory.
// Note: "inst" is short for "instance".
// Note: "it" refers to last new thing created by "*" in CREATE stmt.


(CREATE type inst *distance  & dir item it)

(CREATE type inst *time      & dir item it)

(CREATE type inst *qty       & dir item it)


(CREATE type inst *unit      & dir item it)

// Create 20 ft.
(CREATE distance inst *

      & it qty +20
      & it unit +feet)

// Create 40 ft.
// Note: db auto normalized unit "feet"
(CREATE distance inst *

      & it qty +40
      & it unit +feet)

// Create 20 sec.
// Note: db auto normalized qty "20".
(CREATE time inst *

      & it qty +20
      & it unit +second)

// Create abbreviation for feet and second.
(CREATE type inst *abbr)
(CREATE feet abbr +ft)
(CREATE second abbr +sec)

// Find all things whose qty is 20.
// Returns 20ft and 20sec.
(SELECT * qty 20)

// Find all things whose qty is abbreviated as "ft". // Returns 20ft and 40ft.
(SELECT * unit (* abbr ft))

// Create verb height to relate distance as height of things.
(CREATE verb inst *height)

// Create person John's height is 20 ft.
(CREATE type inst *person & dir item it)
(CREATE person inst *john & it height (SELECT * qty 20 & * unit
feet))

// Create tree1's height is 20 ft.
(CREATE type inst *tree & dir item it)
(CREATE tree inst *tree1 & it height (SELECT * qty 20 & * unit feet))

// Find all things that are 20 ft in height. // Returns john and tree1.
(SELECT * height (SELECT * qty 20 & * unit feet))

// Create conversion rate from feet to meter // and vise versa.
(CREATE type inst *"conversion rate")
(CREATE unit inst *meter)
(CREATE meter abbr +m)
(CREATE feet to meter "conversion rate" +0.3)
(CREATE meter to feet "conversion rate" +3.3)

// Find conversion rate from feet to meter. // Returns 0.3
(SELECT feet to meter "conversion rate" *)
Received on Fri Jul 08 2005 - 05:12:28 CEST