Enumeration

For a long list containing few distinct values, an enumeration can reduce storage requirements. The ‘manual’ way to create an enum (for understanding, not recommended):

q)y:`a`b`c`b`a`b`c`c`c`c`c`c`c
q)x:`a`b`c
q)show e:"i"$x?y;
0 1 2 1 0 1 2 2 2 2 2 2 2i  /these values are what we store instead of y.
q)x e                       /get back the symbols any time from x and e.
`a`b`c`b`a`b`c`c`c`c`c`c`c
q)`x!e / same result as `x$y 
`x$`a`b`c`b`a`b`c`c`c`c`c`c`c

$ (enum)

Syntax: x $ y

Where x and y are lists, x~distinct x, and distinct y are all items of x, returns y as an enumeration of x. Using built-in enum:

q)show e:`x$y;
`x$`a`b`c`b`a`b`c`c`c`c`c`c`c

Values are stored as indexes and so need less space.

q)"i"$e
0 1 2 1 0 1 2 2 2 2 2 2 2i

Changing one lookup value (in x) has the same effect as changing those values in the enumeration, while the indexes backing e are unchanged.

q)x[0]:`o
q)e
x$`o`b`c`b`o`b`c`c`c`c`c`c`c
q)"i"$e
0 1 2 1 0 1 2 2 2 2 2 2 2i

To get x and y from e:

q)(key;value)@\:e
`x
`o`b`c`b`o`b`c`c`c`c`c`c`c

Ensure all items of y are in x

When creating an enumeration using $, the domain of the enumeration must be in x, otherwise a cast error will be signalled.

q)y:`a`b`c`b`a`b`c`c`c`c`c`c`c
q)x:`a`b
q)`x$y
'cast

To expand the domain, use ? (enum-extend) instead of $.

Q for Mortals: Enumerations

? (enum-extend)

Syntax: x ? y

Where y is a list and x is

• a symbol naming a variable, fills in any missing items in x, returns y as an enumeration of the variable named in x with filling in missing items in it. (Unlike enum.)

  q)foo:`a`b
  q)`foo?`a`b`c`b`a`b`c`c`c`c`c`c`c
  `foo$`a`b`c`b`a`b`c`c`c`c`c`c`c
  q)foo
  `a`b`c
  

  Note that ? retains the attr of the right-argument but $ does not.

  q)`foo?`g#y
  `g#`foo$`g#`a`b`c`b`a`b`c`c`c`c`c`c`c
  q)`foo$`g#y
  `foo$`a`b`c`b`a`b`c`c`c`c`c`c`c
  

• a filepath symbol and y is a list, fills in any missing items in file x, loads it into the session as a variable of the same name, and returns y as an enumeration of it.

  q)bar:`c`d  /about to be overwritten
  q)`:bar?`a`b`c`b`a`b`c`c`c`c`c`c`c
  `bar$`a`b`c`b`a`b`c`c`c`c`c`c`c
  q)\ls -l bar
  "-rw-r--r--  1 sjt  staff  16  3 Mar 12:53 bar"
  q)bar
  `a`b`c
  

! (enumerate)

Syntax:`x ! y

Where x is a symbol atom naming a variable containing a symbol vector, and y is an int vector of indexes into x, returns an enumerated symbol list.

q)x:`a`b`c`d
q)`x!1 2 3
`x$`b`c`d