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get, set

Read or set the value of a variable or a kdb+ data file

get

Read or memory-map a variable or kdb+ data file

get x     get[x]

Where x is

  • the name of a global variable as a symbol atom
  • a file or folder named as a symbol atom or vector

returns its value.

Signals a type error if the file is not a kdb+ data file.

Used to map columns of databases in and out of memory when querying splayed databases, and can be used to read q log files, etc.

q)a:42
q)get `a
42

q)\l trade.q
q)`:NewTrade set trade                  / save trade data to file
`:NewTrade
q)t:get`:NewTrade                       / t is a copy of the table
q)`:SNewTrade/ set .Q.en[`:.;trade]     / save splayed table
`:SNewTrade/
q)s:get`:SNewTrade/                     / s has columns mapped on demand
value is a synonym for get

By convention, value is used for other purposes. But the two are completely interchangeable.


q)value "2+3"
5
q)get "2+3"
5

eval, value

set

Assign a value to a global variable
Persist an object as a file or directory

nam set y                  set[nam;y]                     / set global variable nam
fil set y                  set[fil;y]                     / serialize y to fil
dir set t                  set[dir;t]                     / splay t to dir
(fil;lbs;alg;lvl) set y    set[(fil;lbs;alg;lvl);y]       / write y to fil, compressed
(dir;lbs;alg;lvl) set t    set[(dir;lbs;alg;lvl);t]       / splay t to dir, compressed
(dir;dic) set t            set[(dir;dic);t]               / splay t to dir, compressed

Where

alg   integer atom     compression algorithm
dic   dictionary       compression specifications
dir   filesymbol       directory in the filesystem
fil   filesymbol       file in the filesystem
lbs   integer atom     logical block size
lvl   integer atom     compression level
nam   symbol atom      valid q name
t     table
y     (any)            any q object

Compression parameters alg, lbs, and lvl
Compression specification dictionary

Examples:

q)`a set 42                         / set global variable
`a
q)a
42

q)`:a set 42                        / serialize object to file
`:a

q)t:([]tim:100?23:59;qty:100?1000)  / splay table
q)`:tbl/ set t
`:tbl/

q)(`:ztbl;17;2;6) set t             / serialize compressed
`:ztbl

q)(`:ztbl/;17;2;6) set t            / splay table compressed
`:ztbl/

Anymap write detects consecutive deduplicated (address matching) top-level objects, skipping them to save space; since V4.1t 2021.06.12.

q)a:("hi";"there";"world")
q)`:a0 set a
`:a0
q)`:a1 set a@where 1000 2000 3000
`:a1
q)(hcount`$":a0#")=hcount`$":a1#"
0b

Splayed table

To splay a table t to directory dir

  • dir must be a filesymbol that ends with a /
  • t must have no primary keys
  • columns of t must be vectors or compound lists
  • symbol columns in t must be fully enumerated

Splayed tables

Format

set saves the data in a binary format akin to tag+value, retaining the structure of the data in addition to its value.

q)`:data/foo set 10 20 30
`:data/foo
q)read0 `:data/foo
"\376 \007\000\000\000\000\000\003\000\000\000\000\000\000\000"
"\000\000\000\000\000\000\000\024\000\000\000\000\000\000\000\036\000..
Setting variables in the KX namespaces can result in undesired and confusing behavior.

These are .h, .j, .Q, .q, .z, and any other namespaces with single-character names.

Compression

For

(fil;lbs;alg;lvl) set y   / write y to fil, compressed
(dir;lbs;alg;lvl) set t   / splay t to dir, compressed

Arguments lbs, alg, and lvl are compression parameters.

Splay table t to directory ztbl/ with gzip compression:

q)(`:ztbl/;17;2;6) set t
`:ztbl/

For

(dir;dic) set t            / splay t to dir, compressed

the keys of dic are either column names of t or the null symbol `. The value of each entry is an integer vector: lbs, alg, and lvl.

Compression for unspecified columns is specified either by an entry for the null symbol (as below) or by .z.zd.

q)m1:1000000
q)t:([]a:m1?10;b:m1?10;c:m1?10;d:m1?10)

q)/specify compression for cols a, b and defaults for others
q)show dic:``a`b!(17 2 9;17 2 6;17 2 6)
 | 17 2 9
a| 17 2 6
b| 17 2 6
q)(`:ztbl/;dic) set t               / splay table compressed
`:ztbl/

Database: tables in the filesystem
File system
File compression
Compression in kdb+