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KDB/Q (operators (/ amend q) a:3 q) a+:1, highlights operators do not…

KDB/Q

data types

symbol

/ cast a string to symbol
q)`$"A symbol with blanks and `"

temporal data

q)`int$2000.02.01
31i
/ dot notation
q)dat:2018.04.27
q)dat.year
q)dat.mm
q)dat.dd
/ month
q)2015.11m / not 2015.11, which is a float
/ second
q)23:59:59=-1+24*60*60
1b
/ timestamp
q)2014.11.22D17:43:40.123456789

Tip: casting is preferred over dot notation since the latter does not work inside functions. See in #transforming data

guid

/ guids are generated
q)1?0Ng
/ or imported
q)"G"$"61f35174-90bc-a48a-d88f-e15e4a377ec8"

infinities and nulls (typed)

/ pos-float inf, neg-float inf, null float, pos-long inf, neg-long inf, null long
q)0w -0w 0n 0W -0W 0N
q) null 0N
q) null 0Ng
/ In q, on can also create typed infinity
q)0Wh / with the "h" being the type code

A few principles in q with arithmetics:

division of a pos value by 0 results pos infinity
division of zero by zero is undefined, 0n

Here is the list of nulls for all data types in q

data structure

dictionary

creating

/ extendable, unlike list
q) D:(enlist `a)!(enlist 10)
/ empty dictionary
q) d:()!()
/ typed empty dictionary
q)(`symbol$())!`float$()
/ enforce hashing
q) D:(`u#`a`b`c)!10 20 30

query

q) D:`a`b`c!10 20 30
q) D `a`b / justaposition
10 20
q) D?10
`a
q) null D?50
1b

operations

arithmetics

q) d1: 1 2 3!4 5 6
q) d2: 3 4 5!6 5 4
q) d3:d1+d2
q) d3 3
12
/ missing data during operation
q)d:(`a`b`c!0N 20 30)=`b`c`d!20 300 0N
q)d `a
1b

derive

q)d: `a`b`c!10 20 30
q)d `a`c
q)`a`c#d / derive with inclusion
q)(enlist `c)#d
q)`a`c _ d / derive with exclusion
q)`a`c cut d / same as overload of _
/ join, the-right prevails, non-cumulative
q)d1:`a`b`c!1 2 3
q)d2:`b`c`d!20 30 40
q)d1,d2

column dictionary foundation of table

creation

c1...cn!(v1;...;vn)

q)d:`c1`c2!(`a`b`c; 10 20 30)
q)d
c1| a b c
c2| 10 20 30
q)d[`c1][0]
`a

indexing

dc:c1...cn!(v1;...;vn)
the ith element of column cj is retrieved by:
dc[cj][i], or
dc[cj;i]

q)travelers:`name`iq!(`Dent`Beeblebrox`Prefect;42 98 126)
q)travelers[; 2]
name| `Prefect
iq  | 126

table

([] c1:L1; ...; cn:Ln)

creation
([] c1:L1; ...; cn:Ln)

/ from a dictionary
q)t:flip `name`iq!(`Dent`Beeblebrox`Prefect;98 42 126)
/ create an empty table
q)t:([key1:();key2:()] col1:();col2:())
/ create a typed empty table by casting empty list
q)t:([`symbol$key1:`$();`symbol$key2:`$()] col1:`$();col2:`$())

query

/ indexing in depth, the first index specifies record, the second specifies column
q)t:flip `name`iq!(`Dent`Beeblebrox`Prefect;98 42 126)
/ get a row
q)t[0] / which is a dictionary
/ so that you can use key or value builtin on it
q)t[0;`name]

functional form of Q-sql

![t;c;b;a]                /update and delete
?[t;c;b;a]                /select and exec
?[t;i;p]                  /simple exec

key-ed table
key-ed table is not a table, it is a dictionary, demonstrating this:

q)v:flip `name`iq!(`Dent`Beeblebrox`Prefect;98 42 126)
q)k:flip (enlist `eid)!enlist 1001 1002 1003
q)kt:k!v

as a result, you can access the row using dictionary key lookup:

q)kt[(enlist `eid)!enlist 1002] /or
q)kt ([] eid:1001 1002) /or
q)([] eid:1001 1002)#kt / which is drawing

reverse look up as with a dictionary

q)kts:([eid:1001 1002 1003] name:`Dent`Beeblebrox`Prefect)
q)kts?([] name:`Prefect`Dent)
/ introspecting a key'ed table
q)key kt / which gets the key columns
q)value kt
q)keys kt / which gets key names
q)cols kt / which gets column names from bot key table and value table

several operations around keys of a key'ed table

/add a key with a column of a name
q)`eid xkey kt
q)() xkey kt
/add a key with a column at a position
q)1!kt
q)0!kt

gotcha: If xkey is applied with a column that does not contain unique values, the result is not a error but rather a keyed table that does not have a true primary key.
compound key'ed table

q)ktc:([lname:`Dent`Beeblebrox`Prefect; 
          fname:`Arthur`Zaphod`Ford]; iq:98 42 126)
q)ktc[`lname`fname!`Beeblebrox`Zaphod]
q)ktc[`Dent`Arthur]
q)ktc (`Dent`Arthur;`Prefect`Ford)
q)K:([] lname:`Dent`Prefect; fname:`Arthur`Ford)
q)K#ktc

list

inconsistencies

/ cannot use abbreviated notation of simple lists with variables:
q)a:42
q)b:43
q)a b
`: Bad file descriptor|

/ Assignment via index at depth works but assignment does not work with iterated indexing.
q)L[1][2][0]:42
`assign

operations with list

q)first til 2
0
q)last til 2
2
q) 1 2 3,4 5 / combine
/ merging is more complicated than combine
q)L1:10 0N 30
q)L2:100 200 0N
q)L1^L2 / the right prevails unless it is null
100 200 30
/ distinct
q)distinct 1 2 3 2 3 4 6 4 3 5 6
1 2 3 4 6 5
/ where
q)where 101010b
0 2 4
q)L:10 20 30 40 50
q)L[where L>20]:42
/ group, thinking of itertools.counter in Python
q)group "i miss mississippi"

query

q)L:til 3
q)L[2] / for for a value at an index
q)L 2 / juxtaposition
q)L 10 / this gives null
q)L?2 / look for the index of a value
/ eliding indices for nested list
q)L:((1 2 3;4 5 6 7);(`a`b`c`d;`z`y`x`;`0`1`2);("now";"is";"the"))
q)L[;;2]
3 6
`c `y `2
"w e"
/ also notice how q deals missing data in above example

creating

q)() / empty list
q)enlist `a / singleton list
/ you can also create from an existing list:
q)0#10 20 30
`long$()
q)L:til 3
q)L:1 2 3 / simple list
q)L:(::; 1; 2; 3) / generic list
q)/ check this out:
q)count L:(::; 1;2;3;)
1

transforming data

casting

when casting from one type to another, the conversion is exact sometimes, other times information is lost. Casting can take several forms, in the basic cast form, the left operand can be a char type abbreviation, a type short, or a symbol type name:

q)`int$4.2
q)6h$4.2
q)`int`long$4.2
q)`int`long$(4.2;2.4)
q)"ijf'$98.6
q)"ijf"$10 20 30

q)`int$4.2
 / across disparate types:
q)`char$42
q)`int$2001.01.01
/ we can also extract constituents from complex types:
q)`date$2015.01.02D10:20:30.123456789
2015.01.02

to-from string

quirks:

calling string on string gives you nonsense
string always produces a list of characters

to string:

/ create string from symbol
q)string `Life`the`Universe`and`Everything
/ create symbol from string
q)`$"abc"
q)string `$" abc "
"abc"

from string

q)"F"$"42"
42f
/ lots of flexibility in getting a temporal type from string:
q)"D"$"12.31.2014"
q)"D"$"12/31/2014"
q)"D"$"2014/12/31"

IMPORTANT, you can access q interpreter:

q)value "{x*x}"

IMPORTANT, creating a typed container is done by casting an empty container:

q)`float$()