I am still a KDB noob. This page will be kept updating whenever I have new tricks.
Display string to console**
0N!"Hello World"
Cast string to symbol
ticker:"MSFT"
ticker_symbol:`$ticker
List
Define a list
mylist:(1;2;3)
Get n-th element
(1;2;3)[1]
{x[1]}(1;2;3)
Get first n elements
3#(1;2;3;4;5;6;7;8;9;10)
Get last n elements
-3#(1;2;3;4;5;6;7;8;9;10)
Filter list
even_list:{x where not x mod 2}(1;2;3;4;5;6)
Run function on each element
{0N!x} each ("Hello";"World")
({0N!x}')("Hello";"World")
Convert string list to long list
({"J"$x}')("123";"456";"789")
Command line parameters
This is the command we use.
rlwrap q myscript.q -foo "bar" "123"
.z.x
arguments:.z.x
/ arguments is a list (0h) of ("-foo";"bar";"123")
.z.X
arguments:.z.X
/ arguments is a list (0h) of ("q";"myscript.q";"-foo";"bar";"123")
.Q.opt .z.X
arguments:.Q.opt .z.X
/ arguments is a dict (99h) of (enlist `foo)!enlist ("bar";"123")
String
Get substring
6 8 sublist "Hello world"
6 _ "Hello world"
Join strings
"Hello"," ","World"
"" sv ("Hello";enlist " ";"World")
Find occurrences of a substring in a string
ss["Hello world";"wo"]
File IO
Open a file where file name is in variable
filePath:"/tmp/test.txt"
file:hopen hsym[`$filePath]
Delete a file
hdel `:/home/franz/test.txt
Check if a file exists
not () ~ key hsym `:/home/franz/test.txt
Delete a file only if it exists
if[not () ~ key hsym `:/home/franz/test.txt; hdel `:/home/franz/test.txt]
Write a single string to file (Not append)
`:test.txt 0: enlist "Hello world"
Table
Define a table
table1:([] c1:1 2; c2:`abc`def; c3:123 456)
Define a single row table
table1:([] enlist c1:1; c2:`abc; c3:123)
Export a table to csv
File name is same as the table name.
save `$"/home/franz/table1.csv"
Export a table to csv with a different file name
`:/home/franz/test.txt 0: csv 0: table1
Load csv file to table
"jsj" is the data type of each column in the csv. For example, j is long and s is symbol.
table3:("jsj";enlist",")0:`$"/home/franz/table1.csv"
Conditional branching
try-catch
h:@[hopen;`::9999;{show "Unable to connect"; exit 1}]
Early return
Use : without anything on the left
:"Hello World"
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