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Franz Wong
Franz Wong

Posted on • Edited on

KDB cookbook

#kdb

I am still a KDB noob. This page will be kept updating whenever I have new tricks.

Display string to console** 

0N!"Hello World"
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Cast string to symbol 

ticker:"MSFT" ticker_symbol:`$ticker
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List

Define a list 

mylist:(1;2;3)
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Get n-th element 

(1;2;3)[1]
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{x[1]}(1;2;3)
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Get first n elements 

3#(1;2;3;4;5;6;7;8;9;10)
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Get last n elements 

-3#(1;2;3;4;5;6;7;8;9;10)
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Filter list 

even_list:{x where not x mod 2}(1;2;3;4;5;6)
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Run function on each element 

{0N!x} each ("Hello";"World")
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({0N!x}')("Hello";"World")
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Convert string list to long list 

({"J"$x}')("123";"456";"789")
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Command line parameters

This is the command we use.

rlwrap q myscript.q -foo "bar" "123"
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.z.x

arguments:.z.x / arguments is a list (0h) of ("-foo";"bar";"123")
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.z.X

arguments:.z.X / arguments is a list (0h) of ("q";"myscript.q";"-foo";"bar";"123")
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.Q.opt .z.X

arguments:.Q.opt .z.X / arguments is a dict (99h) of (enlist `foo)!enlist ("bar";"123")
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String

Get substring 

6 8 sublist "Hello world"
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6 _ "Hello world"
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Join strings 

"Hello"," ","World"
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"" sv ("Hello";enlist " ";"World")
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Find occurrences of a substring in a string 

ss["Hello world";"wo"]
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File IO

Open a file where file name is in variable 

filePath:"/tmp/test.txt" file:hopen hsym[`$filePath]
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Delete a file 

hdel `:/home/franz/test.txt
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Check if a file exists 

not () ~ key hsym `:/home/franz/test.txt
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Delete a file only if it exists 

if[not () ~ key hsym `:/home/franz/test.txt; hdel `:/home/franz/test.txt]
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Write a single string to file (Not append) 

`:test.txt 0: enlist "Hello world"
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Table

Define a table 

table1:([] c1:1 2; c2:`abc`def; c3:123 456)
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Define a single row table 

table1:([] enlist c1:1; c2:`abc; c3:123)
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Export a table to csv

File name is same as the table name.

save `$"/home/franz/table1.csv"
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Export a table to csv with a different file name 

`:/home/franz/test.txt 0: csv 0: table1
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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"
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Conditional branching

try-catch

Execution Control

h:@[hopen;`::9999;{show "Unable to connect"; exit 1}]
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Early return

Use : without anything on the left

:"Hello World"
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