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Diffstat (limited to 'kdb+.html.markdown')
| -rw-r--r-- | kdb+.html.markdown | 50 |
1 files changed, 26 insertions, 24 deletions
diff --git a/kdb+.html.markdown b/kdb+.html.markdown index ae132451..772c8a47 100644 --- a/kdb+.html.markdown +++ b/kdb+.html.markdown @@ -6,9 +6,8 @@ contributors: filename: learnkdb.q --- -The q langauge and its database component kdb+ were developed by Arthur -Whitney and released by Kx systems in 2003. q is a descendant of -[APL](https://en.wikipedia.org/wiki/APL_(programming_language)) and as such is +The q langauge and its database component kdb+ were developed by Arthur Whitney +and released by Kx systems in 2003. q is a descendant of APL and as such is very terse and a little strange looking for anyone from a "C heritage" language background. Its expressiveness and vector oriented nature make it well suited to performing complex calculations on large amounts of data (while also @@ -69,7 +68,7 @@ at jonny.press@aquaq.co.uk / And exponentiation... 2 xexp 4 / => 16 -/ ...and rounding... +/ ...and truncating... floor 3.14159 / => 3 / ...getting the absolute value... @@ -257,7 +256,7 @@ l / => 5 2 3 4 / Lists can also be used for indexing and indexed assignment l[1 3] / => 2 4 l[1 3]: 1 3 -l / => 1 1 3 3 +l / => 5 1 3 3 / Lists can be untyped/mixed type l:(1;2;`hi) @@ -291,6 +290,7 @@ last 1 2 3 / => 3 / Take (#), drop (_) and find (?) are also useful working with lists l:1 2 3 4 5 6 7 8 9 +l:1+til 9 / til is a useful shortcut for generating ranges / take the first 5 elements 5#l / => 1 2 3 4 5 / drop the first 5 @@ -298,7 +298,7 @@ l:1 2 3 4 5 6 7 8 9 / take the last 5 -5#l / => 5 6 7 8 9 / drop the last 5 --5_l / => 1 2 3 4 5 +-5_l / => 1 2 3 4 / find the first occurance of 4 l?4 / => 3 l[3] / => 4 @@ -312,7 +312,7 @@ d:`a`b`c!1 2 3 / the keyword key returns the first list key d / => `a`b`c / and value the second -value / => 1 2 3 +value d / => 1 2 3 / Indexing is indentical to lists / with the first list as a key instead of the position @@ -398,7 +398,7 @@ k!t / => id| c1 c2 c3 / => --| -------- / => 1 | 1 4 7 -/ => 2 | 2 5 8 +/ => 2 | 3 5 8 / => 3 | 3 6 9 / We can also use this shortcut for defining keyed tables @@ -456,7 +456,7 @@ f[2] / => 4 / however using a dictionary as a single argument can overcome this / allows for optional arguments or differing functionality d:`arg1`arg2`arg3!(1.0;2;"my function argument") -{x[`arg1]+x[`arg2]}[d] / => 3.0 +{x[`arg1]+x[`arg2]}[d] / => 3f / Functions in q see the global scope a:1 @@ -465,6 +465,7 @@ a:1 / However local scope obscures this a:1 {a:2;:a}[] / => 2 +a / => 1 / Functions cannot see nested scopes (only local and global) {local:1;{:local}[]}[] / throws error as local is not defined in inner function @@ -495,11 +496,11 @@ t:([]name:`Arthur`Thomas`Polly;age:35 32 52;height:180 175 160;sex:`m`m`f) / equivalent of SELECT * FROM t select from t / (must be lower case, and the wildcard is not necessary) -/ => name age height -/ => ----------------- -/ => Arthur 35 180 -/ => Thomas 32 175 -/ => Polly 52 160 +/ => name age height sex +/ => --------------------- +/ => Arthur 35 180 m +/ => Thomas 32 175 m +/ => Polly 52 160 f / Select specific columns select name,age from t @@ -524,6 +525,7 @@ select name, feet:floor height*0.032, inches:12*(height*0.032) mod 1 from t / => ------------------ / => Arthur 5 9.12 / => Thomas 5 7.2 +/ => Polly 5 1.44 / Including custom functions select name, growth:{[h;a]h%a}[height;age] from t @@ -535,18 +537,18 @@ select name, growth:{[h;a]h%a}[height;age] from t / The where clause can contain multiple statements separated by commas select from t where age>33,height>175 -/ => name age height -/ => ----------------- -/ => Arthur 35 180 +/ => name age height sex +/ => --------------------- +/ => Arthur 35 180 m / The where statements are executed sequentially (not the same as logical AND) select from t where age<40,height=min height -/ => name age height -/ => ----------------- -/ => Thomas 32 175 +/ => name age height sex +/ => --------------------- +/ => Thomas 32 175 m select from t where (age<40)&(height=min height) -/ => name age height -/ => --------------- +/ => name age height sex +/ => ------------------- / The by clause falls between select and from / and is equivalent to SQL's GROUP BY @@ -586,7 +588,7 @@ t / => Polly 52 160 f / Insert however is in place, it takes a table name, and new data -`t insert (`John;25;178;`m) +`t insert (`John;25;178;`m) / => ,3 t / => name age height sex / => --------------------- @@ -698,7 +700,7 @@ first each (1 2 3;4 5 6;7 8 9) / their behaviour differs based on the number of arguments the function they / are modifying receives. Here I'll summarise some of the most useful cases / a single argument function modified by scan given 2 args behaves like "do" -{x * 2}\[5;1] / => 1 2 4 8 16 3 (i.e. multiply by 2, 5 times) +{x * 2}\[5;1] / => 1 2 4 8 16 32 (i.e. multiply by 2, 5 times) {x * 2}/[5;1] / => 32 (using over only the final result is shown) / If the first argument is a function, we have the equivalent of "while" |