The fundamental building blocks of q are atoms, lists, and functions. Atoms are scalars and include the data types numeric, character, date, and time. Lists are ordered collections of atoms upon which the higher leveldata structuresdictionaries and tables are internally constructed. A dictionary is a map of a list of keys to a list of values. A table is a transposed dictionary of symbol keys and equal length lists as values. A keyed table, analogous to a table with a primary key placed on it, is a dictionary where the keys and values are arranged as two tables. The following code demonstrates the relationships of the data structures. Expressions to evaluate appear prefixed with the q) prompt, with the output of the evaluation shown beneath: q)`john / an atom of type symbol `john q)50 / an atom of type integer 50 q)`john`jack / a list of symbols `john`jack q)50 60 / a list of integers 50 60 q)`john`jack!50 60 / a list of symbols and a list of integers combined to form a dictionary john| 50 jack| 60 q)`name`age! / an arrangement termed a column dictionary name| john jack age | 50 60 q)flip `name`age! / when transposed via the function "flip", the column dictionary becomes a table name age -------- john 50 jack 60 q)!flip !enlist 50 60 / two equal length tables combined as a dictionary become a keyed table name| age ----| --- john| 50 jack| 60
These entities are manipulated via functions, which include the built-in functions that come with Q and user-defined functions. Functions are a data type, and can be placed in lists, dictionaries and tables, or passed to other functions as parameters.
Examples
Like K, Q is interpreted and the result of the evaluation of an expression is immediately displayed, unless terminated with a semi-colon. The Hello world program is thus trivial: q)"Hello world!" "Hello world!"
The following expression sorts a list of strings stored in the variable x descending by their lengths: x@idesc count each x
The expression is evaluated from right to left as follows:
Note that in both cases the function implicitly takes a single argument called x - in general it is possible to use up to three implicit arguments, named x, y and z, or to give arguments local variable bindings explicitly. In the direct implementation, the expression "til x" enumerates the integers from 0 to x-1, "1+" adds 1 to every element of the list and "prd" returns the product of the list. In the recursive implementation, the syntax "$" is a ternary conditional - if the condition is true then expr1 is returned; otherwise expr2 is returned. The expression ".z.s" is loosely equivalent to 'this' in Java or 'self' in Python - it is a reference to the containing object, and enables functions in q to call themselves. When x is an integer greater than 2, the following function will return 1 if it is a prime, otherwise 0:
"2_" drops the first two elements of the enumeration.
"x mod" performs modulo division between the original integer and each value in the truncated list.
"min" find the minimum value of the list of modulo result.
The q programming language contains its own table query syntax called , which resembles traditional SQL but has important differences, mainly due to the fact that the underlying tables are oriented by column, rather than by row. q)show t: / define a simple table and assign to "t" name age -------- john 50 jack 60 jill 50 jane 20
q)select from t where name like "ja*",age>50 name age -------- jack 60 q)select rows:count i by age from t age| rows ---| ---- 20 | 1 50 | 2 60 | 1
