Creating Dictionaries

A dictionary maps a list of keys to a list of values of the same length. Dictionaries are q's general-purpose key-value structure, and every table is built from one.

Syntax

keys!values

d:`a`b`c!10 20 30

a| 10
b| 20
c| 30

Dictionary Literal Syntax

A more concise, self-describing form (kdb+ 4.1+) names each key directly. Keys created this way are always symbols:

d:([a:10;b:20;c:30])

Dot Apply

! is just another function, so a dictionary can also be built with dot apply, passing (keys;values) as a single argument list:

d:(!) . (`a`b`c;10 20 30)

Key Vector Types

Whether keys can have mixed types depends on the key vector used to build the dictionary:

This matters when joining dictionaries: , requires the key vectors on each side to have compatible types, otherwise the join fails with 'type (e.g. joining a symbol-keyed dictionary with a character-keyed one).

Generating Keys with Roll (?)

The Roll operator ? is a quick way to generate random dictionary keys and values:

10?100.0     / 10 random floats, with replacement
-10?100      / 10 distinct random longs from 0-99, without replacement

A negative left argument samples without replacement, guaranteeing unique keys — but the domain must be large enough to supply that many distinct values, or the call fails with 'length.