q

q Language & kdb+

q is a vector-oriented language for kdb+ time-series databases. Right-to-left evaluation, no operator precedence.

Load the matching reference before writing — Python translations and error diagnostics depend on idioms not covered inline:

• Translating from Python? Load python-q-mapping.md
• Debugging a 'type / 'rank / 'assign / 'domain error? Load common-errors.md
• Reviewing existing q code? Load review-checklist.md

The shell-running idiom and the most common syntax traps are inlined below so you don't have to navigate elsewhere for them. For the full operator/type/system-command reference, see reference.md.

Critical Rules

Right-to-left evaluation (no operator precedence):

2*3+4    / = 14 (not 10!) evaluates as 2*(3+4)
/ Use parentheses: (2*3)+4 = 10

% is division, not modulo. The single most dangerous Python→Q mapping.

10%3     / 3.333333 (division)
10 mod 3 / 1 (modulo)

Assignment uses colon, equality uses =:

x: 42        / assignment
x = 42       / comparison, returns 1b

= is element-wise; ~ (Match) compares structures.

(1 2 3)=(1 2 4)  / 1 1 0b (boolean vector, NOT a single bool)
(1 2 3)~(1 2 3)  / 1b (structural match, single bool)

Never use = to check if two lists are the same. Use ~.

No negative indexing. x -1 is subtraction, not last element.

/ WRONG: x -1        → subtraction
/ RIGHT: last x      → last element

Atoms and vectors are different types.

"a"        / char atom, type -10h
"abc"      / char vector (string), type 10h
enlist "a" / one-element char vector, type 10h
5          / long atom, type -7h
enlist 5   / one-element long vector, type 7h

Use (),x or enlist x to promote an atom to a one-element list when needed.

if[] is statement-only — has no return value. Use $[cond;true;false] for conditional expressions.