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Extenders and extensions


Extenders (formerly known as adverbs) are the primary means of iteration in q, and in almost all cases the most efficient way to iterate. Loops are rare in q programs and are almost always candidates for optimization. Mastery of extenders is a core q skill.

The first part of this paper introduces extenders informally. This provides ready access to the two principal forms of iteration: distributive and progressive.

The second part of the paper reviews extenders more formally and with greater attention to syntax. We see how extenders apply not only to functions but also to lists, dictionaries and tables. From their syntax we see when parentheses are required, and why.


Extenders are higher-order unary operators: they take a single argument and return a derived function, known as an extension. The single argument is a map: a list, dictionary, table, or function. The extension extends its normal application.

Extenders are the only operators that can be applied postfix. They almost always are.

For example, the extender Scan, written \, applied to the Add operator + returns the extension Add Scan, written +\, which extends Add to return cumulative sums.

q)(+\)2 3 4
2 5 9

Applied to the Multiply operator * it returns the extension Multiply Scan, written *\, which returns cumulative products.

q)(*\)2 3 4
2 6 24

(Writers of some other programming languages might recognize these uses of Scan as fold.)

Another example. The extender Each, written ', applied to the Join operator ,, returns the extension Join Each, written ,'.

q)a:2 3#"abcdef"
q)b:2 3#"uvwxyz"

Above, a and b are both 2×3 character matrixes. That is to say, they are both 2-lists, and their items are character 3-lists. While a,b joins the two lists to make a 4-list, the extension Join Each a,'b joins their corresponding items to make two character 6-lists.

Scan and Each are the cores of the progressive and distributive extenders. The other extenders are variants of them.

Three kinds of iteration

Atomic iteration

Many native q operators have iteration built into them. They are atomic. They apply to conforming arguments.

q)2+2           / two atoms
q)2 3 4+5 6 7   / two lists
7 9 11
q)2+5 6 7       / atom and list
7 8 9

Two arguments conform if they are lists of the same length, or if one or both is an atom. In atomic iteration this definition recurses to any depth of nesting.

q)(1;2;3 4)+( (10 10 10;20 20); 30; ((40 40; (50 50 50; 60)); 70) )
(11 11 11;21 21)
((43 43;(53 53 53;63));74)

Because atomic iteration burrows deep into nested structure it is not easy to parallelize. A simpler form of it is.

Distributive iteration

Distributive iteration applies a function across items of a list or lists. It does not burrow into a nested structure, but simply iterates across its top level.

q)count ("the";"quick";"brown";"fox")
q)(count') ("the";"quick";"brown";"fox")
3 5 5 3

The Each extender has four variants. An extension of Each Right /: applies its entire left argument to each item of its right argument. Correspondingly, an extension of Each Left \: applies its entire right argument to each item of its left argument.

q)"abc",/:"xyz"     / Join Each Right
q)"abc",\:"xyz"     / Join Each Left

Each Left and Each Right

Remember which is which by the direction in which the slash leans.

Each Prior takes a binary as its argument. The unary extension applies the binary between each item of a list (or dictionary) and its preceding item. The differences between items in a numeric or temporal vector:

q)(-':)1 1 2 3 5 8 13 21 34     / Subtract Each Prior
1 0 1 1 2 3 5 8 13

Each Parallel takes a unary argument and applies it, as Each does, to each item in the extensions argument. Unlike Each, it partitions its work between any available slave processes. Suppose analyze is CPU-intensive and takes a single symbol atom as argument.


With a unary function, the mnemonic keyword each is generally preferred as a cover for the extender Each. Similarly, prior is preferred for Each Prior and peach for Each Parallel.

q)count each ("the";"quick";"brown";"fox")
3 5 5 3
q)(-) prior 1 1 2 3 5 8 13 21 34
1 0 1 1 2 3 5 8 13
q)analyze peach `ibm`msoft`googl`aapl

In distributive iterations the number of iterations is the number of top-level items in the extension’s argument/s. Distributor extensions are right-uniform.

The distributors:

extender name mnemonic keyword
' Each each
\: Each Left
/: Each Right
': Each Prior prior
': Each Parallel peach

Progressive iteration

In progressive iterations the extender’s argument is evaluated repeatedly, first on the entire (left) argument of the extension, next on the result of that evaluation, and so on.

The number of iterations is determined according to the extension’s rank.

For a unary extension, there are three forms:

  • Converge: iterate until a result matches either the previous result or the original argument
  • Do: iterate a specified number of times
  • While: iterate until the result fails a test
q)({x*x}\)0.1                        / Converge
0.1 0.01 0.0001 1e-08 1e-16 1e-32 1e-64 1e-128 1e-256 0
q)5{x*x}\0.1                         / Do
0.1 0.01 0.0001 1e-08 1e-16 1e-32
q)(1e-6<){x*x}\0.1                   / While
0.1 0.01 0.0001 1e-08

For higher-rank extensions the number of iterations is the count of the right argument/s. For example, the result r of applying a ternary extension f\ to arguments x, y, and z:

r[0]:f[x;   y 0; z 0]
r[1]:f[r 0; y 1; z 1]
r[2]:f[r 1; y 2; z 2]

From this we see that the right arguments y and z must conform and that count r – the number of evaluations – is count[y]|count[z].

There are two progressive extenders.

  • Extensions of Scan \ return as a list the results of each evaluation. Scan extensions are thus right-uniform functions: their results conform to their right arguments. They resemble fold in some other programming languages.
  • Extensions of Over / perform the same computation as the Scan extensions, but return only the last result. They resemble map reduce in some other programming languages.
q)(+\)2 3 4    / Add Scan
2 5 9
q)(+/)2 3 4    / Add Over

For binary arguments of Scan and Over, the mnemonic keywords scan and over are generally preferred.

q)(+) scan 2 3 4
2 5 9
q)(+) over 2 3 4

The progressors:

extender name mnemonic keyword
\ Scan scan
/ Over over



Conor Slattery is a Financial Engineer who has designed kdb+ applications for a range of asset classes. Conor is currently working with a New York based investment firm, developing kdb+ trading platforms for the US equity markets.

Stephen Taylor is the Kx Systems librarian.

An earlier version of this paper was published in 2013 by Slattery as “Efficient use of adverbs”.