Variadic syntax

An applicable value is variadic if its rank is not fixed.

Lists and dictionaries of depth ≥2 and tables are variadic.

q)m:4 5#"abcdefghijklmnopqrst"
q)m[1 3]                        / unary
"fghij"
"pqrst"
q)m[1 3;2 4]                    / binary
"hj"
"rt"
q)t:([]name:`Tom`Dick`Harry;city:`London`Paris`Rome)
q)t[`name]                      / unary
`Tom`Dick`Harry
q)t 1                           / unary
name| Dick
city| Paris
q)t[1;`city]                    / binary
`Paris

Some operators are variadic, for example Apply and Amend.

Each Prior, Over and Scan applied to binary values derive variadic functions.

q)+/[2 3 4]                  / unary
9
q)+/[1000000;2 3 4]          / binary
1000009
q)-':[1952 1954 1960]        / unary
1952 2 6
q)-':[1900;1952 1954 1960]   / binary
52 2 6

Keywords defined from such extensions are also variadic.

q)deltas                     / Subtract Each Prior
-':
q)deltas[15 27 93]           / unary
15 12 66
q)deltas[10;15 27 93]        / binary - unsupported
5 12 66
q)-':[10;15 27 93]           / binary - supported
5 12 66

Projection

Variadic values do not project unless the omitted argument/s are specified as nulls in the argument list.

To project a variadic value as a unary, use a 2-item argument list to resolve the binary form.

q)g:+/[100;]       / 2-item argument list resolves the binary form
q)g 2 3 4 5        / the projection is unary
114

Unary forms of binary operators

Many binary operators are variadic: they have unary forms. The unary form can be selected with a suffixed colon.

q)|[2;til 5]        / binary: maximum
2 2 2 3 4
q)|:[til 5]         / unary: reverse
4 3 2 1 0

Binary operators are infixes.

Like an infix extension, the unary form can be parenthesized and applied prefix.

q)2|til 5            / maximum
2 2 2 3 4
q)(|:)"zero"         / reverse
"orez"
q)2#"zero"           / take
"ze"
q)(#:)"zero"         / count
4

Unary forms can also be applied by Apply At.

q)|:["zero"]       / bracket notation
"orez"
q)(|:)"zero"       / prefix
"orez"
q)(|:)@"zero"      / apply-at
"orez"
q)@[|:;"zero"]     / apply-at
"orez"

Unary forms are poor q style

The semantics of the unary and binary forms of an operator are not always closely related.

For better legibility, q provides keywords for unary forms. Good q style prefers them. Write count "zero", not (#:)"zero".