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in

Syntax: x in y, in[x;y]

Where

x y result
atom or vector atom or vector boolean for each item of x: whether it is an item of y
vector or list list boolean atom: whether x is an item of y
list atom or vector x in\:y
q)"x" in "a"                                    / atom in atom
0b
q)"x" in "acdexyz"                              / atom in vector
1b
q)"wx" in "acdexyz"                             / vector in vector
01b
q)"wx" in ("acdexyz";"abcd";"wx")               / vector in list
1b
q)("ab";"cd") in (("ab";"cd");0 1 2)            / list in list
1b

In the third rule above (list in vector) in simply recurses into the list until atoms or vectors are found and the first rule can be applied.

q)("acdexyz";"abcd";"wx") in "wx"               / list in vector
0000100b
0000b
11b
q)("acdexyz";(("abcd";"efg");"wx")) in "wx"     / nested list in vector
0000100b
((0000b;000b);11b)

in is often used with select

q)\l sp.q
q)select from p where city in `paris`rome
p | name  color weight city
--| ------------------------
p2| bolt  green 17     paris
p3| screw blue  17     rome
p5| cam   blue  12     paris

Further examples:

q)1 3 7 6 4 in 5 4 1 6        / which of x are in y
10011b
q)1 2 in (9;(1 2;3 4))        / no item of x is in y
00b
q)1 2 in (1 2;9)              / 1 2 is an item of y
1b
q)1 2 in ((1 2;3 4);9)        / 1 2 is not an item of y
0b
q)(1 2;3 4) in ((1 2;3 4);9)  / x is an item of y
1b

Results for mixed-rank arguments are not intuitive

q)3 in'1 reverse\(1 2;3)
01b

Instead use Match ~ for mixed-rank lists::

q)any ` ~/: (1 2;`)
1b

except, inter, union
Basics: Search