Tips for golfing in K K is a programming language in the APL family designed by Arthur Whitney. While the official interpreter is closed-source and commercial, a trial version with a workspace limit of 32 bits of addressing space (which shouldn't pose problems for code golf) can be found at the Kx Systems website. This version bundled as part of the kdb+ database is colloquially known as "K4". There are also open-source K implementations available, including Kona, which is based on K3, and my own interpreter called oK, which is based on K5 and has a browser based REPL.

Kx Systems has a wiki with K4/kdb+/Q information, and the Kona GitHub page also has an excellent collection of reference materials. I have begun writing a manual for oK/k5 which may be a useful reference.

Like J and APL, K is a very terse and powerful language, and can often make a good showing in code golf. Please share tips, tricks and idioms you discover, and if you haven't tried K before consider giving it a spin! Post one tip per answer, please!
Calling a Dyad

Assuming you had a dyadic (2-argument) function f:

f: {x+2*y}


You would ordinarily call it like so:

f[3;47]


You can save a character by instead currying in the first argument and then applying the resulting partial function to the second argument by juxtaposition:

f[3]47


The same naturally works for array indexing:

  z: (12 17 98;90 91 92)
(12 17 98
 90 91 92)

  z[1;2]
92

  z[1]2
92