Charmap

The monad p {~ q i. ] where p and q
are lists of 1-byte characters performs character mappings. For example:

   'AA aa'=: a.{~ 65 97 +/ i.26
   AA
ABCDEFGHIJKLMNOPQRSTUVWXYZ
   aa
abcdefghijklmnopqrstuvwxyz
   lc=: (aa,a.) {~ (AA,a.) i. ]
   lc 'United States of America, Union of South Africa'
united states of america, union of south africa

The phrase and its variants ( (q i.]){p"_
and q&i.{p"_ )
are now supported by special code and are faster and leaner.
The following benchmarks demonstrate the improvement:

x=: a. {~ 1e6 ?@$ 256
p=: a. {~ 123 (22 b.) i.256
ts=: 6!:2, 7!:2@]  NB. time and space
ts 'Expression'