At

is a primitive dyadic operator that is similar to J and K's Amend in functionality. In Dyalog APL, At handles a subset of the cases of the Under operator that pertain to modifying elements at specific positions in an array.

Description
At takes two operands, each of which can be an array or function, with varying effect: A call to At is of the form  where   is optional. Each possible derived function can be applied monadically or dyadically, except if the left operand is an array, in which case, only the monadic derived function is defined.
 * specifies the indices of the right argument that should be modified.
 * Function: Must apply monadically to, and return a boolean array of the same shape as  , with ones at positions that should be amended.
 * Array: A vector (or scalar) of cell indices (of equal lengths) to amend. If the same index appears multiple times, the last amendment overrules any previous ones, meaning that amendments are not stacked. This is in contrast to modified assignment where multiple specifications of the same parts of the array will be processed sequentially.
 * specifies what happens to the elements at those positions.
 * Function: Is applied once on an array whose major cells are the selected cells. If  is present, then it is bound to    so   is equivalent to.
 * Array: Replaces the selected elements and must either have the same shape as the array whose major cells are the selected cells, or be a scalar, which then replaces all selected cells.

Lessons
APL Cultivation

Documentation

 * Dyalog
 * J Dictionary, NuVoc

Publications
Prefix friendly @ by Aaron Hsu and Roger Hui