At

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At	R←{X}(f@g)Y

This operator substitutes selected items in Y with new values or applies a function to modify selected items in Y.

The right operand g identifies which items of array Y are to be substituted or modified. It is either:

an array that specifies a set of indices in Y. If g is a simple scalar or vector, it selects major cells in Y. If nested, it specifies indices for Choose or Reach indexing.
or a function that when applied to Y returns a Boolean array of the same shape as Y (a mask) in which a 1 indicates that the corresponding item of Y is to be substituted or modified. Note that the ravel of the mask selects from the ravel of the right argument's index array.

The left operand f is either:

an array that contains values to replace those items in Y identified by g
or a function to be applied to those items, the result of which is used to replace them. If this function is dyadic, its left argument is the array X. Note that the function is applied to the sub-array of Y selected by gas a whole and not to each item separately.

The result R is the same as Y but with the items specified by g substituted or modified by f.

Examples (array @ array)

Replace the 2nd and 4th items of ⍳5:

      (10 20@2 4)⍳5   ⍝ 1Note that the expression does not require parentheses because without them, the array 2 4 binds anyway to the @ operator rather than to the ⍳ function.
1 10 3 20 5

      10 20@2 4⍳5
1 10 3 20 5

Replace the 2nd and 4th items of nested vector with ⍬:

      (⊂⍬)@2 4 ⍳¨⍳5
┌─┬┬─────┬┬─────────┐
│1││1 2 3││1 2 3 4 5│
└─┴┴─────┴┴─────────┘

Replace the 2nd and 4th rows (major cells) of a matrix:

      (2 3⍴10 20)(@2 4)4 3⍴⍳12
 1  2  3
10 20 10
 7  8  9
20 10 20

Replace first and last elements with 0 using Choose Indexing:

      (0@(1 1)(4 3))4 3⍴⍳12
 0  2 3
 4  5 6
 7  8 9
10 11 0

Replace nested items using Reach Indexing:

       G
┌───────┬───────┬───────┐
│┌───┬─┐│┌───┬─┐│┌───┬─┐│
││ABC│1│││DEF│2│││GHI│3││
│└───┴─┘│└───┴─┘│└───┴─┘│
├───────┼───────┼───────┤
│┌───┬─┐│┌───┬─┐│┌───┬─┐│
││JKL│4│││MNO│5│││PQR│6││
│└───┴─┘│└───┴─┘│└───┴─┘│
└───────┴───────┴───────┘
       G[((1 2)1)((2 3)2)]
┌───┬─┐
│DEF│6│
└───┴─┘
       ('' '*' @((1 2)1)((2 3)2)) G
┌───────┬───────┬───────┐
│┌───┬─┐│┌┬─┐   │┌───┬─┐│
││ABC│1││││2│   ││GHI│3││
│└───┴─┘│└┴─┘   │└───┴─┘│
├───────┼───────┼───────┤
│┌───┬─┐│┌───┬─┐│┌───┬─┐│
││JKL│4│││MNO│5│││PQR│*││
│└───┴─┘│└───┴─┘│└───┴─┘│
└───────┴───────┴───────┘

Examples (function @ array)

Replace the 2nd and 4th items of ⍳5 with their reciprocals:

      ÷@2 4 ⍳5
1 0.5 3 0.25 5

Replace the 2nd and 4th items of ⍳5 with their reversal:

      ⌽@2 4 ⍳5
1 4 3 2 5

Multiply the 2nd and 4th items of ⍳5 by 10:

      10×@2 4⍳5
1 20 3 40 5

Replace the 2nd and 4th items by their totals:

       +/¨@2 4 ⍳¨⍳5
┌─┬─┬─────┬──┬─────────┐
│1│3│1 2 3│10│1 2 3 4 5│
└─┴─┴─────┴──┴─────────┘

Replace the 2nd and 4th rows (major cells) of a matrix with their accumulatives:

      (+\@2 4)4 3⍴⍳12
 1  2  3
 4  9 15
 7  8  9
10 21 33

Examples (array @ function)

Replace odd elements with 0:

      0@(2∘|)⍳5
0 2 0 4 0

Replace multiples of 3 (note that masked items are substituted in ravel order):

      'abcde'@{0=3|⍵} 4 4⍴⍳16
 1  2  a  4
 5  b  7  8
 c 10 11  d
13 14  e 16

      'abcde'@(0=3|⊢) 4 4⍴⍳16 ⍝ or using a train
 1  2  a  4
 5  b  7  8
 c 10 11  d
13 14  e 16

Examples (function @ function)

Replace odd elements with their reciprocals:

      ÷@(2∘|)⍳5
1 2 0.3333333333 4 0.2

Replace odd items of ⍳5 with themselves reversed:

      ⌽@(2∘|)⍳5
5 2 3 4 1