Assignment (Indexed Modified) {R}←X[I]f←Y

f may be any dyadic function which returns an explicit result. Y may be any array whose items are appropriate to function f. X must be the name of an existing array. I must be a valid index specification. The items of the indexed portion of X must be appropriate to function f.

Y is either an array of the same shape as the indices specified by I or a scalar that is notionally extended to be the same shape as those indices.

The operator loops through the indices specified by I in ravel order. For each successive index i in the set specified by I, it calculates the result of X[i]fY[i] and assigns it back to X[i].

R is the "pass-through" value, that is, the value of Y. If the result of the derived function is not assigned or used, there is no explicit result.

Examples

      A
1 2 3 4 5

      +A[2 4]+←1
1

      A
1 3 3 5 5

      A[3]÷←2

      A
1 3 1.5 5 5

As the operator performs a loop, if an index in I is repeated, function f will be applied that number of times and successively to the same item of X.

Example

      B←3 5⍴0
      B[1 1 3;1 3 3 5]+←1
      B
2 0 4 0 2
0 0 0 0 0
1 0 2 0 1