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