“Replaces” in the sense that you would use it instead, and not feel like you are missing something, not in the sense that it is equivalent char-per-char.

APL has a few expand/compress operators that take a boolean vector; all of these in K use monadic & (where/replicate), either on the at-index e.g. +/a[&x>3] which is read “sum over a where x is greater than 3”, or at the amend-index, e.g. @[&7 9 15;i:&x>3;:;7-x@|i] which is read “in a vector with seven 0s, followed by nine 1s and fifteen 2s, in the indices where x is greater than 3 (call those indices i), place 7 minus those values from the same indices in reverse order.

Obviously contrived examples, but in APL you’d have to compress/expand a couple of times instead. (Sorry, don’t remember the operator names, my APL is rusty, last wrote a real program in 1992....)

NARS2000, Dyalog APL, and dzaima/APL all have the Where function now. See https://aplwiki.com/wiki/Indices.

I'm not familiar with J but it also appears to have something similar. However the resulting code seems a bit more convoluted than the K version. +/ (I. a>3) { a

More idiomatic J might look like this: +/a#~a>3 .

Thanks! Should probably brush up on my APL, as I have last really used APL2 on a mainframe nearly 3 decades ago.

But I have gotten lazy and too comfortable with K.... oh well.