Dan, after your 4 series, I tried this. Also odd: NB. Begin here 5#''
(5#'')*/i.5 ''*/i.5 '' NB. Note the spacing between lines above Linda -----Original Message----- From: programming-boun...@forums.jsoftware.com [mailto:programming-boun...@forums.jsoftware.com] On Behalf Of Dan Bron Sent: Wednesday, September 10, 2014 11:56 AM To: programm...@jsoftware.com Subject: Re: [Jprogramming] Replace one item of a list Linda wrote: > *i.5 > 0 1 1 1 1 > */"0 i.5 > 0 1 2 3 4 Joe wrote: > It looks like /"0 yields the right side without evaluating the left side > > asdfasdf/"0 i.5 > 0 1 2 3 4 > > I don't totally get it myself This boils down to foo/ scalar_value . Note that because of the "0 (meaning "apply to each scalar value individually") asdfasdf/"0 i.5 0 1 2 3 4 is equivalent to (asdfasdf/ 0),(asdfasdf/ 1),(asdfasdf/ 2),(asdfasdf/ 3),(asdfasdf/ 4) 0 1 2 3 4 In Linda's example, asdfasdf is *, so her example is (*/ 0),(*/ 1),(*/ 2),(*/ 3),(*/ 4) 0 1 2 3 4 And this is quite different from the other expression she contrasts it with, which is *i.5 0 1 1 1 1 which, because monad * (signum) is a scalar function, is equivalent to: *"0 i.5 0 1 1 1 1 which, after expanding in the same manner as above, is equivalent to: (* 0),(* 1),(* 2),(* 3),(* 4) 0 1 1 1 1 Note the lack of any / which is the key issue here. That is, the difference Linda spotted boils down to the difference between these two expressions: */ 4 4 * 4 1 Now, *4 is 1 because 4>0 ; no mystery there. But why does */4 produce 4 ? Well, */ is product, and */ 4 4 4 4 NB. Product of four 4s is 4^4, 256 256 */ 4 4 4 NB. Product of three 4s is 4^3, 64 64 */ 4 4 NB. Product of two 4s is 4^2, 16 16 */ 4 NB. Product of one 4 is 4^1, 4 4 And in general, foo/ scalar_value (or asdfasdf/ scalar_value) is simply scalar_value . Because foo/ noun says "insert foo between all _pairs of items_ in noun", but when noun is scalar_value, there _are no pairs_, so no insertion is done. -Dan PS: and, of course, */ '' NB. Product of zero "4s" is 4^0, 1 1 There are no pairs to insert * between here, either, but empty arguments gets into identity functions. ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
