> From: Robert O'Boyle > > I have come across a feature of J that I didn't expect. When I add > rows of a matrix, I use +/"1 > > a > 1 2 3 4 > 1 2 3 4 > 1 2 3 4 > > +/"1 a > 10 10 10 > > And columns, > > +/"2 a > 3 6 9 12
Although the result is the same as summing the columns, I suspect you are misunderstanding rank in this case. Note to start with that not using rank: +/ a 3 6 9 12 Will give you the same result. The adverb insert ( / ) will place the verb to its immediate left between each _item_ of the noun to its right. The noun ( a ) is rank 2, and is therefore a list of rank 1 items. ( +/ a ) will place + in between each item of ( a ), i.e. 1 2 3 4 + 1 2 3 4 + 1 2 3 4 3 6 9 12 You can think of the rank conjunction as a way of feeding chunks of a noun on its right, to the verb on the left. So ( +/"1 a ) will send rank 1 chunks (vectors) of ( a ) to ( +/ ). i.e. similar to: (+/ 1 2 3 4) , (+/ 1 2 3 4) ,: (+/ 1 2 3 4) Insert will now place + between each item of the list: (1+2+3+4) , (1+2+3+4) ,: (1+2+3+4) If you use ( +/"2 a ) then you are saying you want to feed ( a ) to ( +/ ) in rank 2 chunks. Since ( a ) is only rank 2 (a matrix) this is the same as sending all of ( a ) to ( +/ ) at once. i.e. the same as ( +/ a ). > By chance, I happened to 'misplace' insert and got an unexpected result Note also that the sentence ( a *"1/ b ) uses dyadic / and so is "table" not "insert". I see Bob has replied along similar lines, I certainly endorse his recommendation of Henry's book! ---------------------------------------------------------------------- For information about J forums see http://www.jsoftware.com/forums.htm
