Subtraction in Algodiff

[priamfive]

Hi, I have been experimenting with Algodiff and have had some odd behavior with subtraction.
I'm not sure if I am doing something wrong. If I want to differentiate f(x)=1-x, I seem to get an odd result if I do it in the obvious way:
`# #require "owl-top";;

module AD = Owl.Algodiff.D;;

module AD = Owl.Algodiff.D

let f x = AD.Maths.((F 1.0) - x);;

val f : AD.t -> AD.t =

let f1 = AD.diff f;;

val f1 : AD.t -> AD.t =

f1 (F 0.0);;

• : AD.t = [F(1)]

let g x = AD.Maths.((F 1.0) + (neg x));;

val g : AD.t -> AD.t =

let g1 = AD.diff g;;

val g1 : AD.t -> AD.t =

g1 (F 0.0);;

• : AD.t = [F(-1)]
  `
  The second way (using g) works but the first (using f) has the wrong sign.

Perhaps the second way is what is supposed to be used but this wasn't clear from the excellent draft Owl book.

[tachukao]

@priamfive thanks for raising this issue. The culprit was this line

owl/src/base/algodiff/owl_algodiff_ops.ml

Line 998 in 5150b0a

let df_db _cp _ap _bp bt = bt

Instead of bt it should have been neg bt. This is now fixed in the PR #533

[priamfive]

Great - thanks for all the good work on Owl.