R devel - Jun 2002 - Bug in printing symbolic derivative (PR#1697)

A colleague pointed out the following error in D():

 f  <- expression(exp(-2*x*y))
 Dx <- D(f, 'x')
 Dy <- D(f,'y')
 Dxy <- D(Dx, 'y')
 Dyx <- D(Dy, 'x')

Then 
>  Dx    # this is right
-exp(-2 * x * y) * (2 * y)
>  Dy    # this is right
exp(-2 * x * y) * (-2 * x)
>  Dxy   # this is wrong in the sign of the second term!!!
-exp(-2 * x * y) * (-2 * x) * (2 * y) + exp(-2 * x * y) *
2
>  Dyx   # this is also wrong in the sign of the second term!!!
-exp(-2 * x * y) * 2 + exp(-2 * x * y) * (2 * y) * (-2 * x)

In both cases, the answer would be correct if the initial sign were
applied to the whole expression; is this just a case of not printing
some parentheses?  Let's experiment...
> x <- 1
> y <- 2
> eval(Dxy)
[1] 0.1098938  # this is right
> eval(Dyx)
[1] 0.1098938  # this is right
> -exp(-2 * x * y) * (-2 * x) * (2 * y) + exp(-2 * x * y) * 2 # this is how
Dxy prints
[1] 0.1831564  # this is wrong!!
> -(exp(-2 * x * y) * (-2 * x) * (2 * y) + exp(-2 * x * y) * 2) # this is how
it should print
[1] 0.1098938  # this is right

Looks like that's it...

Duncan Murdoch

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murdoch@stats.uwo.ca writes:

> > eval(Dxy)
> [1] 0.1098938  # this is right
> > eval(Dyx)
> [1] 0.1098938  # this is right
> > -exp(-2 * x * y) * (-2 * x) * (2 * y) + exp(-2 * x * y) * 2 # this is
how Dxy prints
> [1] 0.1831564  # this is wrong!!
> > -(exp(-2 * x * y) * (-2 * x) * (2 * y) + exp(-2 * x * y) * 2) # this
is how it should print
> [1] 0.1098938  # this is right
> 
> Looks like that's it...
Yep. This is another long-standing item on the parse/deparse TODO
list. A more minimalistic version is
> "*"(5,2+3)
[1] 25
> quote("*"(5,2+3))
5 * 2 + 3
> eval(quote("*"(5,2+3)))
[1] 25

There are really two issues: we don't strip parentheses when parsing
and we don't insert them where needed on deparsing.

-- 
   O__  ---- Peter Dalgaard             Blegdamsvej 3  
  c/ /'_ --- Dept. of Biostatistics     2200 Cph. N   
 (*) \(*) -- University of Copenhagen   Denmark      Ph: (+45) 35327918
~~~~~~~~~~ - (p.dalgaard@biostat.ku.dk)             FAX: (+45) 35327907
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