What about numeric constants, like `163'?
eval(Pre(exp(sqrt(163)*pi), 120)) does not work.
Thanks,
Ravi
________________________________________
From: David Winsemius <dwinsem...@comcast.net>
Sent: Saturday, July 4, 2015 1:12 PM
To: Duncan Murdoch
Cc: r-help; John Nash; Ravi Varadhan
Subject: Re: [R] : Ramanujan and the accuracy of floating point computations -
using Rmpfr in R
> On Jul 4, 2015, at 12:20 AM, Duncan Murdoch <murdoch.dun...@gmail.com> wrote:
>
> On 04/07/2015 8:21 AM, David Winsemius wrote:
>>
>>> On Jul 3, 2015, at 11:05 PM, Duncan Murdoch <murdoch.dun...@gmail.com>
>>> wrote:
>>>
>>> On 04/07/2015 3:45 AM, David Winsemius wrote:
>>>>
>>>>> On Jul 3, 2015, at 5:08 PM, David Winsemius <dwinsem...@comcast.net>
>>>>> wrote:
>>>>>
>>>>>
>>>>>
>>>>> It doesn’t appear to me that mpfr was ever designed to handle expressions
>>>>> as the first argument.
>>>>
>>>> This could be a start. Obviously one would wnat to include code to do
>>>> other substitutions probably using the all.vars function to pull out the
>>>> other “constants” and ’numeric’ values to make them of equivalent
>>>> precision. I’m guessing you want to follow the parse-tree and then testing
>>>> the numbers for integer-ness and then replacing by paste0( “mpfr(“, val,
>>>> “L, “, prec,”)” )
>>>>
>>>> Pre <- function(expr, prec){ sexpr <- deparse(substitute(expr) )
>>>
>>> Why deparse? That's almost never a good idea. I can't try your code (I
>>> don't have mpfr available), but it would be much better to modify the
>>> expression than the text representation of it. For example, I think
>>> your code would modify strings containing "pi", or variables with those
>>> letters in them, etc. If you used substitute(expr) without the
>>> deparse(), you could replace the symbol "pi" with the call to the Const
>>> function, and be more robust.
>>>
>>
>> Really? I did try. I was fairly sure that someone could do better but I
>> don’t see an open path along the lines you suggest. I’m pretty sure I tried
>> `substitute(expr, list(pi= pi))` when `expr` had been the formal argument
>> and got disappointed because there is no `pi` in the expression `expr`. I
>> _thought_ the problem was that `substitute` does not evaluate its first
>> argument, but I do admit to be pretty much of a klutz with this sort of
>> programming. I don’t think you need to have mpfr installed in order to
>> demonstrate this.
>
> The substitute() function really does two different things.
> substitute(expr) (with no second argument) grabs the underlying
> expression out of a promise. substitute(expr, list(pi = pi)) tries to
> make the substitution in the expression "expr", so it doesn't see "pi”.
Thank you. That was really helpful. I hope it “sticks” to sufficiently durable
set of neurons.
>
> This should work:
>
> do.call(substitute, list(expr = substitute(expr), env=list(pi =
> Const(pi, 120))))
>
> (but I can't evaluate the Const function to test it).
The expression `pi` as the argument to Const only needed to be character()-ized
and then evaluation on that result succeeded:
library(mpfr)
# Pre <- function(expr, prec){ do.call(substitute,
list(expr = substitute(expr),
env=list(pi = Const(pi, prec)))) }
> Pre(exp(pi),120)
Error in Const(pi, prec) :
'name' must be one of 'pi', 'gamma', 'catalan', 'log2'
Pre <- function(expr, prec){ do.call(substitute,
list(expr = substitute(expr),
env=list(pi = Const('pi', prec)))) }
> Pre(exp(pi),120)
exp(list(<S4 object of class "mpfr1">))
> eval( Pre(exp(pi),120) )
1 'mpfr' number of precision 120 bits
[1] 23.140692632779269005729086367948547394
So the next step for delivering Ravi’s request would be to add the rest of the
‘Const’ options:
Pre <- function(expr, prec){ do.call(substitute,
list(expr = substitute(expr),
env=list(pi = Const('pi', prec),
catalan=Const('catalan', prec),
log2=Const('log2', prec),
gamma=Const('gamma', prec)))) }
> eval(Pre(exp(gamma)))
Error in stopifnot(is.numeric(prec)) :
argument "prec" is missing, with no default
> eval(Pre(exp(gamma), 120))
1 'mpfr' number of precision 120 bits
[1] 1.781072417990197985236504103107179549
>
> This works:
>
> do.call(substitute, list(expr = substitute(expr), env = list(pi =
> quote(Const(pi, 120)))))
>
> because it never evaluates the Const function, it just returns some code
> that you could modify more if you liked.
>
> Duncan Murdoch
David Winsemius, MD
Alameda, CA, USA
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