n163 <- mpfr(163, 500)
is how I set up the number.
JN
On 15-07-04 05:10 PM, Ravi Varadhan wrote:
>> 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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