Exponentiate very large numbers

1351+log(.1) -1347

1347 + log(abs(1-exp(1.697415)))

Are there any tricks I can use to get a real result
for exp( ln(a) ) - exp( ln(0.1) + ln(b) ), either in logarithm or
exponential form?

  log.a <- 1347
  log.b <- 1351
  f0 <- function(log.a, log.b) exp(log.a) - exp(log(0.1) + log.b)

will not work because f0(1347,1351) is too big to represent as
a double precision number (abs(result)>10^308)).
If you are satisfied with computing log(result) you can do it
with some helper function:
   addOnLogScale <- function (e1, e2)
   {
       # log(exp(e1) + exp(e2))
       small <- pmin(e1, e2)
       big <- pmax(e1, e2)
       log1p(exp(small - big)) + big
   }
   subtractOnLogScale <- function (e1, e2)
   {
      # log(abs(exp(e1) - exp(e2)))
       small <- pmin(e1, e2)
       big <- pmax(e1, e2)
       structure(log1p(-exp(small - big)) + big, isPositive = e1 > 
e2)
   }

as

   f1 <- function(log.a, log.b)  {
        # log(abs(exp(log.a) - exp( log(0.1) + log.b))), avoiding 
overflow
        subtractOnLogScale( log.a, log(0.1) + log.b )
   }

E.g.,

  > f0(7,3)

  [1] 1094.625

  > exp(f1(7,3))

  [1] 1094.625
  attr(,"isPositive")
  [1] TRUE

With your log.a and log.b we get

  > f1(1347, 1351)

  [1] 1348.495
  attr(,"isPositive")
  [1] FALSE

You can use the Rmpfr package to compute the results to any desired
precision.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

-----Original Message-----
From: r-help-bounces at r-project.org 
[mailto:r-help-bounces at r-project.org] On Behalf
Of francesca casalino
Sent: Monday, February 04, 2013 8:00 AM
To: r-help at r-project.org
Subject: Re: [R] Exponentiate very large numbers

I am sorry I have confused you, the logs are all base e:

ln(a) = 1347
ln(b) = 1351

And I am trying to solve this expression:

exp( ln(a) ) - exp( ln(0.1) + ln(b) )


Thank you.

2013/2/4 francesca casalino <francy.casalino at gmail.com>:

Dear R experts,

I have the logarithms of 2 values:

log(a) = 1347
log(b) = 1351

And I am trying to solve this expression:

exp( ln(a) ) - exp( ln(0.1) + ln(b) )

But of course every time I try to exponentiate the log(a) or 

log(b)

values I get Inf. Are there any tricks I can use to get a real 

result

for exp( ln(a) ) - exp( ln(0.1) + ln(b) ), either in logarithm or
exponential form?


Thank you very much for the help