[R] Exponentiate very large numbers

[Gabor Grothendieck]

On Tue, Feb 5, 2013 at 3:35 PM, Gabor Grothendieck
<ggrothendieck at gmail.com> wrote:
> On Mon, Feb 4, 2013 at 10:11 AM, francesca casalino
> <francy.casalino at gmail.com> wrote:
>> 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?
>>
>
> Assuming you have bc, try this using the r-bc package available at
> http://r-bc.googlecode.com
>
>> source("http://r-bc.googlecode.com/svn/trunk/R/bc.R")
>> bc("e(1347) - e(l(0.1) + 1351)")
> [1] "-4405386005870716963250677607096464495107208188262037831496629962604389947022370210116563260536709412765244230596649428637325731073087741577278759185783174516835759767642509461583320513769415050345878341068570238633056331967056945630593558411688354769032035897813566786044573424610037424319481107653164601301363052343211549433260783299929388275371544460247043814951038945784700209474446687259093777751098627790391356643931274009579136611476739316800835499749286852733758497967615615441909946879410903730244743058347457562287380518284874370438753598667230357006574194406623712901009806747.7875941440505465257638722719393870571587463674102678572599172110259394653657558768303852737288352763"
>
>
Just continuing from where we left off:

> # log of expression
> bc("l(- e(1347) + e(l(0.1) + 1351))")
[1] "1348.4951072860942085476545300380241005507587854108159740610940322438151104344740271849332074211018350282"



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