R help - Jun 2013 - Built-in function for extracting mantissa and exponent of a numeric

Dear all,

Given a number

x<-1.234e12

is there a built-in function for extracting 1.234 and 12 ?

The following "hack" seems clumpsy:
> a<-strsplit(format(x, scientific=T),"e")[[1]]
> a
[1] "1.234" "+12"  
> as.numeric(a[1])
[1] 1.234
> as.integer(a[2])
[1] 12

Regards
S?ren

On 23/06/2013 10:54, S?ren H?jsgaard wrote:
> Dear all,
>
> Given a number
>
> x<-1.234e12
>
> is there a built-in function for extracting 1.234 and 12 ?
No, because that is not how the number is stored (and in fact the value 
stored is a binary fraction with a slightly different value).
> The following "hack" seems clumpsy:
format() is doing a lot of work to produce a decimal approximation to 
'x', including choosing the precision.

It is not clear what you want: if you merely want to express x as a*10^b 
for 1 <= a < 10 then

b <- floor(log10(x))
a <- x/10^b

does the job (but might get values of a very slightly less than 1 or 
above 10, so if you care this might need checking and refinement).
>
>> a<-strsplit(format(x, scientific=T),"e")[[1]]
>> a
> [1] "1.234" "+12"
>> as.numeric(a[1])
> [1] 1.234
>> as.integer(a[2])
> [1] 12
>
> Regards
> S?ren
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

On 13-06-23 5:54 AM, S?ren H?jsgaard wrote:
> Dear all,
>
> Given a number
>
> x<-1.234e12
>
> is there a built-in function for extracting 1.234 and 12 ?
I don't think so, but it is not hard to build them from log10:

mantissa <- function(x) {
   if (x == 0) 0
   else {
     log <- log10(abs(x))
     10^(log - floor(log))
   }
}

exponent <- function(x) {
   if (x == 0) 0
   else floor(log10(abs(x)))
}

Duncan Murdoch


>
> The following "hack" seems clumpsy:
>
>> a<-strsplit(format(x, scientific=T),"e")[[1]]
>> a
> [1] "1.234" "+12"
>> as.numeric(a[1])
> [1] 1.234
>> as.integer(a[2])
> [1] 12
>
> Regards
> S?ren
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

Hello,

Sorry I forgot to Cc the list.
And I had forgotten the case where x == 0.


extract <- function(x){
	e <- ifelse(x == 0, 0, floor(log10(x)))
	m <- x/10^e
	list(mantissa = m, exponent = e)
}

extract(c(0, 1.234e12, 12345678901234, 123e123))


Hope this helps,

Rui Barradas

Em 23-06-2013 10:54, S?ren H?jsgaard escreveu:
> Dear all,
>
> Given a number
>
> x<-1.234e12
>
> is there a built-in function for extracting 1.234 and 12 ?
>
> The following "hack" seems clumpsy:
>
>> a<-strsplit(format(x, scientific=T),"e")[[1]]
>> a
> [1] "1.234" "+12"
>> as.numeric(a[1])
> [1] 1.234
>> as.integer(a[2])
> [1] 12
>
> Regards
> S?ren
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>