[R] automatic SI prefixes as ticklabels on axis

[Jonas Stein]

On 2012-01-05, Jonas Stein <news at jonasstein.de> wrote:
> i want to plot values with frequency on a logarithmic x axis.
> similar to this example that i found in the web:
> http://www.usspeaker.com/jensen%20p15n-graph.gif
>
> I would like to convert long numbers to si prefix notation 
> like in the example
>
> (200000 to 200k, 35000000 to 3.5 M)
>
> Of course i could create labels by hand, but
> i have many files so i need some automatic function.
>
> Has anyone done that in R?
>
> Kind regards and thank you for your help,

my first try looks like this:
==================================

getSIstring <- function(x){

  sistring <- paste(x);
  
  prefixpairs <- data.frame(c(1e24,1e21,1e18,1e15,1e12,1e9,1e6,1e3,1e0,
                              1e-3,1e-6,1e-9,1e-12,1e-15,1e-18,1e-21,1e-24),
                            c("Y", "Z", "E", "P", "T", "G", "M", "k", " ",
                              "m", "u", "n", "p", "f", "a", "z", "y"))
  
  colnames(prefixpairs) <- c("factor", "prefix")
  i=0
  repeat{i=i+1;
         if (x > prefixpairs$factor[i]) {
           sistring <- paste(x/prefixpairs$factor[i], prefixpairs$prefix[i]); break;}
         if (i >= length(prefixpairs$factor)) break}

   return(sistring)
}

==================================

> getSIstring(2e7)
[1] "20 M"

How can i improve this function?
It would be nice if it could handle lists too like sin() can do
> sin(1:4)
[1]  0.8414710  0.9092974  0.1411200 -0.7568025

kind regards,

-- 
Jonas Stein <news at jonasstein.de>