system() in packages

With cygwin bash installed under Windows, one can use pipes in system(),
e.g., like this:

R 2.5.0 under Windows XP:

system("echo foo | sed s/foo/bar/") # this doesn't work under windows

foo | sed s/foo/bar/

# but using 'bash -c' does:
system("bash -c \"echo foo | sed s/foo/bar/\"")

bar

# and some use of quotes within the command actually works!
system("bash -c \"echo foo | sed 's/foo/bar rrr/'\"")

bar rrr

[Beware: complex quoting inside the -c command can be difficult to get
right.]

-- Tony Plate


Prof Brian Ripley wrote:

The first comment is that will only work on Unix-alikes, since '|' needs a
shell.

So, if this is on a Unix-alike you need to establish if the program is in
the path at run time and cache the result.  I have no idea if this would
actually work, but for example system('gp --version') might provide a
suitable test.

On Fri, 27 Apr 2007, Robin Hankin wrote:

Hello

Quite often I need higher precision, or larger numbers,
 than IEEE double precision allows.  One strategy I sometimes
use is to use system() to call pari/gp, which is not constrained by
IEEE.

[pari/gp is a GPL high-level mathematical programming language
geared towards pure mathematics]

Two of my packages contain lines like the following:

system(" echo '1.12^66' | gp -q f",intern=TRUE)

[1]
"1771.697189476241729649767636564084681203806302318041262248838950177194
116346432205160921568393661760"

Note the high precision of the answer.

My question is, how to deal with the possibility that pari/gp is not
installed?

If the system cannot find gp for some reason, I get:

system(" echo '1.12^66' | gp -q f",intern=TRUE)

sh: line 1: gp: command not found
character(0)
What's the recommended way to handle this eventuality gracefully?  The
functions that do use pari/gp have "pure" R equivalents (but much
slower and less accurate)  so I want users to be able to install the
package without pari/gp.


--
Robin Hankin
Uncertainty Analyst
National Oceanography Centre, Southampton
European Way, Southampton SO14 3ZH, UK
 tel  023-8059-7743