How to capture console output in a numeric format

Thank you very much, Jim. ?That works!

I did know that I could process the character strings using regex, but was also wondering if there was a direct way to get this.

Suppose, in the current example I would like to obtain a 3-column matrix that contains the parameters and the function value:

fr <- function(x) { ? ## Rosenbrock Banana function
? ?on.exit(print(cbind(x1, x2, f)))
? ?x1 <- x[1]
? ?x2 <- x[2]
? ?f <- 100 * (x2 - x1 * x1)^2 + (1 - x1)^2
? ?f
}

fvals <- capture.output(ans <- optim(c(-1.2,1), fr))

Now, I need to tweak your solution to get the 3-column matrix. ?It would be nice, if there was a more direct way to get the numerical output, perhaps a numeric option in capture.output().

This works in general assuming that we only wish to keep those output
lines having the same number of numerics as the most frequently output
length, i.e. 3 in your example above and 1 in the original example.
Also it does not require modification of the function to be optimized.

The first line extracts all numerics that have a decimal in them (so
that, for example, the 1 in [1] is not extracted).  The next two lines
count how many numbers are in each extracted line and returns the most
frequently occurring non-zero length as n so that n is 3 here.  The
final line extracts those lines and rbind's them. fvals is from your
post.

library(gsubfn)
fvals.numeric <- strapply(fvals, "[-0-9]+[.][0-9]*", as.numeric)
tab <- table(sapply(fvals.numeric, length))
n <- as.numeric(names(which.max(tab[names(tab) > 0])))
do.call(rbind, Filter(function(x) length(x) == n, fvals.numeric))

If we knew in advance that we wanted only lines with 3 numbers and
that those three numbers are each prefaced by a space (both of which
are the case in your example) then we could simplify it to just this:

library(gsubfn)
fvals.numeric <- strapply(fvals, " [-0-9.]+", as.numeric)
do.call(rbind, fn$Filter(x ~ length(x) == 3, fvals.numeric))

There is more on strapply and fn$ at http://gsubfn.googlecode.com

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