Sorry if I misunderstand your Qs... however when I convert R-dataframes to latex documents. I always use the Hmisc package and the latex function. Seems to work perfect:

library(Hmisc) ###an example of a dataframe dat<-data.frame(cbind(paste("rs",1:5,sep=""),c(0.1,0.00001,0.2,0.000004,0.4))) ###the latex function latex(dat)

all the best Thomas

Expanding on the sprintf comment, you could try something like this:

nums = c(0.1,0.00001,0.2,0.000004,0.4)
r2l = function(n) sub("E", " \times 10^{", sprintf("%0.3E}", n))
latex = lapply(nums, r2l)

You can also use the xtable package in R to write to Latex or HTML. Though this is not for data.frames but for matrixes.

The good thing about xtable is that you can specify how xtable should deal with small numbers in your matrix. You can use the display option to give a vector for each column:

display Character vector of length equal to the number of columns of the resulting table indicating the format for the corresponding columns. Since the row names are printed in the first column, the length of display is one greater than ncol(x) if x is a data.frame. These values are passed to the formatC function. Use "d" (for integers), "f", "e", "E", "g", "G", "fg" (for reals), or "s" (for strings). "f" gives numbers in the usual xxx.xxx format; "e" and "E" give n.ddde+nn or n.dddE+nn (scientific format); "g" and "G" put x[i] into scientific format only if it saves space to do so. "fg" uses fixed format as "f", but digits as number of significant digits. Note that this can lead to quite long result strings. Default depends on the class of x.

See for the full usage of this package the following link: http://rss.acs.unt.edu/Rdoc/library/xtable/html/00Index.html