To answer your first question about read.big.matrix(), we don't know what
your acc3.dat file is, but it doesn't appear to have been detected as a
standard file (like a CSV file) or -- perhaps -- doesn't even exist (or
doesn't exist in your current directory)?

Next:

> In addition, I am planning to do a multiple imputation with MICE package
> using the data read by bigmemory package.
> So usually, the multiple imputation code is like this:
 > imp=mice(data.frame,m=50,seed=1234,print=F)
> the data.frame is required. How can I change the big.matrix class
> generated by bigmemory package to a data.frame?

Please read the help files for bigmemory -- only matrix-like objects are
supported.  However, the more serious problem is that you can't expect to
run just any R function on a big.matrix (or on an ff object, if you check
out ff for some nice features).  In particular, for large data sets you
would likely use up all of RAM (other reasons are more subtle and
important, but out of place in this reply).

Jay

-- 
John W. Emerson (Jay)
Associate Professor of Statistics
Department of Statistics
Yale University
http://www.stat.yale.edu/~jay

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