[R] Sparse dataframes?

[andrewH]

Dear Folks--
Is there a data frame analog to sparse matrices? I am working with a panel
data set that has a large number of variables that are redefined repeatedly
or exist for only a few years (out of 48).  In my current structure, where
variables are columns and rows are years, more than 90 percent of the cells
and more than 3/4 of the total size of my file are NAs.  

I am wondering if there is an alternate file specification currently
available that still allows numeric, character and factor data to be stored. 
Besides just using a database. 

A pointer in the right direction (or a solid "no" if that is the truth)
would be greatly appreciated.

Sincerely, andrewH



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