[R] na.strings in readLines or is.na?

[Benjamin M. Osborne]

When reading a data set into R using readLines, na.strings="-99.99" is ignored. 
Is there an equivalent command for readLines?  Alternatively, either
immediately after reading into R or once the data set has been converted to a
data frame, what is the appropriate command (or appropriate use of is.na) to
convert my -99.99s to NAs?
Thanks,
Ben Osborne

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