How to read a file containing two types of rows - (for the Netflix challenge data format)

grab<- function(file)
{
 fin<- fread(file=file,
 sep=NULL,
 dec=".",
 quote="", nrows=Inf, header=FALSE,
 stringsAsFactors=FALSE, verbose=FALSE,
 col.names=c("record"),
 check.names=FALSE, fill=FALSE, blank.lines.skip=FALSE,
 showProgress=TRUE,
 data.table=FALSE, skip=0,
 nThread=2, logical01=FALSE, keepLeadingZeros=FALSE)
 cat(sprintf("Read '%s'.\n", file))
 #
 substance<- apply(X=fin, MARGIN=1, FUN=function(r) chartr(",", "\t", r[1]))
 cat(sprintf("Translated '%s'.\n", file))
 D<- fread(text=substance,
 sep="\t",
 dec=".",
 quote="", nrows=Inf, header=FALSE,
 stringsAsFactors=FALSE, verbose=FALSE,
 col.names=c("ip", "valid.hits", "err.hits", "megabytes"),
 check.names=FALSE, fill=FALSE, blank.lines.skip=FALSE,
 showProgress=TRUE,
 data.table=FALSE, skip=0,
 nThread=2, logical01=FALSE, keepLeadingZeros=FALSE)
 cat(sprintf("Parsed '%s'.\n", file))
 ip<- D$ip
 withinBlock<- sapply(X=ip, FUN=function(s) as.integer((strsplit(x=s, split=".", fixed=TRUE)[[1]])[4]))
 D$within.block<- withinBlock
 return(D)
}