How to ignore lines when reading files in R

https://stat.ethz.ch/pipermail/r-help/2012-January/301518.html

> tf <- tempfile()
> cat(c("a,b", "1,2", "3,4", "5,,6", "7,8"), file=tf, sep="\n")
> # following will fail
> read.table(tf, sep=",", header=TRUE)
Error in scan(file, what, nmax, sep, dec, quote, skip, nlines, na.strings,  :
  line 1 did not have 3 elements
> # following reads lines with 2 fields only
> textLines <- readLines(tf)
> counts <- count.fields(textConnection(textLines), sep=",")
> read.table(textConnection(textLines[counts == counts[1]]), header=TRUE, sep="\t")
  a b
1 1 2
2 3 4
3 7 8