[R] Dropping columns from data frame

[Mike Harwood]

How does R do it, and should I ever be worried?  I always remove
columns by index, and it works exactly as I would naively expect - but
HOW?  The second illustration, which deletes non contiguous columns,
represents what I do all the time and have some trepidation about
because I don't know the mechanics (e.g. why doesn't the column
formerly-known-as-4 become 3 after column 1 is dropped: doesn't vector
removal from a df/list invoke a loop in C?).  Can I delete a named
list of columns, which are examples 4 and 5 and which generate the
"unary error' mesages, without resorting to "orig.df$num1.10 <- NULL"?

Thanks!

orig.df <- data.frame(cbind(
	1:10
	,11:20
	,letters[1:10]
	,letters[11:20]
	,LETTERS[1:10]
	,LETTERS[11:20]
	))
names(orig.df) <- c(
	'num1.10'
	,'num11.20'
	,'lc1.10'
	,'lc11.20'
	,'uc1.10'
	,'uc11.20'
	)
# Illustration 1: contiguous columns at beginning of data frame
head(orig.df[,-c(1:3)])

# Illustration 2: non-contiguous columns
head(orig.df[,-c(1,3,5)])

# Illustration 3: contiguous columns at end of data frame
head(orig.df[,-c(4:6)]) 	## as expected

# Illustrations 4-5: unary errors
head(orig.df[,-c(as.list('num1.10', 'lc1.10', 'uc1.10'))])
head(orig.df[,-c('num1.10', 'lc1.10', 'uc1.10')])


Mike