indexing in data frames

And if you are extremely concerned with speed, do
not compute a$b and a$c in every iteration of the loop.
E.g., change
  lapply(seq_along(a$c),function(x) a$b[x]-a$c[[x]])
to something like
  with(a, lapply(seq_along(c), function(x)b[x] - c[[x]]))

a <- data.frame(b = 1:2, c = I(list(2, 1:2)))
a <- within(a, dif <- mapply("-", b, c))
a

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
Of R. Michael Weylandt
Sent: Thursday, August 09, 2012 4:22 PM
To: arun
Cc: R help; jimi adams
Subject: Re: [R] indexing in data frames

On Thu, Aug 9, 2012 at 5:30 PM, arun <smartpink111 at yahoo.com> wrote:

lapply(1:length(a$c),function(x) a$b[x]-a$c[[x]])

Arun,

I've seen you use this idiom a few times lately and I'd just like to note that

seq_along()

is an (underutilized) primitive and a safer and faster alternative
(avoiding the pathological length(x) = 0 case).

Cheers,
Michael