How to write efficient R code

From: Tom Blackwell

Sebastian  -

For successive differences within a single column 'x'

differences <- c(NA, diff(x)),

same as

differences <- c(NA, x[-1] - x[-length(x)]).

See  help("diff"), help("Subscript").  The second version also
works when  x  is a matrix or a data frame, except now the result
is a matrix or data frame of the same size.

x <- data.frame(matrix(rnorm(1e+5), 1e+4))
dim(x)               # 10000    10
differences <- rbind(rep(NA, 10), x[-1, ] - x[-dim(x)[1], ])
dim(differences)     # 10000    10

However, you write "I need to do this for all the subsets of data
created by the numbers in one of the columns of the data frame ..."
and I'm not sure I understand how an 'id' column would create many
subsets of the data.  So the simple examples above may not answer
the question you are asking.

-  tom blackwell  -  u michigan medical school  -  ann arbor  -

On Tue, 17 Feb 2004, Sebastian Luque wrote:

Hi,

In fact, I've been trying to get rid of loops in my code for more
than a week now, but nothing I try seems to work. It sounds as if
you have lots of experience with loops, so would appreciate any
pointers you may have on the following.

I want to create a column showing the difference between the ith
row and i-1. Of course, the first row won't have any value in it,
because there is nothing above it to subtract to. This is fairly
easy to do with a simple loop, but I need to do this for all the
subsets of data created by the numbers in one of the columns of
the data frame (say, an id column). I would greatly appreciate
any idea you may have on this.

Thanks in advance.

Best regards,
Sebastian
--
  Sebastian Luque

sluque at mun.ca