Accessing the index of factor in by() function

You can convert my data into a dataframe simply by dat <- data.frame(id,
month, value). That doesn't help though.

for. What I intend to do is for by to loop through the data based on levels
of "id" factor (1,2, and 3), and for each level, for my function to printout
the values of "value" and "month" belonging to the section of data with that
"id".

Right now, I achieve this with a for loop but I want to avoid looping in the
data as much as possible.

On Tue, Jul 26, 2011 at 12:18 AM, Ista Zahn <izahn at psych.rochester.edu>
wrote:

Hi Merik,
by() works most easily with data.frames. Is this what you are after?

my.plot <- function(dat) { print(dat$value);
print(dat$month[dat$id==dat$value]) }
by(dat.tmp, id, my.plot)

Best,
Ista

On Mon, Jul 25, 2011 at 9:19 PM, Merik Nanish <merik.nanish at gmail.com>
wrote:

Hello,

Here are three vectors to give context to my question below:

*id ? ?<- c(1,1,1,1,1,2,2,2,3,3,3))
month <- c(1, 1, 2, 3, 6, 2, 3, 6, 1, 3, 5)
value <- c(10, 12, 11, 14, 16, 12, 10, 8, 14, 11, 15)*

and I want to plot "value" over "month" separately for each "id". Before
I
can do that, I need to section both month and value, based on ID. I
create a
my.plot function like this (at this point, it doesn't draw any plots, it
is
just an effort to help my understand what I'm doing):

*my.plot <- function(y) { print(y); print(month[id==y]) }*

Now, I tried:

*by(value, id, my.plot)*

But of course, it didn't do what I wanted. I realized that the parameter
passed to my.plot, is a "secion of value" per ID, and not the ID value
itself. Question is, how can I get the value of factor ID at each level
of
by()?

Please advise,

Merik

? ? ? ?[[alternative HTML version deleted]]

--
Ista Zahn
Graduate student
University of Rochester
Department of Clinical and Social Psychology
http://yourpsyche.org