[R] If...while...or what else??

David Winsemius
Tue, 13 Jan 2009 15:06:11 -0000

Generally extraction of subsets is better done by subset().

myDat <- subset(dataset, subset= (variable==i) | (variable==j) )  #, or

myDat <- dataset[which( (dataset$variable==i) | (dataset
$variable==j) ), ]
#note the need to add the name of the dataframe in the second version.

Both of these avoid the difficult to debug problem that might arise
from the sometimes
incomplete evaluation of logical expressions. See the example from
this morning:

---------
z <- data.frame( x = c(5,6,5,NA,7,5,4,NA),
                 y = c(1,2,2,2,2,2,2,2) )

p <- (z$x <= 5) & (z$y == 1)
p
z[p, "p1"] <-5
z
# ok, this works fine     (no NA's)  I suppose b/c (z$x <= 5) only
gets evaluated once
z <- z[,-3]

p <- (z$x <= 5) & (z$y == 2)   # creates NA
p
z[p, "p2"] <-5            #throws an error due to the NA's
z
# this failed
--------------

The two versions of p above give different results using simple
extraction, but
the subset function deals with the two conditions in what seems to be
a sensible manner.

Then, of course, there is the standard warning: avoid naming objects
"data".

You might gain efficiency by selecting once, and then not repeating
the calls to "analysis" I * J times. As I understand the R efficiency
strategies,
one generally tries to avoid unnecessary repetition of "setting up".

--
David Winsemius


On Jan 13, 2009, at 8:20 AM, Niccol? Bassani wrote:

> Dear R users,I come to you with a quite silly question I think, but
> I hope
> you can answer me...
> That is, I've got some problems in using the if and while conditions
> in a
> loop.

Sorry,did not see that you used "if" or "while" in the
example.

>
> Substantially, I want to select rows in a dataset depending on an
> index
> variable (suppose it ranges from 1 to 5), so to make a specific
> analysis
> (homemade of course) on thie new dataset. Mi first tought has been
> to do a
> double loop like this:
>
> for i in 1:5{
> for j in (i+1):5{
> data = dataset[(variable==i) | (variable==j),]

That effort at extraction might fail. If "variable" is a column within
dataset, it is not a full-fledged object.

--
David Winsemius

>
> ##analysis
> ##analysis
> }
> }
>
> This way I should select all the couples only once (gaining in
> efficiency I
> hope). The fact is that this arrangement has a problem: that j ends
> up with
> ranging from 2 to 6, not from to 2 to 5. So when I do a subsetting
> on the
> dataset to obtain only the rows corresponding to the values of i and
> j I
> want, when the loop comes to j = 6 I have an error, of course.
> What I want to know is: how can I use the if or while condition in
> such a
> loop to avoid the routine doing the computations for this case?
> I.e., can I
> tell somehow R "Hey, if j=6, let it go and move on with the other
> computations"?
> Or maybe you can see a faster and better way of using the for
> conditions??