If...while...or what else??

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.
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),]
##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??

I hope I made myself clear, if not I'll carify myself!!

Thanks in advance
Niccol?

Presumably the foloowing summarises the situation you want to avoid:

  for(i in (1:5)){for(j in ((i+1):5)){ print(c(i,j))}}
# [1] 1 2
# [1] 1 3
# [1] 1 4
# [1] 1 5
# [1] 2 3
# [1] 2 4
# [1] 2 5
# [1] 3 4
# [1] 3 5
# [1] 4 5
# [1] 5 6
# [1] 5 5

I.e. you get (5,6) when case 6 is not wanted. If (as seems likely)
you don't want (5,5) either (i.e. you only want all pairs (i,j)
from 1:5 with i<j), then the following does it:

  for(i in (1:4)){for(j in ((i+1):5)){ print(c(i,j))}}
# [1] 1 2
# [1] 1 3
# [1] 1 4
# [1] 1 5
# [1] 2 3
# [1] 2 4
# [1] 2 5
# [1] 3 4
# [1] 3 5
# [1] 4 5

However, if for some reason you do want (5,5) as well, then:

  for(i in (1:5)){for(j in (min(5,(i+1)):5)){ print(c(i,j))}}
# [1] 1 2
# [1] 1 3
# [1] 1 4
# [1] 1 5
# [1] 2 3
# [1] 2 4
# [1] 2 5
# [1] 3 4
# [1] 3 5
# [1] 4 5
# [1] 5 5

Also (though I suspect it was simply due to hasty typing), the syntax
of what you wrote above:

  for i in 1:5{
   for j in (i+1):5{
     ...
   }
  }

will not work, since you must write "for( ... )" and not "for ... ":

  for i in (1:5)
# Error: unexpected symbol in "for i"

Hoping this helps,
Ted.

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E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 13-Jan-09                                       Time: 15:09:58
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