R help - May 2008 - inconsistent output when using variable substitution

I am extremely puzzled by this behavior in R. I have a data frame called 
Trials in which I have results from an experiment. I am trying to do a 
subjects analysis, but getting weird results. Each row has 1 trial in it, 
which includes a column for the subject number I get the list of subject 
numbers like so:
> Subj=unique(sort(Trials$Subj))
Then I loop over them. But I get strange results. As a test, I tried the 
following:
> i=1
> Subj[i]
[1] 49
> thisSubj = subset(Trials,Trials$Subj==Subj[i])
> thisSubj$Ansr[1]
[1] "abacus"
> thisSubj = subset(Trials,Trials$Subj==49)
> thisSubj$Ansr[1]
[1] "able"

I am expecting to get a result of "able" both times, since I know that
Subj[i]
is 49, when i=1. Clearly the two different "thisSubj=..." lines are
not
returning the same values, as I would expect them to.

Is this the normal and correct behavior for R, or am I missing something? And 
if it is normal and correct, what do I need to do to get the behavior I am 
expecting.

Thanks in advance for any advice.

Rob
--
Robert Felty
http://robfelty.com

"In this house we obey the laws of thermodynamics!"
  -- Homer Simpson

Mark,

Thanks for the reply. 
> hi:  subset doesn't know what i  is but I don't have enough
knowledge
> about scope to know what it's actually doing in that case. my
> point is that i wouldn't put i inside a subset command and expect it to
> know the value. scope is  very complex in R so
> doing things in the simplest manner is best like the way you do it with
> 49. hopfully someone else will reply with
> a more detaild answer but i have a feeling that that is where your
> problem lies. good luck.
>
I still get the wrong result without i:
Subj[1]
[1] "49"
> thisSubj = subset(Trials,Trials$Subj=="49")
> thisSubj$Ansr[1]
[1] "able"
> thisSubj = subset(Trials,Trials$Subj==Subj[1])
> thisSubj$Ansr[1]
[1] "abacus"

The problem is that I want to use this in a loop, so that I can get a score 
for every subject. Maybe there is an alternative way to do this?

Rob
--
Robert Felty
http://robfelty.com

Reality is known to have a liberal bias.
  -- Stephen Colbert

Robert Felty <robfelty <at> indiana.edu> writes:
> 
> I am extremely puzzled by this behavior in R. I have a data frame called 
> Trials in which I have results from an experiment. I am trying to do a 
> subjects analysis, but getting weird results. Each row has 1 trial in it, 
> which includes a column for the subject number I get the list of subject 
> numbers like so:
> > Subj=unique(sort(Trials$Subj))
> Then I loop over them. But I get strange results. As a test, I tried the 
> following:
> > i=1
> > Subj[i]
> [1] 49
> > thisSubj = subset(Trials,Trials$Subj==Subj[i])
> > thisSubj$Ansr[1]
> [1] "abacus"
> > thisSubj = subset(Trials,Trials$Subj==49)
> > thisSubj$Ansr[1]
> [1] "able"
> 
> I am expecting to get a result of "able" both times, since I know
that Subj[i]
> is 49, when i=1. Clearly the two different "thisSubj=..." lines
are not
> returning the same values, as I would expect them to.
Yes, this is "expected behavior", but nevertheless sonfusing and a
source of
some unexpected errors.

The sorting leads to a re-arrangement of the levels, so that Subj is a different
beast after the sorting. 

Please, do post complete, self-running examples next time that can be pasted
into RGui. It needs 2 minutes to find the problem in the example below, and 10
minutes to resolve the "what did he think" and remove the >

Dieter

--------------------------------------
Trials = data.frame(Subj=c(7,5,5,3,3,7),
                    Ansr = letters[1:6])
Trials$Subj = as.factor(Trials$Subj)
Subj = unique(Trials$Subj)

str(Subj)
# Factor w/ 3 levels "3","5","7": 3 2 1
Subj[1]
subset(Trials, Trials$Subj==Subj[1])


Subj = sort(unique(Trials$Subj))
str(Subj)
# Factor w/ 3 levels "3","5","7": 1 2 3
Subj[1]
subset(Trials, Trials$Subj==Subj[1])