priority of operators in the FOR ( ) statement

Dear All,
I spent an entire evening in debugging a small, fairly simple program in R 
- without success. It was my Guru in Bayesian Analysis, Thomas Fridtjof, 
who was able to diagonose the problem. He said that it took a long time 
for him also to locate the problem.
This program illustrates in some ways the shortcomings of the error 
messages that R responds with. In this case, it was quite misleading and 
directs attention to a location far removed the actual problem statement.
Without any more introductory comments, let me directly discuss the 
essential problem. I am enclosing the entire program after a brief 
discussion.

The problem arises from the following statement (nr is an integer 
constant) :
for ( i in 1:nr-1) {.......}
The unexpected problem (at least for me) is that R reads the above 
statement as (i in (1:nr)-1) {.....}. This makes i be initially as zero 
which leads to an error because the for loop in R starts from 1. The 
problem is easily fixed by writing the for loop as  ( i in 1:(nr-1)) 
{.......}. This would be an easy problem to fix if R directly indicates 
what the problem is. Instead, it gives mystifying error messages which are 
totally misleading. For example, to the program given below, I got the 
following error message (these point to commands elsewhere in the program) 
:
Error in if ((x >= 0) & (x < s2)) return(x/2) else if ((x >= s2) & (x <  : 

       missing value where TRUE/FALSE needed

I would like clarifications on the following points :
1. I am just curious to know if the priority of operators in the for 
statement ( the colon before the minus operator, for example) is a 
deliberate design decision. I have tested Matlab and found that it 
interprets my original statement correctly without an extra paranthesis.
2. Faced with a similiar problem in the future, what is a smart way of 
debugging in R to locate a problem. With this problem, I checked and 
double checked every single statement in the program, except the for 
statement because I just did not expect any problem there. I have seen 
that there is a debug package but I have not used it. Can such tools be 
used to locate a problem with greater ease? Can somebody give a concrete 
example (for the following program, for example) of a debugging routine.

*************************************************************************'
# Bayesian Data Analysis
## source("M:/programming/Rfolder/Assignments/fortest.txt")

# #Remove all objects from the workspace
rm(list=ls())
# #We will also try to note the time that the program takes
# #We will start the clock at starttime
starttime <- proc.time()[3];

my.function<-function(x) {
s2<-sqrt(2);
if ((x>=0) & (x<s2)) return(x/2)
else 
if ((x>=s2) & (x<1+s2)) return(0.2)
else 
if ((x>=1+s2) & (x<1.5+s2)) return(0.6)
else 
if ((x>1.5+s2) | (x<0)) return(0)
}

alphayx<-function(y,x) {
fy<-my.function(y)
fx<-my.function(x)
fyx<-fy/fx
# to account for 0/0 division
if (is.na(fyx)) fyx<-0
#fyx<-ifelse(is.na(fyx),0,fyx);
alpha<-min(1,fyx)
return(alpha)
}

sigma<-0.5;
#nr is the number of iterations
nr<-20
x<-numeric(nr);
x[1]<-1;
t<-1:nr;

for (i in 1:nr-1) {
xi<-x[i];
yi<-rnorm(1,mean=xi,sd=sigma);
ui<-runif(1,0,1);
ualphai<-alphayx(yi,xi);
xn<-ifelse(ui<=ualphai,yi,xi);
x[i+1]<-xn;
}

plot(t,x,type="p")

endtime<-proc.time()[3];
elapsedTime<-endtime-starttime;
cat("Elapsed time is", elapsedTime, "seconds", "\n")
*****************************************************************************'



 
This message is meant for the addressee only and may contain 
confidential and legally privileged information. Any unauthorised 
review, use, copying, storage, disclosure or distribution of this e-
mail and any attachments is strictly prohibited. If you are not the 
named recipient or have otherwise received this communication in 
error, please destroy this message from your system and kindly notify 
the sender by e-mail. Thank you for your co-operation.

[[alternative HTML version deleted]]