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.