survival bug? - solved

I _think_ the relevant section of the C standard is 6.5.6 Additive Operators Par 8, excerpted here:

If both the pointer operand and the result point to elements 
of the same array object, or one past the last element of the 
array object, the evaluation shall not produce an overflow; 
otherwise, **the behavior is undefined**.

This is from the [C11 draft][1], though I imagine has been part of the standard for a while.? So by doing id[-1], in this case the pointer operand is id, and the result is one element _before_ the array object, thus undefined behavior which is bad news.

I'm not an expert in these matters though.

[1]: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf

?I ended up finding the issue by a focused code review.

Once in the past, I had a version that would fail under one architecture but not another, 
in that case some help from Brian Ripley pointed me to the offending line of C code.?? 
That line read, but did not write, at an invalid memory location.?? Starting with the 
question of "what C routines have I added or modified most recently" along with where the 
fault appeared to occur in my test suite, I started reading C code and found one.?? 
Revised code passes tests on the winbuilder site.

For the curious, I had a line asking "is this patient id different than the last patient 
id" in the C routine underneath survcheck(); I'm making sure that patients don't go 
backwards in time. Essentially
?for (i=0; i< n; i) {
? ?? if (id[i] != id[i-1] )? { ...}

It is still a surprise to me that just LOOKING at this out of range element would cause a 
failure,? [i-1] never appears on the left hand side of any expressions in the ... chunk 
above. Nevertheless, it was an error. ? Que sera sera

A strong thanks to those who gave solid suggestions for bringing up a local copy of Windows.

Terry T

My latest submission of survival3.1-10 to CRAN fails? a check, but only on
windows, which
I don't use.
How do I track this down?
The test in question works fine on my Linux box.

Terry



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