[R] Help with error in "if then" statement

[Andrew Rominger]

Dear list,

I'm afraid this is a mundane question.  Here's the background: I've  
produced a function which allows me to sequentially measure angles and  
distances from a specified reference point to a curve defined by  
empirical data points while (i.e. using a while loop) the angle being  
measured is within certain bounds.  Because the curve is circular I  
need to pars the data into basically an "upper" curve and "lower"  
curve.  I tried to do this with an if statement, specifically:

ycrit<-subset(data,subset=data$x==min(data$x)
y.max<-length*sin(angle)+y.ref #length, angle and y.ref are given
if(y.max<ycrit){
........
}

Now the problem:  The while loop works for 4 iterations, until I get  
the error message:

"Error in if (y.max < ycrit) { : missing value where TRUE/FALSE needed"

When I forced the function to print y.max and ycrit for each iteration  
of the while loop, it returns finite real numbers, including ycrit =  
153.5 and y.max = 245.16 for the step which returns the error message.

Any ideas about what's going on here--why does R "think" that  
245.16<153.5 is "missing," or is anything other than TRUE/FALSE?  Am I  
using "if" incorrectly?  In which case would it be more appropriate to  
perhaps create subsets of the data points based on < or > ycrit?

Thanks in advance for any guidance--
Andy

-- 
Andrew J. Rominger
Department of Biological Sciences
Stanford University
rominger at stanford.edu