TRUE/FALSE

What's pos?
What's o?
What is your loop looping over?
Where's your reproducible example provided using dput()?

And more generally, have you read R FAQ 7.31 and the posting guide?

Most likely either o is wrong, or there are NA values in pos, but we
have no way to diagnose that.

Sarah

Any reason I'd get an error like this?

Error in if (round(pos[o + 1]) == (pos[o + 1] - 0.4)) { :
  missing value where TRUE/FALSE needed

but when i do it individually, out of the for loop,

(round(pos[o+1])==(pos[o+1]-.4) )

  65
TRUE

Sarah Goslee
http://www.functionaldiversity.org

Any reason I'd get an error like this?

Error in if (round(pos[o + 1]) == (pos[o + 1] - 0.4)) { :
   missing value where TRUE/FALSE needed

but when i do it individually, out of the for loop,

(round(pos[o+1])==(pos[o+1]-.4) )

   65
TRUE

Elementary, my dear cm. pos is a vector of length n. From your 
description of the problem, I suspect that you are trying to run a loop 
that looks something like this:

for(o in 1:n)) {
  if(round(pos[o+1]) == (pos[o+1]-0.4))
   print(o)
}

The error occurs on the nth loop. The additional clue that you have 
unwittingly provided in the form of the number 65 that shouldn't be 
there brings me to the conclusion that n=65.

Jim