r - Error: missing value where TRUE/FALSE needed in while loop -

I am trying to write programs using a while loop:

  n = 50 sum = array (0, n) sum [1] = 0 (for 1: n) {sum [k + 1] = sum [k] + k while ((sum [k + 1] -sum [k] Sample:   
 Its display: ;)      
 Can anyone say? What is wrong with this code  
 
 In the second instance,  sum [k + 1] = NA < / Code> it will be evaluated by:  
  (amount [2 + 1] -sim [1]) <10>  where  sum [2 +1] = Yoga [3]  is  NA . None of   TRUE / FALSE  of  (sum [k + 1] -sum [k]) <10  will not be evaluated.  
  change (s) | Zodiac of [1] -som [k] -------------- + ------------------ 1. Zodiac [2] - zodiac [1] They both know 2 | Zodiac [3] - Zodiac [2] Amount [3] What's that? (NA)