My first idea will only bring speedup if in the records in SCAN NEXT 80 there are
quite a few duplicate high/low pairings, for instance
10 times high 100, low 60
08 times high 100, low 65

and so on.

If that is the case, check if

lnMagic = 80
select high, low, count(*) as r_cnt ;
   from (this.alias_name) ;
   where between(recno(), m.nCount, m.nCount+m.lnMagic) ;
   group by 1,2 ;
   into cursor t_from

lnRecPairs = _Tally

scan
  replace f2 WITH f2+r_cnt ;
      FOR BETWEEN(foo.f1, t_from.low, t_from.high) ;
      IN foo
endscan
? m.lnMagic - m.lnRecPairs," operations saved"

saves enough replace cycles to balance the added sql before.

Try also adding Craig's idea of using seek/replace while instead of replace for - in most multiuser situations I believe this would be the best way. Since you are working on a cursor I am not sure if vfp has not some added caching tricks, since the cdx is guaranteed to be "clean". Another option might be to set deleted off before and on after the part, as these records should always be there and some more cpu cycles could be saved.

>this code runs when ever the ncount is changed.... think like it is for scrolling of a graph ... there is a glitch in the smooth movement .... this process shows a average of 0.5 sec the maximum in the whole class other process including paint of the graph takes 0.07 secs.. so i though is there a faster was to do this ???

If it is as you describe(some kind of derivative from movings average as from the last 80 days of stock market range) then you should save your cursor "foo", subtract the very last record (nCount+80) and add the data from nCount: the rest should still be the same and all those cycles saved <g>. The records nCount to nCount +79 should be nCount+1 and nCount+80 in the next iteration, so use the already calculated sums for them. You could speed up to a runtime of perhaps 1% of the original time <bg>. Something like (all code NON tested, the lnOld -values you will have to take care for yourself in your code structure)

lparameter ncount
local ncount, llCreate

if !used('foo')
        llCreate = .t.
	CREATE CURSOR foo (f1 n(20,2),f2 i)
endif
if not m.llCreate and this.highlow1.ymin=lnOldyMin and this.highlow1.ymax= lnOldyMax
  SELECT (this.alias_name)
  GOTO ncount
  high = high
  low = low
  replace f2 WITH f2+1 FOR BETWEEN(foo.f1,m.low,m.high) IN foo

  GOTO ncount+81
  high = high
  low = low
  replace f2 WITH f2-1 FOR BETWEEN(foo.f1,m.low,m.high) IN foo

else

  select foo
  if reccount()>0
     zap
  endif
  FOR m.xi = this.highlow1.ymin TO this.highlow1.ymax STEP (ROUND((this.highlow1.ymin+this.highlow1.ymax)/2,0)*0.1)/100
        	INSERT INTO foo (f1) values(m.xi)
  NEXT
  SELECT foo
  INDEX ON f1 TAG f1


  SELECT (this.alias_name)
  GOTO ncount
  SCAN NEXT 80
	        m.high = high
        	m.low = low
*	        replace f2 WITH f2+1 FOR foo.f1 >= m.low and foo.f1 <= m.high IN foo
        	replace f2 WITH f2+1 FOR BETWEEN(foo.f1,m.low,m.high) IN foo
  ENDSCAN
endif

regards

thomas