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st: Efficiency: was RE: Taking averages, etc.

From   Allan Reese <[email protected]>
To   Stata distribution list <[email protected]>
Subject   st: Efficiency: was RE: Taking averages, etc.
Date   Wed, 17 Dec 2003 11:51:06 +0000 (GMT)

On Wed, 17 Dec 2003, Nick Cox wrote:
> As it happens,
> . count if mi(mpg)
> . su mpg if r(N) == 0
> is not only legal, but produces the correct result.
> What happens is that Stata looks at the condition
> 	if r(N) == 0
> and says in turn: is this true for observation 1?
> for observation 2? for observation 3? and so
> forth.

By coincidence I wrote to Tech Support just yesterday on this point.
While it is possible to replace one value in a variable (say the 4th)

  replace  x = y if _n == 4

that code tests _n for every element.  It's very wasteful, though you may
not notice in most interactive sessions.  In a program however, the

  replace  x = y in 4

will be about _N times faster.  As this will typically be inside a loop
dealing will each element of the variable, it may reduce execution time
from O(_N squared) to O(_N).

R. Allan Reese                       Email:     [email protected]
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