That said, what Roger is referring to in his
second comment is fairly rare in my experience.
More generally, not changing the data is
easier than you imply. You just don't do it,
unless that is the purpose of the program.
Nick
[email protected]
Roger Newson
> Re Point 1, file input and output is usually MUCH more
> time-consuming than
> in-memory operations such as checking `touse'. However, re
> Point 2, Richard
> is right, and a block of code which modifies the data
> *permanently* should
> indeed start with a single -preserve- and end with a single
> -restore, not-,
> just in case the user presses -break-.
Richard Williams
> I'd be curious what thoughts people have about this one:
>
> "Avoid preserve if possible. preserve is attractive to the
> programmer but
> can be expensive in time for the user with large data files.
> Programmers
> should learn to master marksample."
>
> Suppose I ignore that advise and instead do something like
>
> preserve
> keep if `touse'
>
> 1. Will I gain some speed, because Stata will be working
> with a smaller
> data set and won't have to repeatedly process -if `touse'- commands?
>
> 2. Isn't there a potential tradeoff between this and another rule,
> respect the dataset? If you don't preserve, there is a
> greater burden on
> the programmer to make sure all is well at the end of the
> un. But, isn't
> there also a greater risk if, say, the user stops the program before
> completion?
>
> I suppose good, careful programmers can guarantee both speed
> and security
> of data. But, I would add the caveat that, if you are not
> 100% confident
> of your programming, you might want to favor safety over speed.
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