While it is doubtless clear from the example what was meant, there is
nonetheless a typo that might throw off a newcomer to the wonderful world of
variable indexing. In the nicely worked example presented by Nick he translates
the expression "age[mlineno]" as "age[3]" when, in fact, it should have been
"age[2]." I mention this because it's really a tricky concept for someone not
accustomed to using variables as indexes. If you didn't already grasp the
principles involved, taking the values literally might leave you hopelessly
confused. So, in the excerpt quoted below, substitute "age[2]" for "age[3]" and
"second" for "third" to see the correct connections.
> -----Original Message-----
> From: [email protected]
> [mailto:[email protected]]On Behalf Of Nick Cox
> Sent: Friday, September 17, 2004 4:52 AM
> To: [email protected]
> Subject: st: RE: RE: RE: Re: How to speed up loop
>
<snip>
> Take
>
> hhid lineno age mlineno mage
> 1 1 32 . .
> 1 2 30 . .
> 1 3 5 2 30
>
> Each expression within [ ] is evaluated
> separately for each observation. For
> the first and second
>
> age[mlineno] becomes age[.]
>
> which is taken as missing. For the third,
>
> age[mlineno] becomes age[3]
>
> which by the wrinkle rule above is 30.
> It is the third observation _within that group_.
<snip>
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