I guess you meant $$,
x rho y in APL is equivalent to x ($,) y in J if I remember correctly.
Roger suggested other idioms for copy-by-value but no promise that they
will work for ever.
Птн, 27 Май 2011, Ian Clark писал(а):
> Been experimenting with mapped files...
> Looking for a reliable way to detect if a mapped file has changed
> since I last inspected it.
>
> If pronoun: TITLE is mapped to a file, then
> sav=: TITLE
> will create: sav -as not a copy but an alias of TITLE. (Not
> surprisingly, because they share the same memory.)
>
> Thus reassigning sav will change TITLE too, and vice-versa. Which
> makes: sav=:TITLE no good as a way of snapshotting TITLE to detect
> changes.
>
> 1. Can you think of a low-overhead verb: own -used like so:
> sav=: own TITLE
> -which will force J to allocate sav its own separate space in memory?
>
> I've tried these:
> own=: ]
> own=: [: > [: {: 0 ; <
> own=: $ $ ]
>
> The first 2 don't work: sav keeps in step with TITLE, thanks to J's
> optimisation.
> The third defn works --but for how much longer? At some future date it
> might break, thanks to improved J optimisation.
>
> 2. Is there a more elegant way of checking whether a mapped file has changed?
>
> When I change a mapped noun I cannot see any changes to:
> showmap_jmf_'' --this is on a Mac.
> I can repeatedly read the directory entry using 1!:0 and watch for
> it to change (as its timestamp gets updated). The processing overhead
> is tolerable, but is it the best I can do?
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regards,
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