| Re: Preserving Variables [message #10183] |
Fri, 31 October 1997 00:00  |
Stein Vidar Hagfors H
Messages: 32
Registered: May 1997
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William Thompson wrote:
>
> Matthew Hanson <matt@ktaadn.com> writes:
>
>> Hello,
>> Anyone know how to make statis variables? That is, upon returning to a
>> function, how can i create a variable that has the same value as when i
>> left the function before? I really don't want to use Common Blocks.
>> -matt
>
> I understand your concern about common blocks, but if a common block is
> restricted to a single routine for the sole purpose of retaining information
> between calls, then I think most of those concerns disappear. I make such a
> "private" common block have the same name as the routine, to avoid running into
> a common block of the same name from another routine.
I agree - this is a very good use of common blocks. I often use
similar constructs to make e.g., a cache for stuff that is
computationally expensive, or needs to load data etc. from files.
It's really one of the best things to use common blocks for.
E.g.:
FUNCTION pick_entry,id
common pick_entry_cache,ids,values
if n_elements(ids) eq 0 then begin
<read ids, values from a file>
endif
return,values(where(ids eq id))
end
> The other way to preserve data, of course, is to use DEFSYSV to define a global
> system variable, but that can be rather awkward.
Yep - you basically have to put the defsysvar statement in the startup
file, because IDL refuses to compile any program referring to an
as yet undefinded system variable (unless you package it into an
execute statement - but that's even more cumbersome).
Stein Vidar
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