| Re: Optimizing a lookup table [message #52805 is a reply to message #52804] |
Fri, 02 March 2007 17:30  |
mmeron
Messages: 44
Registered: October 2003
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In article <1172884513.057451.183700@t69g2000cwt.googlegroups.com>, ianpaul.freeley@gmail.com writes:
> I would like to write a function that uses a look-up table. Now the
> easiest (coding-wise), is to just save the look-up table and have the
> function restore it. However, I plan to be calling this function in a
> loop, so this would result in numerous unnecessary disk-reads.
> Anyway, what is the most efficient way to get a lookup table into an
> IDL function?
>
> My thoughts so far:
> 1) Just print the look-up table to the screen, copy/paste into the
> function and add some brackets and commas, presto, variable is in the
> function and will stay loaded as long as the function is compiled.
> This will work for my present problem, but would be unwieldy for
> really large look-up tables and I worry about double-precision getting
> truncated on the print.
> 2) have a procedure that reads in the table and puts it in a common
> block, then just start my function with an if-statement to see if the
> common block exists and if not, call the reading procedure. My
> question here is, how can I check to see if a common block has already
> been created? I know I could call the common block maker outside the
> loop, but that seems lame and makes it more complicated if I want to
> share the code.
>
> Unless someone comes up with something really witty, I'll just use
> option 1. Just seems like this would be a common problem that
> someone's solved before.
>
As for checking whether a common block has already been defined,
that's pretty simple. Have your function check whether some test
variable within the procedure exists (N_ELEMENTS will do for the
testing) and have the routine which loads the table assign some value
(doesn't matter what) to same variable.
Personally, thoug, I would prefer to have a procedure which creates a
system variable (using DEFSYSV) and loads the table into it. And, I
would call it at startup. This way the table is accessible everywhere
with no need for any testing.
Mati Meron | "When you argue with a fool,
meron@cars.uchicago.edu | chances are he is doing just the same"
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