| Re: using WHERE to search for multiple values [message #30391] |
Fri, 26 April 2002 09:05 |
Pavel A. Romashkin
Messages: 531
Registered: November 2000
|
Senior Member |
|
|
Oh boy. It looks to me that the poster in fact wanted to avoid looping
through every element of the array.
Why not try SET_INTERSECTION or what is it called, from David's web
site? The one, real fast, that uses HISTOGRAM?
No loops at all. And you can get either the elements that match or the indices.
Good luck,
Pavel
Doug Martin wrote:
>
> I'm trying to do something like the following:
>
> w=where(im eq val)
>
> with im a byte array (say, 640x480)
> and val an array (say, 10 to 40 long)
>
> I want to find where im is equal to _any_ of the values in val. But,
> val is not always the same size.
>
> Is there a way to do this using one WHERE call? There seems to be a
> giant time difference between:
>
> for i=0,(size(val))[1]-1 do begin
> w=where(im eq val(i))
> ...
> endfor
>
> and
>
> w=where(im eq val(0) or val(1) or val(2) or ...)
>
> (but I can't do the above since val has different sizes from call to
> call)
>
> Sorry for the run-on nature of this post.
>
> Thanks,
> Doug
|
|
|
|
|
|
| Re: using WHERE to search for multiple values [message #30396 is a reply to message #30394] |
Fri, 26 April 2002 05:15  |
James Kuyper
Messages: 425
Registered: March 2000
|
Senior Member |
|
|
Doug Martin wrote:
> I'm trying to do something like the following:
>
> w=where(im eq val)
>
> with im a byte array (say, 640x480)
> and val an array (say, 10 to 40 long)
>
> I want to find where im is equal to _any_ of the values in val. But,
> val is not always the same size.
>
> Is there a way to do this using one WHERE call? There seems to be a
> giant time difference between:
>
> for i=0,(size(val))[1]-1 do begin
> w=where(im eq val(i))
> ...
> endfor
>
> and
>
> w=where(im eq val(0) or val(1) or val(2) or ...)
> (but I can't do the above since val has different sizes from call to
> call)
mask = where(im ne im)
for i=0, count=(size(val))[1]-1 do begin
mask = mask OR (im eq val[i])
endfor
w = where(mask)
|
|
|
|
| Re: using WHERE to search for multiple values [message #30398 is a reply to message #30396] |
Fri, 26 April 2002 06:08 |
Paul Van Delst[1]
Messages: 1157
Registered: April 2002
|
Senior Member |
|
|
Doug Martin wrote:
>
> I'm trying to do something like the following:
>
> w=where(im eq val)
>
> with im a byte array (say, 640x480)
> and val an array (say, 10 to 40 long)
>
> I want to find where im is equal to _any_ of the values in val. But,
> val is not always the same size.
>
> Is there a way to do this using one WHERE call? There seems to be a
> giant time difference between:
>
> for i=0,(size(val))[1]-1 do begin
> w=where(im eq val(i))
> ...
> endfor
why not do it using the loop but using the COMPLEMENT keyword to WHERE so that subsequent
searches don't search the values of IM that you've already matched to the previous VAL(i)? Each
subsequent search will be on a smaller part of the IM array.
Of course:
a) you will have to reconstruct the index values to reflect their IM index rather than their
IM(complement) indices and
b) this may be even slower if the values you're looking for are randomly distributed throughout
the array (i.e. searching an array that is increasingly non-contiguous in memory). You're
arrays aren't that large tho'.
paulv
--
Paul van Delst Religious and cultural
CIMSS @ NOAA/NCEP purity is a fundamentalist
Ph: (301)763-8000 x7274 fantasy
Fax:(301)763-8545 V.S.Naipaul
|
|
|
|
| Re: using WHERE to search for multiple values [message #30405 is a reply to message #30396] |
Thu, 25 April 2002 17:05 |
Robert Stockwell
Messages: 74
Registered: October 2001
|
Member |
|
|
Doug Martin wrote:
> I'm trying to do something like the following:
>
> w=where(im eq val)
>
> with im a byte array (say, 640x480)
> and val an array (say, 10 to 40 long)
>
> I want to find where im is equal to _any_ of the values in val. But,
> val is not always the same size.
>
> Is there a way to do this using one WHERE call? There seems to be a
> giant time difference between:
>
> for i=0,(size(val))[1]-1 do begin
> w=where(im eq val(i))
> ...
> endfor
>
> and
>
> w=where(im eq val(0) or val(1) or val(2) or ...)
>
> (but I can't do the above since val has different sizes from call to
> call)
>
>
> Sorry for the run-on nature of this post.
>
> Thanks,
> Doug
>
Hi Doug,
how about using an execute statement to call the where() a la
for i = 0,n_elements(val) do valstring = valstring + 'or val('+string(i)+')'
r = execute( 'w=where(im eq' + valstring+')')
should be ok unless there are a lot of vals.
cheers
bob
PS YAY, two execute() posts in one day.
|
|
|
|
comp.lang.idl-pvwave archive
Members
Search
Help
Login
Home
