Re: findng array[3] in array[3,n]

[Craig Markwardt <craigmnet(at)cow.physics.wisc.edu>]

"tbowers" <tbowers@nrlssc.navy.mil> writes:

> If I have
> 
> a=[ $
>  [0,1,2], $
>  [3,4,5], $
>  [6,7,8]]
> 
> b=[3,4,5]
> 
> how do I find where in a is the row vector b? The answer should
> be 1, the 2nd row of a. I've tried many
> permutations of where(), but I just don't get it. The only way I
> can get an answer is to loop through the rows till i find a match.
> What I'm really tryin' to do is to find a color in a color table,
> e.g. load RAINBOW color table, (loadct,13) then identify
> where a color is. For the color that's listed 6th in the color table
> (an almost black) that'd be the 5th row index. Like:
...
>

Your problem is that WHERE only does a 1D search.  So you need to
somehow convert your triplets to a single number.

Solution 1:

1. Convert your 3 BYTE values to a single LONG value:
    colorwords = long(r) + ishft(long(g),8L) + ishft(long(b),16L)
  do the same for your target, and use WHERE to find the match

  This will be fast if you need a few matches, slow if you need a ton
  of matches.

2. Search on R value alone using WHERE, then use a FOR loop to scan
   the resulting matches.

   wh = where(r EQ rtarg, ct)
   if ct GT 0 then for i = 0, ct-1 do if ...

   Probably overkill.

3. Use a Euclidean distance to find the color table entry with the
   smallest distance from you target value [rtarg, gtarg, btarg].  You
   should convert R G and B to vectors of type LONG to prevent
   overflow:

   dist = (r-rtarg)^2 + (g-gtarg)^2 + (b-btarg)^2
   wh = where(dist EQ min(dist))

   This will be the most robust to small variations in the color table
   (ie, if an exact match doesn't exist).

Any other ideas?

Craig

-- 
--------------------------------------------------------------------------
Craig B. Markwardt, Ph.D.         EMAIL:    craigmnet@cow.physics.wisc.edu
Astrophysics, IDL, Finance, Derivatives | Remove "net" for better response
--------------------------------------------------------------------------