| Re: advise for saving a for-loop [message #67986 is a reply to message #67984] |
Tue, 15 September 2009 02:21   |
Bernhard Reinhardt
Messages: 26
Registered: October 2008
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Junior Member |
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Jean H. wrote:
> Bernhard Reinhardt wrote:
>> Hi,
>>
>> I don't know if there's a special term for what I'm trying to do:
>>
>> I have two 2D arrays of the same size (msg_x and msg_y) which contain
>> x- and y-values. So msg_y consists of rows which contain mainly the
>> same values and msg_y consists of columns which contain mainly the
>> same values. But it has to mentioned that values are slightly changing
>> in a row or column. That's what make's things nasty.
>> For msg_y it means, it may look like:
>> 1000 1000 1000 1000 [..] 1001 1001 1001 1001 [..] 1002 1002
>> 1001 1001 1001 1001 [..] 1002 1002 1002 1002 [..] 1003 1003
>> 1002 1002 1002 [..] 1003 1003 1003 1003 [..] 1004 1004 1004
>>
>> I also have two linear arrays li_x and li_y of the same size. I now
>> want to make a map with the same dimensions of msg_x with a 1 where
>> the points in the linear arrays match into the pseudo-grid and 0
>> elsewhere.
>>
>> Here's how I do it at the moment:
>>
>> for i=0, N_ELEMENTS(li_x)-1 do begin
>> ind=WHERE(msg_x eq li_x[i] AND msg_y eq li_y[i])
>> if ind[0] ne -1 then ligrid[ind] = 1
>> endfor
>>
>> The 2-D arrays have sizes of 600x600 or 1800x1800 and the linear
>> arrays are of size 10000.
>>
>> This means where has to search 10000 over the two 2D-arrays which
>> takes some time.
>>
>> I guess there must be a smarter way to do. I thought about some
>> solutions involving sort and histogram but so far I couldn't come up
>> with a solution without for-loops.
>>
>> I'd be pleased if someone of you could enlighten me.
>>
>> Regards,
>>
>> Bernhard
>
> Hi Bernhard,
>
> yes, histogram is the way to go. You will want to 1) intersect msg_x and
> li_x, then 2) msg_y and li_y and 3) the ouptut of 1 and 2 (index based)
>
> Have a look at
> http://www.dfanning.com/tips/set_operations.html
>
> you can get ri from the 1st histogram and return the following, to get
> the index:
>
> r = Where((Histogram(a, Min=mina, Max=maxa, reverse_indices=ri) NE 0)
> AND (Histogram(b, Min=mina, Max=maxa) NE 0), count)
>
> IF count eq 0 THEN RETURN, -1
> toReturn = ri[ri[r[0]]:ri[r[0]+1]-1]
>
> for Rcount = 1, count-1 do begin
> toReturn = [toReturn,ri[ri[r[Rcount]]:ri[r[Rcount]+1]-1]]
> endfor
Hi Jean,
I tamed the beast - well a least it seems it's doing what I expect for
now. I modified your suggested code a bit. Intersecting the resulting
indices is the wrong way to go. It would yield way to much "hits".
I now look at indices of values that exist in both msg_x and li_x and
then where the corresponding y-values in both arrays match, too. But I
have to do it for one single x-value at a time. But although I have a
double for-loop, speed-up is about 200x :)
mina=min(msg_x)
maxa=max(msg_x)
r = Where((Histogram(msg_x, Min=mina, Max=maxa,
reverse_indices=rim) $ NE 0) AND (Histogram(li_x, Min=mina, Max=maxa,$
REVERSE_INDICES=ril) $
NE 0), count)
IF count gt 0 THEN begin
for Rcount = 0, count-1 do begin
lind=ril[ril[r[Rcount]]:ril[r[Rcount]+1]-1]
mind=rim[rim[r[Rcount]]:rim[r[Rcount]+1]-1]
for i=0, N_ELEMENTS(lind)-1 do begin
ind=where(msg_y[mind] eq li_y[lind[i]])
if ind[0] ne -1 then ligrid[mind[ind]] = 1
;ligrid is the map, same dim as msg_x and msg_y
endfor
endfor
endif
Bernhard
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