| Re: Array multiplication: implicit loop query [message #26243 is a reply to message #26185] |
Mon, 13 August 2001 07:43   |
John-David T. Smith
Messages: 384
Registered: January 2000
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Senior Member |
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Richard Younger wrote:
>
> "Bill B." wrote:
>>
>> george@apg.ph.ucl.ac.uk (george Millward) wrote in message
>
>>> It
>>> would seem that, to get this to work I need to make
>>> Pres=fltarr(30,91,40).
>>
>> Yes.
>>
>> IDL> a = indgen(20,20)
>> IDL> b = indgen(20)
>> IDL> c = b * a
>> IDL> help, c
>> C INT = Array[20]
>>
>> I believe you need to REPLICATE 'Pres' as needed.
>>
>
> I've been converted to REBIN, myself.
> (see the group archives for the dimensional juggling tutorial by JD
> Smith this past spring)
>
Don't abandon those subscripting array inflation techniques just yet
though! While rebin/reform is conceptually simpler (especially for more
than 2 dimensions), the old lindgen() method still has its place. When,
you ask? Well, rebin works only with numeric data. If you have an
array of structures, pointers, or objects, you'll need to fall back on
the ancestral methods.
The idea is simple. Construct an array of indices of the size you're
after, and use "mod" and "/" to massage it into the correct form for
indexing into the original array. If you have many such arrays to
inflate, it may even be competitive in speed (since you have to
precompute the index array only once).
In 2D it's simple.
IDL> a=findgen(5)
IDL> inds=lindgen(5,10)
IDL> big_a=a[inds mod 5] ; across
IDL> inds=lindgen(10,5)
IDL> big_a=a[inds/10] ; down
for higher dimensions, it quickly becomes cumbersome (try it and see).
JD
P.S. Here's an example of this method's use in the field... a little
function I cooked up to find where in one vector elements of another
vector do *not* exist. As a bonus, not a histogram in there.
function where_not_array,A,B,cnt,IA_IN_B=iA_in_B
Na = n_elements(a)
Nb = n_elements(b)
l = lindgen(Na,Nb)
AA = A(l mod Na)
BB = B(l / Na)
if keyword_set(iA_in_B) then $
wh = where(total(AA ne BB,2) eq Nb,cnt) $
else wh = where(total(AA ne BB,1) eq Na,cnt)
return,wh
end
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