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Use of ++ operator to count frequency of an entry (Was: Majority Voting) [message #65473]

Sun, 08 March 2009 06:26

Allan Whiteford
Messages: 117
Registered: June 2006

Senior Member

Hi,

A couple of weeks ago it was pointed out that the ++ operator used in a
vectorised form would work on repeated indices meaning that it's
possible to do this:

IDL> a=[1,2]
IDL> ++a[[0,0,0,1,0,0]]
IDL> print,a
6 3

here a[0] has been incremented five times because the index 0 appears
five times.

It was pointed out that this meant that David's mode calculation example of:

array = [1, 1, 2 , 4, 1, 3, 3, 2, 4, 5, 3, 2, 2, 1, 2, 6, -3]
distfreq = Histogram(array, MIN=Min(array))
maxfreq= Max(distfreq)
mode = Where(distfreq EQ maxfreq) + Min(array)
Print, mode

could be re-written as:

array = [1, 1, 2 , 4, 1, 3, 3, 2, 4, 5, 3, 2, 2, 1, 2, 6,-3]
f=intarr(max(array)-min(array)+1)
f[array-min(array)]++
junk=max(f,idx)
mode=idx + min(array)
print,mode

(in fact I see David's article on this has already been updated to
include the ++ solution). It was also shown that this solution wasn't
any slower than using histogram. It's not necessarily better but in
certain situations could be easier to read.

Anyway, it was pointed out that this meant that a++ behaves differently
from a+=1 which some people didn't like and others didn't mind. Some of
us (ok, mainly me) worried that using this behaviour of the ++ operator
leading to problems later if it turned out that the behaviour wasn't
intended by ITTVIS and they would try to rationalise ++ with +=.

I volunteered to contact them to query if the behaviour was intended and
if it could be considered future-proof. The response included:

"the output of the following commands seem to be the expected one (and
should not change in the future)

a = [1, 2]
a [[0, 0, 1, 0]]++
print,a"

so, as promised, this is me sharing the result of the query with the
rest of the group. Everyone can feel safe about relying on the ++
operator to work on repeated indices.

Thanks,

Allan

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[Message index]

		Use of ++ operator to count frequency of an entry (Was: Majority Voting) By: Allan Whiteford on Sun, 08 March 2009 06:26
		Re: Use of ++ operator to count frequency of an entry (Was: Majority Voting) By: Allan Whiteford on Mon, 09 March 2009 08:40
		Re: Use of ++ operator to count frequency of an entry (Was: Majority Voting) By: Carsten Lechte on Mon, 09 March 2009 07:06
		Re: Use of ++ operator to count frequency of an entry (Was: Majority Voting) By: Jeremy Bailin on Mon, 09 March 2009 05:33

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