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Re: Inverting indices? [message #2498 is a reply to message #2448]

Thu, 07 July 1994 08:57

dball
Messages: 4
Registered: June 1994

Junior Member

In article <2uslco$m4k@highway.LeidenUniv.nl>, robijn@Strw.LeidenUniv.NL (Frank Robijn) writes:
> In article <kkobayas.772907009@husc9.harvard.edu>,
> Ken Kobayashi <kkobayas@husc9.harvard.edu> wrote:
>>
>> Am I making sense? I just need to do j=OPPOSITE(i,10) so that when
>> i=[1,3,5] it'll return j=[0,2,4,6,7,8,9].
>>
>> I couldn't find a built-in routine that does this. I tried the following
>> code:
>>
>> FUNCTION opposite,idx,n
>> k = bytarr(n)
>> k(idx)=1
>> return,where(not k)
>> end
>>
>> but this didn't seem to work for some reason.
>
> A 'false' value in IDL is -1, not 1. So change k(idx)=1 to k(idx)=-1 and
> it will work.
>
> Frank

Well, to be correct, a 'true' value is any 'odd' value, i.e. LSB is 1.
A 'false' value is any 'even' value, i.e. LSB is 0. The NOT operator
does a bitwise NOT, so this works out. WHERE, however, returns indices
of all *nonzero* elements, so use -1 because (NOT -1) is 0.

-- Dave

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[Message index]

		Inverting indices? By: kkobayas on Wed, 29 June 1994 09:23
		Re: Inverting indices? By: robijn on Wed, 29 June 1994 13:26
		Re: Inverting indices? By: dball on Thu, 07 July 1994 08:57
		Language Documentation; was: Re: Inverting indices? By: caron on Fri, 08 July 1994 08:49

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