| Re: Succinct way of testing array membership [message #81201] |
Thu, 30 August 2012 12:16  |
wlandsman
Messages: 743
Registered: June 2000
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Senior Member |
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For very large vectors, WHERE() methods are slow because (1) you compute the index of where the member is found, even if you just care about membership, and more important (2) WHERE operates on the entire vector, and does not exit as soon as the member is found. A faster method would be
function array_contains,v,x
;Is x an element of V?
return,~array_equal( (v eq x), 0b)
end
since ARRAY_EQUAL exits as soon as the first match is found. I can't imagine the speed difference is significant for normal size vectors, though.
--Wayne
On Thursday, August 30, 2012 12:51:48 PM UTC-4, godber wrote:
> Is there a more succinct way of testing array membership than using where and n_elements on the indexes? Something like array_contains(a, 'pancakes')?
>
>
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> Austin
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| Re: Succinct way of testing array membership [message #81204 is a reply to message #81203] |
Thu, 30 August 2012 09:59  |
Russell Ryan
Messages: 122
Registered: May 2012
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Senior Member |
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On Thursday, August 30, 2012 12:51:48 PM UTC-4, godber wrote:
> Is there a more succinct way of testing array membership than using where and n_elements on the indexes? Something like array_contains(a, 'pancakes')?
>
>
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> Austin
wouldn't that just be liek this:
suppose:
t=['pancakes','eggs','milk','sausage'] ;array to test
then:
g=where(strmatch(t,'pancakes',/fold),n) ;the line testing it
if n ne 0 then print,'Its in the array!'
-russell
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| Re: Succinct way of testing array membership [message #81298 is a reply to message #81203] |
Thu, 30 August 2012 14:12 |
Helder Marchetto
Messages: 520
Registered: November 2011
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Senior Member |
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On Thursday, August 30, 2012 7:09:35 PM UTC+2, Mike Galloy wrote:
> On 8/30/12 10:51 AM, godber wrote:
>
>> Is there a more succinct way of testing array membership than using where and n_elements on the indexes? Something like array_contains(a, 'pancakes')?
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>>
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>> Austin
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>>
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> In general, how about:
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> print, where(a eq 'pancakes', /null) ? 'found' : 'not found'
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> Mike
>
> --
>
> Michael Galloy
>
> www.michaelgalloy.com
>
> Modern IDL: A Guide to IDL Programming (http://modernidl.idldev.com)
>
> Research Mathematician
>
> Tech-X Corporation
Hi Mike,
I never noticed the presence of the /null keyword. Nice tip.
However, the version you provided will not work (at least not on my pc...).
How about:
print, (where(a EQ 'pancakes', /null) NE !NULL) ? 'found' : 'not found'
or
print, ((where(a EQ 'pancakes'))[0] GE 0) ? 'found' : 'not found'
Not as clean, but still doing the job.
Cheers,
Helder
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| Re: Succinct way of testing array membership [message #81299 is a reply to message #81205] |
Thu, 30 August 2012 13:57 |
godber
Messages: 4
Registered: January 2012
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Junior Member |
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On Thursday, August 30, 2012 9:57:02 AM UTC-7, Coyote wrote:
> Godber writes:
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>
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>> Is there a more succinct way of testing array membership than using where and n_elements on the indexes? Something like array_contains(a, 'pancakes')?
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> It might take you 30 seconds to write such a thing!
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> Cheers,
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> David
Yeah David it does, but look at all the nice trys by people below. In fact some of them were very informative. I like Mike's suggestion, and it points out /null to me and wayne is kind enough to point out a quicker solution. Therefor I declare the question a success.
I mean, it still is a little surprising to me that there isn't an inbuilt way to test list or array membership.
Thanks for the alternatives guys.
Austin
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