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Re: Efficiently perform histogram reverse indices like procedure on a string array? [message #80990 is a reply to message #80989]

Thu, 26 July 2012 10:30

ben.bighair
Messages: 221
Registered: April 2007

Senior Member

On Wednesday, July 25, 2012 10:17:16 PM UTC-4, Jeremy Bailin wrote:
> On 7/25/12 9:09 PM, Bogdanovist wrote:
> > I have an array of a data structure, one tag of which is a string identifier indicating which location the data belongs to. There are many thousands of data points, but only about a dozen or so unique locations.
> >
> > I make frequent use of the HISTOGRAM function with the reverse_indices in order to carve up data by some identifier, most commonly the time. In this case, I want to divide out the data by site efficiently. I can't use HISTOGRAM on strings, so I need some other approach. There are plenty of ways this can be done, but I'd like some views on the better and most efficient ways to do it.
> >
> > Take an example, say we have a simple string array
> >
> > foo=['a','b','c& #39;,'b','b','a& #39;,'a','c']
> >
> > To determine the list of unique strings we could do
> >
> > sfoo = foo[sort(foo)]
> > print,sfoo[uniq(sfoo)]
> >
> > We can then repeatedly use WHERE to find the indices in the data array(s) corresponding to each site.
> >
> > Is there a quicker/better way to do this? Repeatedly calling WHERE seems inefficient (certainly HISTOGRAM is way faster when it is usable)
>
> Use VALUE_LOCATE to find where in the list of unique indices the
> elements belong to, and use that index as a number that you can run
> HISTOGRAM on.
>
> (raise your hand everyone who saw that coming...)
>
> -Jeremy.

Not me. I had no idea VALUE_LOCATE works on strings. Now that is cool!

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[Message index]

		Re: Efficiently perform histogram reverse indices like procedure on a string array? By: Craig Markwardt on Thu, 26 July 2012 14:33
		Re: Efficiently perform histogram reverse indices like procedure on a string array? By: Jeremy Bailin on Thu, 26 July 2012 10:41
		Re: Efficiently perform histogram reverse indices like procedure on a string array? By: ben.bighair on Thu, 26 July 2012 10:30
		Re: Efficiently perform histogram reverse indices like procedure on a string array? By: Jeremy Bailin on Wed, 25 July 2012 19:17
		Re: Efficiently perform histogram reverse indices like procedure on a string array? By: ben.bighair on Wed, 25 July 2012 18:34

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