| Re: Histogram and bin sizes [message #58772] |
Thu, 21 February 2008 09:19  |
Conor
Messages: 138
Registered: February 2007
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Senior Member |
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On Feb 21, 11:29 am, jeffnettles4...@gmail.com wrote:
> On Feb 21, 9:05 am, Conor <cmanc...@gmail.com> wrote:
>
>
>
>> On Feb 20, 2:43 pm, pgri...@gmail.com wrote:
>
>>> jeffnettles4...@gmail.com wrote:
>>>> I've always wondered why you have to use a constant bin size with
>>>> HISTOGRAM().
>>>> To quote J.D.'s famous tutorial: "a histogram
>>>> represents nothing more than a fancy way to count." Doesn't an
>>>> imposed constant bin size imply that this is the only way it's ok to
>>>> count? I can think of several reasons i wouldn't want to do this - I
>>>> used logarithmic bin sizes in my dissertation, for example (now i'm
>>>> hoping someone isn't going to answer this post saying i screwed up in
>>>> my dissertation :-) ).
>
>>> I use logarithmic bins myself quite often, and the fact that a
>>> logarithmic bin
>>> size is the same as a constant bin size in log space, makes it is easy
>>> to use histogram to get that. Less regulars binning don't work with
>>> histogram, but nobody stops you from writing your own version to work
>>> with them (it will not be as fast as histogram though).
>
>>> Ciao,
>>> Paolo
>
>>>> And besides, Excel lets you use arbitrary bin
>>>> sizes....and if Excel lets you do it, it has to be ok, right???? ;-)
>
>>>> Jeff
>
>> You can always do whatever binning you want, you just have to
>> transform your data to the new space and then bin it constantly. Why
>> doesn't histogram let you use aribtrary binsizes? Not being an IDL
>> developer I don't know for sure, but I would guess it's a speed
>> issue. The simpler a program is the faster it is. I use histogram
>> all the time because it's one of the speedier programs in IDL. It
>> would make me very sad if in order to make histogram more flexible, it
>> also became much slower, especially since by transforming my data set
>> I can use aribtrary bin sizes for histogram.
>
> That actually sounds like what i've done in the past. For my
> dissertation i needed two kinds of histograms: logarithmic bins
> (which was fine, no trouble there) and bins that had arbitrary sizes.
> For the latter, i would either do the histograms in Excel (yuck) or
> compute two or three histograms in IDL using histogram() with
> different bin sizes and sort of do some "mixing and matching" of the
> resulting arrays to get what i wanted. Of course, David hadn't
> written his awesome histoplot routine yet then either :( Anyway, i'm
> up against the arbitrary bin sizes problem again for a project i'm
> doing for someone, and it got me wondering whether this situation is
> just so rare it wasn't worth supporting in histogram(). I wouldn't
> want to lose histogram's speed either though.
>
> Jeff
Arbitrary bin sizes should be pretty easy to program. You just need
to map your data points appropriately. For instance if you had the
data set:
x = randomu(seed,100)
and you wanted bins from:
[0-.1,.1-.3,.3-.35,.35-.8,.8-1]
you might do something like this:
x = randomu(seed,100)
bins = [ [0,.1], [.1,.3], [.3,.35], [.35,.8], [.8,1] ]
newx = fltarr(n_elements(x))
for i=0,n_elements(bins[0,*])-1 do begin
w = where( x ge bins[0,i] and x lt bins[1,i], c )
if c gt 0 then newx[w] = i+.5
endfor
hist = histogram(newx,binsize=1.0,min=0)
plothist,newx
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