[Numpy-discussion] puzzle: generate index with many ranges
Raik Gruenberg
raik.gruenberg@crg...
Sun Feb 1 05:02:04 CST 2009
Beautiful! That should do the trick. Now let's see how this performs against the
list comprehension...
Thanks a lot!
Raik
Rick White wrote:
> Here's a technique that works:
>
> Python 2.4.2 (#5, Nov 21 2005, 23:08:11)
> [GCC 4.0.0 20041026 (Apple Computer, Inc. build 4061)] on darwin
> Type "help", "copyright", "credits" or "license" for more information.
> >>> import numpy as np
> >>> a = np.array([0,4,0,11])
> >>> b = np.array([-1,11,4,15])
> >>> rangelen = b-a+1
> >>> cumlen = rangelen.cumsum()
> >>> c = np.arange(cumlen[-1],dtype=np.int32)
> >>> c += np.repeat(a[1:]-c[cumlen[0:-1]], rangelen[1:])
> >>> print c
> [ 4 5 6 7 8 9 10 11 0 1 2 3 4 11 12 13 14 15]
>
> The basic idea is that the difference of your desired output from a
> simple range is an array with a bunch of constant values appended
> together, and that is what repeat() does. I'm assuming that you'll
> never have b < a. Notice the slight ugliness of prepending the
> elements at the beginning so that the cumsum starts with zero.
> (Maybe there is a cleaner way to do that.)
>
> This does create a second array (via the repeat) that is the same
> length as the result. If that uses too much memory, you could break
> up the repeat and update of c into segments using a loop. (You
> wouldn't need a loop for every a,b element -- do a bunch in each
> iteration.)
>
> -- Rick
>
> Raik Gruenberg wrote:
>
>> Hi there,
>>
>> perhaps someone has a bright idea for this one:
>>
>> I want to concatenate ranges of numbers into a single array (for
>> indexing). So I
>> have generated an array "a" with starting positions, for example:
>>
>> a = [4, 0, 11]
>>
>> I have an array b with stop positions:
>>
>> b = [11, 4, 15]
>>
>> and I would like to generate an index array that takes 4..11, then
>> 0..4, then
>> 11..15.
>>
>> In reality, a and b have 10000+ elements and the arrays to be
>> "sliced" are very
>> large so I want to avoid any for loops etc. Any idea how this could
>> be done? I
>> thought some combination of *repeat* and adding of *arange* should
>> do the trick
>> but just cannot nail it down.
>>
>> Thanks in advance for any hints!
>>
>> Greetings,
>> Raik
>
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>
--
________________________________
Dr. Raik Gruenberg
http://www.raiks.de/contact.html
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