[Numpy-discussion] bitwise view on numpy array

Vincent Schut schut@sarvision...
Wed May 6 06:28:11 CDT 2009


Hi,

I'm gonna have large (e.g. 2400x2400) arrays of 16 and 32 bit bitfields. 
  I've been searching in vain for an efficient and convenient way to 
represent these array's individual bit's (or, even better, configureable 
bitfields of 1-4 bits each).
Of course I know I can 'split' the array in its separate bitfields using 
bitwise operators and shifts, but this will greatly increase the memory 
usage because it'll create one byte array for each bitfield.
So I was looking for a way to create a bitwise view on the original 
array's data. I've been looking at recarray's, but the smallest element 
these can use are bytes, correct?. I've been looking at ctypes arrays of 
  Structure subclasses, which can define bitfields. However, these will 
give me an object array of elements with the Structure class subclass, 
and only allow me to access the bits per array element instead of for 
the entire array (or a subset), e.g. data[:].bit17-19 or someting like that.
After searching the net in vain for some hours, the list is my last 
resort :-) Anyone having ideas of how to get both memory-efficient and 
convenient access to single bits of a numpy array?

On a slightly related note, during my search I found some comments 
saying that numpy.bool arrays use an entire byte for each element. Could 
someone confirm (or, better, negate) that?

Thanks,
Vincent.



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