[PYTHON MATRIX-SIG] Array -> String: array length

[viennet@ura1507.univ-paris13.fr]

Jean-Bernard ADDOR writes:
> 
> I am very happy to hear about the length of the numpy arrays other users
> uses because I uses these day pow(2,22) = len(array) = 4194304 and I
> seriously ask me questions about python memory managment. My program uses
> hundreds Megs of RAM. My question is how many memory uses a
> Numeric.array(pow(2,22), Numeric.Float) is it much more than pow(2,24)
> bytes ?

No. You mean: 

   x = zeros( pow(2,22), Float )

which takes exactly 8 x 2^22 = 32Mo.

I frequently use NumPy with large arrays (like 10^5 x 256 matrix).

One must be aware of the following points:

 - there is currently no way to create an unitialized array;

 - operations like x = x + b allocate temporary arrays for
   intermediate results. ufuncs (add, multiply) an be called
   with a third argument specifying the destination array:
   the form           add( x, b, x )
   will avoid unnecessary memory allocations.

 - I/O usualy involve duplication of the data (conversion from/to
   strings). A few months ago, there were interesting discussions
   about a general "buffer" mecanism, but nobody took the time to
   implement it. 

 - If you computes with float values, you may want to use the Float32
   typecode (single precision) to save space (in this case, be careful
   with automatic conversions towards double precision during
   operation with scalar values).


Bonne chance
Emmanuel

-- 
Emmanuel Viennet: <viennet@ura1507.univ-paris13.fr>
LIPN - Institut Galilee - Universite Paris-Nord       
93430 Villetaneuse -  France

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