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Re: numpy.frombuffe r != unpack() ??

Marlin Rowley wrote:

Say I have an array:

a = (['rrrrggggbbbbaa aa'],['rrrrggggbbbbaa aa'])

How do I make it so that I now have:

starting with first element (a[0])
new_arr[0] = 'r'
new_arr[1] = 'g'
new_arr[2] = 'b'
new_arr[3] = 'a'
new_arr[4] = 'r'

continuing "through" a[1] with the same new_arr
new_arr[N] = 'r'
new_arr[N+1] = 'g'

Numpy can do this for you. First, do you really mean the array to
contain lists of one string each? If so:
>>import numpy
a = (['rrrrggggbbbbaa aa'],['rrrrggggbbbbaa aa'])
b = numpy.frombuffe r(''.join(sum(a ,[])),dtype='S1') # Kind of a
kludge here
array(['r', 'r', 'r', 'r', 'g', 'g', 'g', 'g', 'b', 'b', 'b', 'b', 'a',
'a', 'a', 'a', 'r', 'r', 'r', 'r', 'g', 'g', 'g', 'g', 'b', 'b',
'b', 'b', 'a', 'a', 'a', 'a'],
>>b.shape=(2,4, 4)
array([[['r', 'r', 'r', 'r'],
['g', 'g', 'g', 'g'],
['b', 'b', 'b', 'b'],
['a', 'a', 'a', 'a']],

[['r', 'r', 'r', 'r'],
['g', 'g', 'g', 'g'],
['b', 'b', 'b', 'b'],
['a', 'a', 'a', 'a']]],
>>c = b.transpose((2, 0,1))
array([[['r', 'g', 'b', 'a'],
['r', 'g', 'b', 'a']],

[['r', 'g', 'b', 'a'],
['r', 'g', 'b', 'a']],

[['r', 'g', 'b', 'a'],
['r', 'g', 'b', 'a']],

[['r', 'g', 'b', 'a'],
['r', 'g', 'b', 'a']]],
>>d=c.copy() # To make it contiguous
d.shape = (32,)
array(['r', 'g', 'b', 'a', 'r', 'g', 'b', 'a', 'r', 'g', 'b', 'a', 'r',
'g', 'b', 'a', 'r', 'g', 'b', 'a', 'r', 'g', 'b', 'a', 'r', 'g',
'b', 'a', 'r', 'g', 'b', 'a'],

Done. Cool no?

Gary Herron

From: ma***********@h otmail.com
To: ro*********@gma il.com; py*********@pyt hon.org
Subject: RE: numpy.frombuffe r != unpack() ??
Date: Fri, 16 May 2008 17:31:30 -0500

Thank you! That solved it!

To: py*********@pyt hon.org
From: ro*********@gma il.com
Subject: Re: numpy.frombuffe r != unpack() ??
Date: Fri, 16 May 2008 17:25:00 -0500
Marlin Rowley wrote:

I'm getting different floating point values when I use numpy
vs. unpack().

frgba = numpy.frombuffe r(<string of bytes>, dtype=float32)
buffer = unpack("!f", byte)

frgba[0] != buffer[0]

why? This is forcing me use the unpack() function since it's
giving me
the correct values. What am I doing wrong?
Endianness, perhaps? '!' specifies big-endian data (an alias for
'>'). Most
likely, you are on a little-endian platform. All of the dtypes
in numpy default
to the native-endianness unless specified. If you want to read
big-endian data
using numpy, do this:
frgba = numpy.frombuffe r(<string of bytes>, dtype='>f')
If you have any more problems with numpy, please join us on the
numpy mailing
list. When reporting problems, please try to provide a small but
snippet of self-contained code, the output that you got, and
explain the output
that you expected to get. Thank you.
Robert Kern
"I have come to believe that the whole world is an enigma, a
harmless enigma
that is made terrible by our own mad attempt to interpret it as
though it had
an underlying truth."
-- Umberto Eco

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Jun 27 '08 #1
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