Re: Reverse order of bit in repeating seqence of byte string

[MRAB]

Francesco Bochicchio wrote:

imageguy ha scritto:

I am looking for the most efficient method of replacing a repeating
sequence in a byte string returned from a imaging .dll, connected via

I receive the byte string with the following sequence 'bgrbgrbgrbgr'
and I would like to convert this to 'rbgrbgrbgrbg'
FWIW, the string is created using ctypes.create_string_buffer function

The following code works but feels a bit clunk and is rather slow too.

blist = list(buffer)
for start in xrange(0,len(blist), 3):
   try:
        blue = blist[start]
        red = blist[start+2]
        blist[start] = red
        blist[start+2] = blue
   except IndexError:
       pass

new_buffer = ''.join(blist)

new_buffer is then passed to a wx program to create and image.

Any thoughts comments would be appreciated.

geoff.

PS:  I started this post earlier, but I think I hit the send button
too soon.  My apologies to the group for sloppy typing.

You could try the same algorithm on an array.array object : it might be faster.

>>> s = "012345678"
>>> from array import array
>>> a = array("b", s)
>>> a
array('b', [48, 49, 50, 51, 52, 53, 54, 55, 56])
>>> a[0::3], a[2::3] = a[2::3], a[0::3]
>>> a
array('b', [50, 49, 48, 53, 52, 51, 56, 55, 54])
>>> a.tostring()
'210543876'

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