On Sunday, April 28, 2013 8:04:04 PM UTC+8, Jens Thoms Toerring wrote:
> Tim Roberts <t...@probo.com> wrote:
>
> > Jimmie He <jimmie...@gmail.com> wrote:
>
>
>
> > >When I run the readbmp on an example.bmp(about 100k),the Shell is become
> > >to "No respose",when I change f.read() to f.read(1000),it is ok,could
> > >someone tell me the excat reason for this?
>
> > >Thank you in advance!
>
> > >
>
> > >Python Code as below!!
>
> > >
>
> > >import binascii
>
> > >
>
> > >def read_bmp():
>
> > > f = open('example.bmp','rb')
>
> > > rawdata = f.read() #f.read(1000) is ok
>
> > > hexstr = binascii.b2a_hex(rawdata) #Get an HEX number
>
> > > bsstr = bin (int(hexstr,16))[2:]
>
>
>
> > I suspect the root of the problem here is that you don't understand what
>
> > this is actually doing. You should run this code in the command-line
>
> > interpreter, one line at a time, and print the results.
>
>
>
> > The "read" instruction produces a string with 100k bytes. The b2a_hex then
>
> > produces a string with 200k bytes. Then, int(hexstr,16) takes that 200,000
>
> > byte hex string and converts it to an integer, roughly equal to 10 to the
>
> > 240,000 power, a number with some 240,000 decimal digits. You then convert
>
> > that integer to a binary string. That string will contain 800,000 bytes.
>
> > You then drop the first two characters and print the other 799,998 bytes,
>
> > each of which will be either '0' or '1'.
>
>
>
> > I am absolutely, positively convinced that's not what you wanted to do.
>
> > What point is there in printing out the binary equavalent of a bitmap?
>
>
>
> > Even if you did, it would be much quicker for you to do the conversion one
>
> > byte at a time, completely skipping the conversion to hex and then the
>
> > creation of a massive multi-precision number. Example:
>
>
>
> > f = open('example.bmp','rb')
>
> > rawdata = f.read()
>
> > bsstr = []
>
> > for b in rawdata:
>
> > bsstr.append( bin(ord(b)) )
>
> > bsstr = ''.join(bsstr)
>
>
>
> > or even:
>
> > f = open('example.bmp','rb')
>
> > bsstr = ''.join( bin(ord(b))[2:] for b in f.read() )
>
>
>
> Exactly my idea at first. But then I started to time it (using
>
> the timeit module) by comparing the following functions:
>
>
>
> # Original version
>
>
>
> def c1( rawdata ) :
>
> h = binascii.b2a_hex( rawdata )
>
> z = bin( int( h, 16 ) )[ 2 : ]
>
> return '0' * ( 8 * len( r ) - len( z ) ) + z
>
>
>
> # Convert each byte directly
>
>
>
> def c2( rawdata ) :
>
> return ''.join( bin( ord( x ) )[ 2 : ].rjust( 8, '0' ) for x in r )
>
>
>
> # Convert each byte using a list for table look-up
>
>
>
> def c3( rawdata ) :
>
> h = [ bin( i )[ 2 : ].rjust( 8, '0' ) for i in range( 256 ) ]
>
> return ''.join( h[ ord( x ) ] for x in rawdata )
>
>
>
> # Convert each byte using a dictionary for table look-up (avoids
>
> # lots of ord() calls)
>
>
>
> def c4( rawdata ) :
>
> h = { chr( i ) : bin( i )[ 2 : ].rjust( 8, '0' ) for i in range( 256 ) }
>
> return ''.join( h[ x ] for x in rawdata )
>
>
>
> As you can see I even in c3() and c4() tried to speed things up
>
> further by using a table look-up instead if calling bin() etc.
>
> on each byte. But the results was that c2() is nearly 15 times
>
> slower than c1(), c3() about 3 times and c4() still more than 2
>
> times slower! So the method the OP uses seems to be quite a bit
>
> more efficient than one might be tempted to assume.
>
>
>
> I would guess that the reason is that c1() does just a small
>
> number of calls of functions that probably aren't implemented
>
> in Python but in C and thus can be a lot faster then anything
>
> you could achieve with Python, while the other functions use a
>
> for loop in Python, which seems to account for a good part of
>
> the CPU time used. To test for that I split the 'rawdata' string
>
> into a list of character (i.e. single letter strings) and re-
>
> assembled it using join() and a for loop:
>
>
>
> r = list( rawdata( )
>
> z = ''.join( x for x in r )
>
>
>
> The second line alone took about 1.7 times longer than the
>
> whole, seemingly convoluted c1() function!
>
>
>
> What I take away from this is that a lot of the assumption one
>
> is prone to make when coming from e.g. a C/C++ background can
>
> be quite misleading when extrapolating to Python (or other in-
>
> terpreted languages)...
>
> Best regards, Jens
>
> --
>
> \ Jens Thoms Toerring ___ j...@toerring.de
>
> \__________________________ http://toerring.de
Hi,Jens &Peter &Tim,
Thank you very much for your wonderful analysis for my newbie question.
I admit that I throw this question to much early because I just want some
guru to inspire me;-) If it really confuse you,excuse my noise:-)
What I intend to do is to make an BMP Font Maker(Covert the BMP to an data
array,what I did wrong is print it directly to screen and had not understand it
at all firstly.
C1()~C4() which Jens provided deeply indicate that we should think about the
effiency because it is an interpreted language.
Anyway thanks for all your kindly help :-)
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