casevh wrote:
On Apr 28, 5:39 pm, Li Wang <li.wan...@gmail.com> wrote:
2009/4/29 Tim Chase <python.l...@tim.thechases.com>:
I want to concatenate two bits string together: say we have '1001' and
'111' which are represented in integer. I want to concatenate them to
'1001111' (also in integer form), my method is:
('1001' << 3) | 111
which is very time consuming.
You omit some key details -- namely how do you know that "1001" is 4 bits
and not "00001001" (8-bits)? If it's a string (as your current code shows),
you can determine the length. However, if they are actually ints, your code
should work fine & be O(1).
Actually, what I have is a list of integer numbers [3,55,99,44], and
by using Huffman coding or fixed length coding, I will know how the
bits-length for each number. When I try to concatenate them (say
10,000 items in the list) all together, the speed is going down
quickly (because of the shifting operations of python long).
This can be abstracted if you need:
def combine_bits(int_a, int_b, bit_len_b):
return (int_a << bit_len_b) | int_b
a =x09
b =x07
print combine_bits(a, b, 3)
However, if you're using gargantuan ints (as discussed before), it's a lot
messier. You'd have to clarify the storage structure (a byte string? a
python long?)
I am using a single python long to store all the items in the list
(say, 10,000 items), so the work does become messier...
Using GMPY (http://code.google.com/p/gmpy/) may offer a performance
improvement. When shifting multi-thousand bit numbers, GMPY is several
times faster than Python longs. GMPY also support functions to scan
for 0 or 1 bits.
-tkc
PS: You may want to CC the mailing list so that others get a crack at
answering your questions...I've been adding it back in, but you've been
replying just to me.
Sorry, this is the first time I am using mail-list....and always
forgot "reply to all"
Thank you very much:D
--
Li
------
Time is all we have
and you may find one day
you have less than you think
Seems to me you'd do better with a byte array than a Python long. At least
while you're doing all those shifts. That way you do the shifts in an int,
then update a byte in the array whenever you have 8 bits accumulated.
You can then look up a bit in the array trivially. And if you really need a
Python long, I predict the single conversion will be much faster than doing all
those shifts on the long as you go.
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