Fastest way to convert a byte of integer into a list

[Godzilla]

Hello,

I'm trying to find a way to convert an integer (8-bits long for
starters) and converting them to a list, e.g.:

num = 255
numList = [1,1,1,1,1,1,1,1]

with the first element of the list being the least significant, so
that i can keep appending to that list without having to worry about
the size of the integer. I need to do this because some of the
function call can return a 2 lots of 32-bit numbers. I have to find a
way to transport this in a list... or is there a better way?

[Matimus]

On Jul 12, 3:34 pm, Godzilla <godzillais...@ gmail.comwrote:

Hello,

I'm trying to find a way to convert an integer (8-bits long for
starters) and converting them to a list, e.g.:

num = 255
numList = [1,1,1,1,1,1,1,1]

with the first element of the list being the least significant, so
that i can keep appending to that list without having to worry about
the size of the integer. I need to do this because some of the
function call can return a 2 lots of 32-bit numbers. I have to find a
way to transport this in a list... or is there a better way?

num = 255
numlist = [num >i & 1 for i in range(8)]

[Godzilla]

On Jul 13, 9:54 am, Matimus <mccre...@gmail .comwrote:

On Jul 12, 3:34 pm, Godzilla <godzillais...@ gmail.comwrote:

Hello,

I'm trying to find a way to convert an integer (8-bits long for
starters) and converting them to a list, e.g.:

num = 255
numList = [1,1,1,1,1,1,1,1]

with the first element of the list being the least significant, so
that i can keep appending to that list without having to worry about
the size of the integer. I need to do this because some of the
function call can return a 2 lots of 32-bit numbers. I have to find a
way to transport this in a list... or is there a better way?

num = 255
numlist = [num >i & 1 for i in range(8)]

Thanks matimus! I will look into it...

[Paul Rubin]

Godzilla <go***********@ gmail.comwrites :

num = 255
numlist = [num >i & 1 for i in range(8)]

Thanks matimus! I will look into it...

numlist = lookup_table[num]

where lookup_table is a precomputed list of lists.

[John Machin]

On Jul 13, 10:28 am, Paul Rubin <http://phr...@NOSPAM.i nvalidwrote:

Godzilla <godzillais...@ gmail.comwrites :

num = 255
numlist = [num >i & 1 for i in range(8)]

Thanks matimus! I will look into it...

numlist = lookup_table[num]

where lookup_table is a precomputed list of lists.

Ummm ... didn't the OP say he had 32-bit numbers???

[bsneddon]

On Jul 12, 8:49 pm, John Machin <sjmac...@lexic on.netwrote:

On Jul 13, 10:28 am, Paul Rubin <http://phr...@NOSPAM.i nvalidwrote:

Godzilla <godzillais...@ gmail.comwrites :

num = 255
numlist = [num >i & 1 for i in range(8)]

Thanks matimus! I will look into it...

numlist = lookup_table[num]

where lookup_table is a precomputed list of lists.

Ummm ... didn't the OP say he had 32-bit numbers???

List comprehension would be faster, lookup would be even faster but
would have to generate list or dictionary ahead of time
but this will work on any length int up 2 limit of int does not pad
with zeros on most significant end to word length.
n=input()
l=[]
while(n>0):
l.append(str(n& 1)); n=n>>1

I posted this here http://www.uselesspython.com/download.php?script_id=222
a while back.

[Godzilla]

On Jul 13, 11:13 am, bsneddon <bsned...@yahoo .comwrote:

On Jul 12, 8:49 pm, John Machin <sjmac...@lexic on.netwrote:

On Jul 13, 10:28 am, Paul Rubin <http://phr...@NOSPAM.i nvalidwrote:

Godzilla <godzillais...@ gmail.comwrites :
num = 255
numlist = [num >i & 1 for i in range(8)]

Thanks matimus! I will look into it...

numlist = lookup_table[num]

where lookup_table is a precomputed list of lists.

Ummm ... didn't the OP say he had 32-bit numbers???

List comprehension would be faster, lookup would be even faster but
would have to generate list or dictionary ahead of time
but this will work on any length int up 2 limit of int does not pad
with zeros on most significant end to word length.

n=input()
l=[]
while(n>0):
l.append(str(n& 1)); n=n>>1

I posted this herehttp://www.uselesspyth on.com/download.php?sc ript_id=222
a while back.

Thanks all... I will have a look at it soon.

Regarding to the 32-bit number, the lenght is variable but it is
usually defined at design time...

[Paul Rubin]

Godzilla <go***********@ gmail.comwrites :

Regarding to the 32-bit number, the lenght is variable but it is
usually defined at design time...

That you're trying to represent it as a list of bits at all is
weird, and probably likely to slow the program down. You do know
that python has arbitrary sized ints, right?

[Paul McGuire]

On Jul 12, 5:34 pm, Godzilla <godzillais...@ gmail.comwrote:

Hello,

I'm trying to find a way to convert an integer (8-bits long for
starters) and converting them to a list, e.g.:

num = 255
numList = [1,1,1,1,1,1,1,1]

with the first element of the list being the least significant, so
that i can keep appending to that list without having to worry about
the size of the integer. I need to do this because some of the
function call can return a 2 lots of 32-bit numbers. I have to find a
way to transport this in a list... or is there a better way?

Standing on the shoulders of previous posters, I put this together.

-- Paul
# init list of tuples by byte
bytebits = lambda num : [num >i & 1 for i in range(8)]
bytes = [ tuple(bytebits( i)) for i in range(256) ]

# use bytes lookup to get bits in a 32-bit integer
bits = lambda num : sum((bytes[num >i & 255] for i in range(0,32,8)),
())

# use base-2 log to find how many bits in an integer of arbitrary
length
from math import log,ceil
log_of_2 = log(2)
numBits = lambda num : int(ceil(log(nu m)/log_of_2))

# expand bits to integers of arbitrary length
arbBits = lambda num : sum((bytes[num >i & 255] for i in
range(0,numBits (num),8)),())

print arbBits((1<<34)-1)
print arbBits(37)

# prints
#(1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,
1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0)
#(1, 0, 1, 0, 0, 1, 0, 0)

[mensanator]

On Jul 13, 5:17 am, Paul McGuire <pt...@austin.r r.comwrote:

On Jul 12, 5:34 pm, Godzilla <godzillais...@ gmail.comwrote:

Hello,

I'm trying to find a way to convert an integer (8-bits long for
starters) and converting them to a list, e.g.:

num = 255
numList = [1,1,1,1,1,1,1,1]

with the first element of the list being the least significant, so
that i can keep appending to that list without having to worry about
the size of the integer. I need to do this because some of the
function call can return a 2 lots of 32-bit numbers. I have to find a
way to transport this in a list... or is there a better way?

Standing on the shoulders of previous posters, I put this together.

-- Paul

But aren't we moving backwards? The OP did ask for the fastest way.

I put this together (from other posters and my own):

import gmpy
import time

y = 2**177149 - 1

# init list of tuples by byte
bytebits = lambda num : [num >i & 1 for i in range(8)]
bytes = [ tuple(bytebits( i)) for i in range(256) ]
# use bytes lookup to get bits in a 32-bit integer
bits = lambda num : sum((bytes[num >i & 255] for i in range(0,32,8)),
())
# use base-2 log to find how many bits in an integer of arbitrary
length
from math import log,ceil
log_of_2 = log(2)
numBits = lambda num : int(ceil(log(nu m)/log_of_2))
# expand bits to integers of arbitrary length
arbBits = lambda num : sum((bytes[num >i & 255] for i in
range(0,numBits (num),8)),())
t0 = time.time()
L = arbBits(y)
t1 = time.time()
print 'Paul McGuire algorithm:',t1-t0

t0 = time.time()
L = [y >i & 1 for i in range(177149)]
t1 = time.time()
print ' Matimus algorithm:',t1-t0

x = gmpy.mpz(2**177 149 - 1)
t0 = time.time()
L = [gmpy.getbit(x,i ) for i in range(177149)]
t1 = time.time()
print ' Mensanator algorithm:',t1-t0

## Paul McGuire algorithm: 17.4839999676
## Matimus algorithm: 3.28100013733
## Mensanator algorithm: 0.125