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How to generate (enumerate) 2**N tuples representing all vertices of unit hypercube in N-dimensional hyperspace ?

 
 
Dr. Colombes
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      01-04-2006
I'm looking for a good Python way to generate (enumerate) the 2**N
tuples representing all vertices of the unit hypercube in N-dimensional
hyperspace.

For example, for N=4 the Python code should generate the following 2**N
= 16 tuples:

(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),
(-1,1,1,1), (-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)

Maybe converting each integer in the range(2**N) to binary, then
converting to bit string, then applying the "tuple" function to each
bit string?

Thanks for your help.

 
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Paul Rubin
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      01-04-2006
"Dr. Colombes" <(E-Mail Removed)> writes:
> I'm looking for a good Python way to generate (enumerate) the 2**N
> tuples representing all vertices of the unit hypercube in N-dimensional
> hyperspace.
> For example, for N=4 the Python code should generate the following 2**N
> = 16 tuples:


Here's a recursive generator:

def hypercube(ndims):
if ndims == 0:
yield ()
return
for h in 1, -1:
for y in hypercube(ndims-1):
return (h,)+y


>>> print list(hypercube(4))


[(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, 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)]
>>>


> Maybe converting each integer in the range(2**N) to binary, then
> converting to bit string, then applying the "tuple" function to each
> bit string?


Yeah you could do that too.
 
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Heiko Wundram
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      01-04-2006
Dr. Colombes wrote:
> Maybe converting each integer in the range(2**N) to binary, then
> converting to bit string, then applying the "tuple" function to each
> bit string?


A direct translation of that:

def perm(n):
rv = []
for i in xrange(2L**n):
cur = []
for j in range(n):
cur.append(1-2*(bool(i & (1<<j))))
# cur is in reversed order LSB first, but as you seemingly don't
# care about order of the returned tuples, this is irrelevant.
rv.append(tuple(cur))
return rv

modelnine@phoenix ~ $ python
Python 2.4.2 (#1, Dec 22 2005, 17:27:39)
[GCC 4.0.2 (Gentoo 4.0.2-r2, pie-8.7.] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from test1 import perm
>>> perm(5)

[(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, 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, -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, -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, -1, 1,
-1, -1), (1, 1, -1, -1, -1), (-1, 1, -1, -1, -1), (1, -1, -1, -1, -1), (-1,
-1, -1, -1, -1)]
>>>

modelnine@phoenix ~ $

--- Heiko.
 
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Paul Rubin
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      01-04-2006
Heiko Wundram <(E-Mail Removed)> writes:
> def perm(n):
> rv = []
> for i in xrange(2L**n):
> cur = []
> for j in range(n):
> cur.append(1-2*(bool(i & (1<<j))))
> # cur is in reversed order LSB first, but as you seemingly don't
> # care about order of the returned tuples, this is irrelevant.
> rv.append(tuple(cur))
> return rv


def perm(n):
return [tuple([(1,-1)[(t>>i)%2] for i in xrange(n)])
for t in xrange(2L**n)]

>>> perm(4)

[(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, 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)]
>>>

 
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Heiko Wundram
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      01-04-2006
Paul Rubin wrote:
> def perm(n):
> return [tuple([(1,-1)[(t>>i)%2] for i in xrange(n)])
> for t in xrange(2L**n)]


or replace that with:

def perm(n):
return (tuple(((1,-1)[(t>>i)%2] for i in xrange(n)))
for t in xrange(2L**n))

to get a generator like in Paul's first example. Only works with Python 2.4+
though.

--- Heiko.
 
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Claudio Grondi
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      01-04-2006
Heiko Wundram wrote:
> Paul Rubin wrote:
>
>>def perm(n):
>> return [tuple([(1,-1)[(t>>i)%2] for i in xrange(n)])
>> for t in xrange(2L**n)]

>
>
> or replace that with:
>
> def perm(n):
> return (tuple(((1,-1)[(t>>i)%2] for i in xrange(n)))
> for t in xrange(2L**n))
>
> to get a generator like in Paul's first example. Only works with Python 2.4+
> though.
>
> --- Heiko.


Isn't this kind of coding beeing the result of suffering from the
post-pyContest illness syndrom?

Or is there another reason behind writing it that way sacrificing
readability like usage of less CPU time to run the code?

Or am I alone having trouble to read this kind of code because not
experienced enough in writing Python scripts?

Claudio
 
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bonono@gmail.com
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      01-04-2006

Dr. Colombes wrote:
> I'm looking for a good Python way to generate (enumerate) the 2**N
> tuples representing all vertices of the unit hypercube in N-dimensional
> hyperspace.
>
> For example, for N=4 the Python code should generate the following 2**N
> = 16 tuples:
>
> (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),
> (-1,1,1,1), (-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)
>
> Maybe converting each integer in the range(2**N) to binary, then
> converting to bit string, then applying the "tuple" function to each
> bit string?
>
> Thanks for your help.


Is this just a special case for the list of list combine() function
posted not long ago ?

def combine_lol(seq):
return reduce(lambda x,y: (a+(b,) for a in x for b in y), seq,
[()])

list(combine_lol([(1,-1)]*4))

 
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Heiko Wundram
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      01-04-2006
Claudio Grondi wrote:
> Heiko Wundram wrote:
>> def perm(n):
>> return (tuple(((1,-1)[(t>>i)%2] for i in xrange(n)))
>> for t in xrange(2L**n))

>
> Isn't this kind of coding beeing the result of suffering from the
> post-pyContest illness syndrom?


I don't think what Paul Rubin posted is the sign of pyContest illness, as I
use two-level generator expressions such as this quite often in my code.
Why I didn't give this as the reponse to the OP was because it seems that
he's not that familiar with Python to be able to read this efficiently and
grap the concept behind the implementation immediately, that's why I
thought an explicit loop was in order.

But for any sufficiently advanced Python coder, I don't think this is
unreadable in the least.

--- Heiko.
 
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Dr. Colombes
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      01-04-2006
Paul, Heiko:

Thank you for the quality, parsimony and promptness of your
excellent suggestions.

I wasn't familiar with the Python "yield" function.

Dr. Colombes

 
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Claudio Grondi
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      01-04-2006
Heiko Wundram wrote:
> Claudio Grondi wrote:
>
>>Heiko Wundram wrote:
>>
>>>def perm(n):
>>> return (tuple(((1,-1)[(t>>i)%2] for i in xrange(n)))
>>> for t in xrange(2L**n))

>>
>>Isn't this kind of coding beeing the result of suffering from the
>>post-pyContest illness syndrom?

>
>
> I don't think what Paul Rubin posted is the sign of pyContest illness, as I
> use two-level generator expressions such as this quite often in my code.
> Why I didn't give this as the reponse to the OP was because it seems that
> he's not that familiar with Python to be able to read this efficiently and
> grap the concept behind the implementation immediately, that's why I
> thought an explicit loop was in order.
>
> But for any sufficiently advanced Python coder, I don't think this is
> unreadable in the least.
>
> --- Heiko.

List comprehension is a nice thing when standalone.
I am usually able to read it from the left to the right and understand
directly.
But with nested list comprehensions I don't see directly what is going
on, because there are more than one identifier I don't know about at the
time of reading the expression and I am forced to read the entire
nestings with eventually some more code spread over it. The trouble is,
that there are no visual hints toward understanding of the used
algorithm like it is the case in the non-list-comprehension version and
I usually have to re-read the nested list comprehension from the right
to the the left in order to get the idea. I don't want to be forced to
read from the right to the left as I am used to read left-right,
top-down and I expect any text (also source code) to be that way.
Similar critics comes sometimes up when someone learns the German
language, because you are forced to read also a very, very long sentence
up to its end, before it is clear what is the action. I consider it as a
problem which would be nice to get rid of if possible, but it is much
easier to try to influence coding style than the usage of a language.

Can usage of deep (i.e. more than two levels) nested list comprehensions
be considered bad coding style?

Claudio
 
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