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# Little novice program written in Python

Hi, All.

I'm just getting my feet wet on Python and, just for starters, I'm coding some
elementary number theory algorithms (yes, I know that most of them are already
implemented as modules, but this is an exercise in learning the language idioms).

As you can see from the code below, my background is in C, without too much
sophistication.

What I would like is to receive some criticism to my code to make it more
Python'esque and, possibly, use the resources of the computer in a more
efficient way (the algorithm implemented below is the Sieve of Eratosthenes):

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
#!/usr/bin/env python

n = int(raw_input() )
a = [i for i in range(0,n+1)]
a[1] = 0 # not a prime
prime = 1 # last used prime
finished = False

while (not finished):
prime = prime + 1
# find new prime
while prime*prime <= n and a[prime] == 0:
prime += 1
# cross the composite numbers
if prime*prime <= n:
j = 2*prime
while j <= n:
a[j] = 0
j += prime
else:
finished = True

# print out the prime numbers
i = 2
while i <= n:
if a[i] != 0:
print a[i]
i += 1
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Thank you for any help in improving this program,

--
Rogério Brito : rbrito@{mackenz ie,ime.usp}.br : GPG key 1024D/7C2CAEB8
http://www.ime.usp.br/~rbrito : http://meusite.mackenzie.com.br/rbrito
Projects: algorithms.berl ios.de : lame.sf.net : vrms.alioth.deb ian.org
Jun 27 '08 #1
21 1382
What I would like is to receive some criticism to my code to make it more
Python'esque and, possibly, use the resources of the computer in a more
efficient way (the algorithm implemented below is the Sieve of Eratosthenes):
It looks like straight-forward code and is fine as it stands.
If you want to tweak it a bit, you can avoid using a flag like
"finished" by using a break-statement.
Raymond
Jun 27 '08 #2
Rogério Brito wrote:
Hi, All.

I'm just getting my feet wet on Python and, just for starters, I'm
coding some elementary number theory algorithms (yes, I know that most
of them are already implemented as modules, but this is an exercise in
learning the language idioms).

As you can see from the code below, my background is in C, without too
much sophistication.

What I would like is to receive some criticism to my code to make it
more Python'esque and, possibly, use the resources of the computer in a
more efficient way (the algorithm implemented below is the Sieve of
Eratosthenes):

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
#!/usr/bin/env python

n = int(raw_input() )
a = [i for i in range(0,n+1)]
a[1] = 0 # not a prime
prime = 1 # last used prime
finished = False

while (not finished):
prime = prime + 1
# find new prime
while prime*prime <= n and a[prime] == 0:
prime += 1
# cross the composite numbers
if prime*prime <= n:
j = 2*prime
while j <= n:
a[j] = 0
j += prime
else:
finished = True

# print out the prime numbers
i = 2
while i <= n:
if a[i] != 0:
print a[i]
i += 1
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Thank you for any help in improving this program,
Your Python is actually pretty good - if Raymond Hettinger pronounces it
OK then few would dare to disagree.

As for your English, though, the word you sought was "Pythonic" (not
that you will ever find such a word in Webster's dictionary). To suggest
that your code is Pythonesque would mean you found it farcical or
ridiculous (like a Monty Python sketch), which it clearly is not.

Another wrinkle you might consider is simply printing the primes out as
they are generated rather than doing the printing in a separate loop,
though whether that approach would be preferable in "real life" would
depend on the application, of course.

regards
Steve

PS: I think either my mailer or yours has mangled the indentation.
--
Steve Holden +1 571 484 6266 +1 800 494 3119
Holden Web LLC http://www.holdenweb.com/

Jun 27 '08 #3
On Apr 24, 11:09*pm, Dennis Lee Bieber <wlfr...@ix.net com.comwrote:
On Thu, 24 Apr 2008 21:31:15 -0300, Rogério Brito <rbr...@ime.usp .br>
declaimed the following in comp.lang.pytho n:
a = [i for i in range(0,n+1)]

* * * * Uhm... At least in 2.4 and earlier, range() returns a list.... No
need for the list-comp in that era... range() also begins with 0
>n = 5
a = range(n+1)
a
[0, 1, 2, 3, 4, 5]

* * * * So just

* * * * a = range(n+1)

could be used. Of course, if using a version where range() and xrange()
have been unified...
>c = list(xrange(n+1 ))
c
[0, 1, 2, 3, 4, 5]

--
* * * * Wulfraed * * * *Dennis Lee Bieber * * * * * * * KD6MOG
* * * * wlfr...@ix.netc om.com * * * * * * *wulfr...@besti aria.com
* * * * * * * * HTTP://wlfraed.home.netcom.com/
* * * * (Bestiaria Support Staff: * * * * * * * web-a...@bestiaria. com)
* * * * * * * * HTTP://www.bestiaria.com/
You're talking hardware-native, which machines don't guarantee.
Python can in another dimension of machine compatibility. Stacks are
hardware native, the location of an array is not. Python can retrieve
your stack in higher dimensions.

Fortunately, Python's community is sturdy against counterproducti vity
en masse, so it's okay to hairbrain it. Cover features of
improvements, though, and you might get a Bayes Net change to make and
courses to steer. The community values the flexibility of machine-
independency too.

However, real numbers are not integers, so opinion mass of integer
algorithms may favor C. But you just need micro-sales (and scales!)
to examine the future of Python. Welcome to our group.
Jun 27 '08 #4
Peter Otten wrote:

>i = 2
while i <= n:
if a[i] != 0:
print a[i]
i += 1

You can spell this as a for-loop:

for p in a:
if p:
print p

It isn't exactly equivalent, but gives the same output as we know that a[0]
and a[1] are also 0.
If the OP insists in not examining a[0] and a[1], this will do exactly
the same as the while version:

for p in a[2:]:
if p:
print p
Cheers,
RB
Jun 27 '08 #5
On Apr 25, 5:44 pm, Robert Bossy <Robert.Bo...@j ouy.inra.frwrot e:
Peter Otten wrote:
Rogério Brito wrote:
i = 2
while i <= n:
if a[i] != 0:
print a[i]
i += 1
You can spell this as a for-loop:
for p in a:
if p:
print p
It isn't exactly equivalent, but gives the same output as we know that a[0]
and a[1] are also 0.

If the OP insists in not examining a[0] and a[1], this will do exactly
the same as the while version:

for p in a[2:]:
if p:
print p
... at the cost of almost doubling the amount of memory required.
Jun 27 '08 #6
John Machin wrote:
On Apr 25, 5:44 pm, Robert Bossy <Robert.Bo...@j ouy.inra.frwrot e:
>Peter Otten wrote:
>>Rogério Brito wrote:

i = 2
while i <= n:
if a[i] != 0:
print a[i]
i += 1

You can spell this as a for-loop:

for p in a:
if p:
print p

It isn't exactly equivalent, but gives the same output as we know that a[0]
and a[1] are also 0.
If the OP insists in not examining a[0] and a[1], this will do exactly
the same as the while version:

for p in a[2:]:
if p:
print p

... at the cost of almost doubling the amount of memory required.
Indeed. Would it be a sensible proposal that sequence slices should
return an iterator instead of a list?

RB
Jun 27 '08 #7

Rogério Brito:
Hi, All.

I'm just getting my feet wet on Python and, just for starters, I'm coding some
elementary number theory algorithms (yes, I know that most of them are already
implemented as modules, but this is an exercise in learning the language idioms).

As you can see from the code below, my background is in C, without too much
sophistication.

What I would like is to receive some criticism to my code to make it more
Python'esque and, possibly, use the resources of the computer in a more
efficient way (the algorithm implemented below is the Sieve of Eratosthenes):
my variant of the sieve

def GetPrimes(N):
arr = []
for i in range(1,N+1):
arr.append(i)
#Set first item to 0, because 1 is not a prime
arr[0]=0
#sieve processing
s=2
while s < math.sqrt(N):
if arr[s-1] != 0:
j = s*s
while j <= N:
arr[j-1] = 0
j += s
s += 1
return [x for x in arr if x != 0]
Jun 27 '08 #8
also, i would recommend you to visit projecteuler.ne t
you can solve math tasks and then see how others have done the same.

you can fetch very good and pythonic solution there.

Jun 27 '08 #9
hellt <Do*********@gm ail.comwrites:
my variant of the sieve
Since you posted it, you are also looking for advice to improve your
code ;)
def GetPrimes(N):
arr = []
for i in range(1,N+1):
arr.append(i)
This is the same as:
arr = range(1, N+1)
!-)
#Set first item to 0, because 1 is not a prime
arr[0]=0
#sieve processing
s=2
remove this line
while s < math.sqrt(N):
for s in xrange(2, int(math.sqrt(N ))+1):
if arr[s-1] != 0:
if arr[s-1]:
j = s*s
remove this line
while j <= N:
for j in xrange(s*s, N+1, s):
arr[j-1] = 0
j += s
remove this line
s += 1
remove this line
return [x for x in arr if x != 0]
return filter(None, arr)
Altogether now:

def getprimes(N):
arr = range(1, N+1)
arr[0] = 0
for s in xrange(2, int(math.sqrt(N ))+1):
if arr[s-1]:
for j in xrange(s*s, N+1, s):
arr[j-1] = 0
return filter(None, arr)

It's the same, but it looks a bit less like the litteral translation
of some C code.
Lastly, the lines:

for j in xrange(s*s, N+1, s):
arr[j-1] = 0

from above can be condensed using extended slices:

arr[s*s-1 : N+1 : s] = [0] * (N/s - s + 1)

(If I can count correctly)

Giving the following, slightly shorter and probably faster:

def getprimes(N):
arr = range(1, N+1)
arr[0] = 0
for s in xrange(2, int(math.sqrt(N ))+1):
if arr[s-1]:
arr[s*s-1 : N+1 : s] = [0] * (N/s - s + 1)
return filter(None, arr)
If it was me, I would include 0 in the array, giving the slightly simpler:

def getprimes(N):
arr = range(N+1)
arr[1] = 0
for s in xrange(2, int(math.sqrt(N ))+1):
if arr[s]:
arr[s*s : N+1 : s] = [0] * (N/s - s + 1)
return filter(None, arr)

(I think)

This all needs to be tested.

--
Arnaud
Jun 27 '08 #10

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