Re: Need help in understanding a python code

2008-11-17 Thread Steven D'Aprano
On Sun, 16 Nov 2008 04:34:40 -0800, John Machin wrote:

 Nothing to do with style. It was the screaming inefficiency of:
if non_trivial_condition: return x
if not non_trivial_condition: return y
 that fired me up.

Screaming inefficiency?

Try micro-optimization. The difference in execution time between if 
x... if not x... versus if x... else... on my slow, underpowered 
machine is about 10**-8 seconds. If that's your idea of screaming 
inefficiency I can't understand why you're programming in Python in the 
first place.

Of course, if x is an expensive function call (say, a network lookup or 
database query rather than a relatively cheap list indexing operation) 
then the more readable, Pythonic solution will also be significantly 
faster. There's no doubt that it should be preferred -- I'm not defending 
it, as such, just pointing out the over-reaction of dismissing what is a 
generally well-written and thought-out article on the basis of a 
triviality.


 
 http://en.wikipedia.org/wiki/Style_over_substance_fallacy
 
 Quoted Wikipedia - instant disqualification - you lose. Good night.


Oh gosh, well, you've certainly proven your case, how could I be so 
stupid? My apology for thinking that you were acting like an arrogant, 
bad-tempered dick. I don't know *what* I was thinking.



-- 
Steven
who would link to Wikipedia for the definition of sarcasm except I've 
already lost once in this thread and that's enough.
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Re: Need help in understanding a python code

2008-11-16 Thread John Machin
On Nov 16, 4:15 pm, Meryl Silverburgh [EMAIL PROTECTED]
wrote:
 This is the full source code:
 def A(w, v, i,j):
     if i == 0 or j == 0: return 0
     if w[i-1]  j:  return A(w, v, i-1, j)
     if w[i-1] = j: return max(A(w,v, i-1, j), v[i-1] + A(w,v, i-1, j - 
 w[i-1]))

Huh??? There is only a very slight resemblance to the code that you
posted previously ... both contain 'max, 'i', and 'j'

 I am reading this blog

 http://20bits.com/articles/introduction-to-dynamic-programming/

I suggest that you don't bother reading a blog written by somebody who
(presumably consciously) keyed in that if w[i-1] = j:  above.

Oh, very interesting, it contains:
def msum(a):
return max([(sum(a[j:i]), (j,i)) for i in range(1,len(a)+1) for j
in range(i)])

Would you care to tell us which part of which function you are now
trying to understand?
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Re: Need help in understanding a python code

2008-11-16 Thread Steven D'Aprano
On Sun, 16 Nov 2008 01:50:16 -0800, John Machin wrote:

  def A(w, v, i,j):
      if i == 0 or j == 0: return 0
      if w[i-1]  j:  return A(w, v, i-1, j)
      if w[i-1] = j: return max(A(w,v, i-1, j), v[i-1] + 
A(w,v, i-1, j - w[i-1]))


 I am reading this blog

 http://20bits.com/articles/introduction-to-dynamic-programming/
 
 I suggest that you don't bother reading a blog written by somebody who
 (presumably consciously) keyed in that if w[i-1] = j:  above.

That is a translation of standard terminology for a hybrid function. 
Mathematics doesn't have an else, so you write hybrid functions by 
enumerating each branch as an if.

While it's not especially good Python technique, it's a perfectly 
idiomatic mathematical expression, and shouldn't be the basis for 
dismissing an entire blog.


-- 
Steven
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Re: Need help in understanding a python code

2008-11-16 Thread John Machin
On Nov 16, 9:31 pm, Steven D'Aprano [EMAIL PROTECTED]
cybersource.com.au wrote:
 On Sun, 16 Nov 2008 01:50:16 -0800, John Machin wrote:
   def A(w, v, i,j):
       if i == 0 or j == 0: return 0
       if w[i-1]  j:  return A(w, v, i-1, j)
       if w[i-1] = j: return max(A(w,v, i-1, j), v[i-1] +
         A(w,v, i-1, j - w[i-1]))
  I am reading this blog

 http://20bits.com/articles/introduction-to-dynamic-programming/

  I suggest that you don't bother reading a blog written by somebody who
  (presumably consciously) keyed in that if w[i-1] = j:  above.

 That is a translation of standard terminology for a hybrid function.
 Mathematics doesn't have an else, so you write hybrid functions by
 enumerating each branch as an if.

An else is not required.
if w[i-1]  j:
   return A(w, v, i-1, j)
return max(A(w,v, i-1, j), v[i-1] + A(w,v, i-1, j - w[i-1]))

 While it's not especially good Python technique, it's a perfectly
 idiomatic mathematical expression, and shouldn't be the basis for
 dismissing an entire blog.

He's meant to be writing Python code, not mathematical expressions.
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Re: Need help in understanding a python code

2008-11-16 Thread Steven D'Aprano
On Sun, 16 Nov 2008 02:41:03 -0800, John Machin wrote:

 On Nov 16, 9:31 pm, Steven D'Aprano [EMAIL PROTECTED]
 cybersource.com.au wrote:
 On Sun, 16 Nov 2008 01:50:16 -0800, John Machin wrote:
   def A(w, v, i,j):
       if i == 0 or j == 0: return 0
       if w[i-1]  j:  return A(w, v, i-1, j) if w[i-1] = j: return
       max(A(w,v, i-1, j), v[i-1] +
         A(w,v, i-1, j - w[i-1]))
  I am reading this blog

 http://20bits.com/articles/introduction-to-dynamic-programming/

  I suggest that you don't bother reading a blog written by somebody
  who (presumably consciously) keyed in that if w[i-1] = j:  above.

 That is a translation of standard terminology for a hybrid function.
 Mathematics doesn't have an else, so you write hybrid functions by
 enumerating each branch as an if.
 
 An else is not required.
 if w[i-1]  j:
return A(w, v, i-1, j)
 return max(A(w,v, i-1, j), v[i-1] + A(w,v, i-1, j - w[i-1]))

Which is also not valid terminology for hybrid functions.


 While it's not especially good Python technique, it's a perfectly
 idiomatic mathematical expression, and shouldn't be the basis for
 dismissing an entire blog.
 
 He's meant to be writing Python code, not mathematical expressions.

And he's written Python code. Perfectly valid Python code. Just because 
it is not what you consider to be idiomatic Python code isn't a good 
reason to dismiss his entire blog. 

What you've done is rather like me saying that because you failed to use 
a colon after required, and therefore haven't written what *I* consider 
good English style, not only is your specific post best avoided, but 
*all* your posts should be avoided. I trust you understand the logical 
fallacy I would be making, which you have already made.

http://en.wikipedia.org/wiki/Style_over_substance_fallacy



-- 
Steven
and now begins the arguments as to whether it is a fallacy, and if so, if 
it is the fallacy I have said it is...

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Re: Need help in understanding a python code

2008-11-16 Thread John Machin
On Nov 16, 11:04 pm, Steven D'Aprano [EMAIL PROTECTED]
cybersource.com.au wrote:
 On Sun, 16 Nov 2008 02:41:03 -0800, John Machin wrote:
  On Nov 16, 9:31 pm, Steven D'Aprano [EMAIL PROTECTED]
  cybersource.com.au wrote:
  On Sun, 16 Nov 2008 01:50:16 -0800, John Machin wrote:
def A(w, v, i,j):
    if i == 0 or j == 0: return 0
    if w[i-1]  j:  return A(w, v, i-1, j) if w[i-1] = j: return
    max(A(w,v, i-1, j), v[i-1] +
      A(w,v, i-1, j - w[i-1]))
   I am reading this blog

  http://20bits.com/articles/introduction-to-dynamic-programming/

   I suggest that you don't bother reading a blog written by somebody
   who (presumably consciously) keyed in that if w[i-1] = j:  above.

  That is a translation of standard terminology for a hybrid function.
  Mathematics doesn't have an else, so you write hybrid functions by
  enumerating each branch as an if.

  An else is not required.
      if w[i-1]  j:
         return A(w, v, i-1, j)
      return max(A(w,v, i-1, j), v[i-1] + A(w,v, i-1, j - w[i-1]))

 Which is also not valid terminology for hybrid functions.

I couldn't care less. It's valid and efficient (compared to the
original) Python.

  While it's not especially good Python technique, it's a perfectly
  idiomatic mathematical expression, and shouldn't be the basis for
  dismissing an entire blog.

  He's meant to be writing Python code, not mathematical expressions.

 And he's written Python code. Perfectly valid Python code. Just because
 it is not what you consider to be idiomatic Python code isn't a good
 reason to dismiss his entire blog.

 What you've done is rather like me saying that because you failed to use
 a colon after required, and therefore haven't written what *I* consider
 good English style, not only is your specific post best avoided, but
 *all* your posts should be avoided. I trust you understand the logical
 fallacy I would be making, which you have already made.

Nothing to do with style. It was the screaming inefficiency of:
   if non_trivial_condition: return x
   if not non_trivial_condition: return y
that fired me up.

 http://en.wikipedia.org/wiki/Style_over_substance_fallacy

Quoted Wikipedia - instant disqualification - you lose. Good night.

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Re: Need help in understanding a python code

2008-11-16 Thread George Sakkis
On Nov 16, 7:34 am, John Machin [EMAIL PROTECTED] wrote:
 On Nov 16, 11:04 pm, Steven D'Aprano [EMAIL PROTECTED]

 http://en.wikipedia.org/wiki/Style_over_substance_fallacy

 Quoted Wikipedia - instant disqualification - you lose. Good night.

When quoting wikipedia became the new Godwin's law ?? :)

George
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Re: Need help in understanding a python code

2008-11-16 Thread alex23
On Nov 17, 5:26 am, George Sakkis [EMAIL PROTECTED] wrote:
 When quoting wikipedia became the new Godwin's law ?? :)

Probably at the point the editors started becoming revisionists and
culling anything they didn't consider notable enough.
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Re: Need help in understanding a python code

2008-11-16 Thread Benjamin Kaplan
On Sun, Nov 16, 2008 at 7:34 AM, John Machin [EMAIL PROTECTED] wrote:

 On Nov 16, 11:04 pm, Steven D'Aprano [EMAIL PROTECTED]
 cybersource.com.au wrote:
  On Sun, 16 Nov 2008 02:41:03 -0800, John Machin wrote:
   On Nov 16, 9:31 pm, Steven D'Aprano [EMAIL PROTECTED]
   cybersource.com.au wrote:
   On Sun, 16 Nov 2008 01:50:16 -0800, John Machin wrote:
 def A(w, v, i,j):
 if i == 0 or j == 0: return 0
 if w[i-1]  j:  return A(w, v, i-1, j) if w[i-1] = j: return
 max(A(w,v, i-1, j), v[i-1] +
   A(w,v, i-1, j - w[i-1]))
I am reading this blog
 
   http://20bits.com/articles/introduction-to-dynamic-programming/
 
I suggest that you don't bother reading a blog written by somebody
who (presumably consciously) keyed in that if w[i-1] = j:  above.
 
   That is a translation of standard terminology for a hybrid function.
   Mathematics doesn't have an else, so you write hybrid functions by
   enumerating each branch as an if.
 
   An else is not required.
   if w[i-1]  j:
  return A(w, v, i-1, j)
   return max(A(w,v, i-1, j), v[i-1] + A(w,v, i-1, j - w[i-1]))
 
  Which is also not valid terminology for hybrid functions.

 I couldn't care less. It's valid and efficient (compared to the
 original) Python.

   While it's not especially good Python technique, it's a perfectly
   idiomatic mathematical expression, and shouldn't be the basis for
   dismissing an entire blog.
 
   He's meant to be writing Python code, not mathematical expressions.
 
  And he's written Python code. Perfectly valid Python code. Just because
  it is not what you consider to be idiomatic Python code isn't a good
  reason to dismiss his entire blog.
 
  What you've done is rather like me saying that because you failed to use
  a colon after required, and therefore haven't written what *I* consider
  good English style, not only is your specific post best avoided, but
  *all* your posts should be avoided. I trust you understand the logical
  fallacy I would be making, which you have already made.

 Nothing to do with style. It was the screaming inefficiency of:
   if non_trivial_condition: return x
   if not non_trivial_condition: return y
 that fired me up.

  http://en.wikipedia.org/wiki/Style_over_substance_fallacy

 Quoted Wikipedia - instant disqualification - you lose. Good night.


If you really believe that, you haven't been following this list long
enough. Every terminology dispute always includes at least 1 Wikipedia link.


Also, you might want to look at this study:
http://news.cnet.com/2100-1038_3-5997332.html




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Re: Need help in understanding a python code

2008-11-16 Thread Matt Nordhoff
Benjamin Kaplan wrote:
 If you really believe that, you haven't been following this list long
 enough. Every terminology dispute always includes at least 1 Wikipedia
 link.
 
 Also, you might want to look at this study:
 http://news.cnet.com/2100-1038_3-5997332.html

That study has been disputed; see the links at the top of
http://en.wikipedia.org/wiki/Reliability_of_Wikipedia.

/me ducks
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Re: Need help in understanding a python code

2008-11-16 Thread alex23
On Nov 17, 11:40 am, Matt Nordhoff [EMAIL PROTECTED] wrote:
 That study has been disputed; see the links at the top of
 http://en.wikipedia.org/wiki/Reliability_of_Wikipedia.

Now, if there was any independent refutation of the original study
that isn't based on Britannica's - not that I'm outright accusing them
of any bias here :) - that might make a reasonable disputation...
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Re: Need help in understanding a python code

2008-11-15 Thread Chris Rebert
On Sat, Nov 15, 2008 at 8:41 PM, [EMAIL PROTECTED]
[EMAIL PROTECTED] wrote:
 Hi,

 I am trying to understand the following line:
 # a is an integer array

 max([(sum(a[j:i]), (j,i))

This code isn't valid. You have a [ with no closing ].

Cheers,
Chris
-- 
Follow the path of the Iguana...
http://rebertia.com


 Can you please tell me what that means,
 I think sum(a[j:i] means find the some from a[j] to a[i]
 But what is the meaning of the part (j,i)?

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Re: Need help in understanding a python code

2008-11-15 Thread John Machin
On Nov 16, 3:41 pm, [EMAIL PROTECTED]
[EMAIL PROTECTED] wrote:
 Hi,

 I am trying to understand the following line:
 # a is an integer array

 max([(sum(a[j:i]), (j,i))

 Can you please tell me what that means,
 I think sum(a[j:i] means find the some from a[j] to a[i]
 But what is the meaning of the part (j,i)?

0. integer array is a very loose term in Python. Fortunately the
answer to your question is not affected by that.
1. Sorry, the max... line is not syntactically correct; there are two
[s and only one ]; there are 4 (s and only 3 )s. Try copying the line
and pasting, not re-typing.
2. I'm not going to try to guess how to fix the bracket mismatches.
3. Note that you have left off a ) from your question about sum ...
it probably should be sum([j:i]).
4. That is the sum (not some!!) of a[j] to a[i-1] both inclusive.
It's a standard idiom in Python for the end of a range to be expressed
as the first unused element.
5. Even after fixing the bracket mismatches, it looks like you will
have an expression whose value is thrown away. Perhaps you might like
to give us a few lines of context before and after the line of
interest.
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Re: Need help in understanding a python code

2008-11-15 Thread Meryl Silverburgh
This is the full source code:
def A(w, v, i,j):
if i == 0 or j == 0: return 0
if w[i-1]  j:  return A(w, v, i-1, j)
if w[i-1] = j: return max(A(w,v, i-1, j), v[i-1] + A(w,v, i-1, j - w[i-1]))

I am reading this blog

http://20bits.com/articles/introduction-to-dynamic-programming/


On Sat, Nov 15, 2008 at 10:54 PM, Chris Rebert [EMAIL PROTECTED] wrote:
 On Sat, Nov 15, 2008 at 8:41 PM, [EMAIL PROTECTED]
 [EMAIL PROTECTED] wrote:
 Hi,

 I am trying to understand the following line:
 # a is an integer array

 max([(sum(a[j:i]), (j,i))

 This code isn't valid. You have a [ with no closing ].

 Cheers,
 Chris
 --
 Follow the path of the Iguana...
 http://rebertia.com


 Can you please tell me what that means,
 I think sum(a[j:i] means find the some from a[j] to a[i]
 But what is the meaning of the part (j,i)?

 --
 http://mail.python.org/mailman/listinfo/python-list


--
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Re: Need help in understanding a python code

2008-11-15 Thread Aaron Brady
See below.

On Nov 15, 11:15 pm, Meryl Silverburgh [EMAIL PROTECTED]
wrote:
 This is the full source code:
 def A(w, v, i,j):
     if i == 0 or j == 0: return 0
     if w[i-1]  j:  return A(w, v, i-1, j)
     if w[i-1] = j: return max(A(w,v, i-1, j), v[i-1] + A(w,v, i-1, j - 
 w[i-1]))

 I am reading this blog

 http://20bits.com/articles/introduction-to-dynamic-programming/

 On Sat, Nov 15, 2008 at 10:54 PM, Chris Rebert [EMAIL PROTECTED] wrote:
  On Sat, Nov 15, 2008 at 8:41 PM, [EMAIL PROTECTED]
  [EMAIL PROTECTED] wrote:
  Hi,

  I am trying to understand the following line:
  # a is an integer array

  max([(sum(a[j:i]), (j,i))

  This code isn't valid. You have a [ with no closing ].

  Cheers,
  Chris
  --
  Follow the path of the Iguana...
 http://rebertia.com

  Can you please tell me what that means,
  I think sum(a[j:i] means find the some from a[j] to a[i]
  But what is the meaning of the part (j,i)?

  --
 http://mail.python.org/mailman/listinfo/python-list




 if w[i-1] = j: return max(A(w,v, i-1, j), v[i-1] + A(w,v, i-1, j - 
 w[i-1]))

This means:

Calculate 'A(w,v, i-1, j)', calculate 'v[i-1] + A(w,v, i-1, j - w
[i-1])', and return whichever is larger.
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Re: Need help in understanding a python code

2008-11-15 Thread bearophileHUGS
silverburgh:
 max([(sum(a[j:i]), (j,i))

Other people have already answered you so I'll add only a small note:
today the max() function has a key optional attribute, so that code
can also be written as:

max(((j, i) for ...), key=lambda (j, i): sum(a[j : i]))

I think you have copied that part from code that runs in O(n^2);
remember that you can find the max subarray with a well known O(n)
algorithm too.

Bye,
bearophile
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