Velocity Reviews > Need help in understanding a python code

# Need help in understanding a python code

silverburgh.meryl@gmail.com
Guest
Posts: n/a

 11-16-2008
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)?

Chris Rebert
Guest
Posts: n/a

 11-16-2008
On Sat, Nov 15, 2008 at 8:41 PM, http://www.velocityreviews.com/forums/(E-Mail Removed)
<(E-Mail Removed)> 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
>

John Machin
Guest
Posts: n/a

 11-16-2008
On Nov 16, 3:41*pm, "(E-Mail Removed)"
<(E-Mail Removed)> 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
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.

Meryl Silverburgh
Guest
Posts: n/a

 11-16-2008
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]))

http://20bits.com/articles/introduct...c-programming/

On Sat, Nov 15, 2008 at 10:54 PM, Chris Rebert <(E-Mail Removed)> wrote:
> On Sat, Nov 15, 2008 at 8:41 PM, (E-Mail Removed)
> <(E-Mail Removed)> 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
>>

>

Guest
Posts: n/a

 11-16-2008
See below.

On Nov 15, 11:15*pm, "Meryl Silverburgh" <(E-Mail Removed)>
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/introduct...c-programming/
>
> On Sat, Nov 15, 2008 at 10:54 PM, Chris Rebert <(E-Mail Removed)> wrote:
> > On Sat, Nov 15, 2008 at 8:41 PM, (E-Mail Removed)
> > <(E-Mail Removed)> 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.

bearophileHUGS@lycos.com
Guest
Posts: n/a

 11-16-2008
silverburgh:
> max([(sum(a[j:i]), (j,i))

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

John Machin
Guest
Posts: n/a

 11-16-2008
On Nov 16, 4:15*pm, "Meryl Silverburgh" <(E-Mail Removed)>
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/introduct...c-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?

Steven D'Aprano
Guest
Posts: n/a

 11-16-2008
On Sun, 16 Nov 2008 01:50:16 -0800, John Machin wrote:

> > def A(w, v, i,j):
> > Â* Â* if i == 0 or j == 0: return 0
> > Â* Â* if w[i-1] > j: Â*return A(w, v, i-1, j)
> > Â* Â* 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/introduct...c-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

John Machin
Guest
Posts: n/a

 11-16-2008
On Nov 16, 9:31*pm, Steven D'Aprano <st...@REMOVE-THIS-
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/introduct...c-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.

Steven D'Aprano
Guest
Posts: n/a

 11-16-2008
On Sun, 16 Nov 2008 02:41:03 -0800, John Machin wrote:

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

>>
>> >>http://20bits.com/articles/introduct...c-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

http://en.wikipedia.org/wiki/Style_o...stance_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...