[Tutor] Luke, thanks a lot , here is the perfect code (as per ur suggestion)

Asrarahmed Kadri ajkadri at googlemail.com
Fri Oct 6 11:51:10 CEST 2006

```Thanks for the detailed explanation. I read it and understood a bit. Rest in
the leisure time.

Enjoy the recipes..........
Regards,
Asrar

On 10/5/06, Kent Johnson <kent37 at tds.net> wrote:
>
> Asrarahmed Kadri wrote:
> > What is this??? I cannot understand a single character.. Explain this in
> > length.
> >
> >     *list1 = [ [ locals()["_[1]"][i-1][j-1]+locals()["_[1]"][i-1][j] if
> >     (j !=
> >     0 and j != i) else 1 for j in range(i+1) ] for i in
> >     range(num_of_lines) ]
>
> OK, I guess I asked for that. Remember, I did say this was hideous, I
> would never use this for anything other than a newsgroup posting.
>
> Here is your original code, more or less:
>
> list1 = []
> for i in range(5):
>     flag = 0
>     tmp = []
>     for j in range(i+1):
>         if flag == 0 or j == i:
>             tmp.append(1)
>             flag = 1
>         else:
>             tmp.append(list1[i-1][j-1]+list1[i-1][j])
>     list1.append(tmp)
>
> First let's get rid of flag, it isn't needed:
>
> list1 = []
> for i in range(5):
>     tmp = []
>     for j in range(i+1):
>         if j == 0 or j == i:
>             tmp.append(1)
>         else:
>             tmp.append(list1[i-1][j-1]+list1[i-1][j])
>     list1.append(tmp)
>
> Now replace the inner if/else with a conditional expression inside the
> call to append(). A conditional expression has the form (a if b else c)
> where b is the condition being tested and a and c are the two values.
> The if/else becomes this monster:
>
> tmp.append(list1[i-1][j-1]+list1[i-1][j] if (j!=0 and j!=i) else 1)
>
> I inverted the condition so I could put the more common case first. Now
> the whole program looks like this:
>
> list1 = []
> for i in range(5):
>     tmp = []
>     for j in range(i+1):
>         tmp.append(list1[i-1][j-1]+list1[i-1][j] if (j!=0 and j!=i) else
> 1)
>     list1.append(tmp)
>
> The inner loop is now ready to be replaced with a list comprehension. In
> general, a loop of the form
>
> tmp = []
> for i in x:
>   tmp.append(f(i))
>
> can be replaced with the equivalent list comprehension
>
> tmp = [ f(i) for i in x ]
>
> With this change the program is down to this:
>
> list1 = []
> for i in range(5):
>     tmp = [ list1[i-1][j-1]+list1[i-1][j] if (j!=0 and j!=i) else 1 for
> j in range(i+1) ]
>     list1.append(tmp)
>
> This is again in the form of a loop that can be replaced by a list
> comprehension, this time to create list1. The problem is that the
> expression in the list comprehension has to refer to the list being
> built, and this list is not normally available because the name has not
> yet been bound. This is where the cookbook hack comes in play - within a
> list comprehension, the list being built can be referenced as
> locals()["_[1]"]. Refer to the cookbook recipe for details.
>
> So within the list comp,
> list1[i-1][j-1] becomes locals()["_[1]"][i-1][j-1] and
> list1[i-1][j] becomes locals()["_[1]"][i-1][j].
>
> This brings us to the final form:
>
> list1 = [ [ locals()["_[1]"][i-1][j-1]+locals()["_[1]"][i-1][j] if (j!=0
> and j!=i) else 1 for j in range(i+1) ] for i in range(5) ]
>
> or, with slightly nicer formatting:
>
> list1 = [
>     [ locals()["_[1]"][i-1][j-1]+locals()["_[1]"][i-1][j] if (j!=0 and
> j!=i) else 1
>         for j in range(i+1)
>     ] for i in range(5)
> ]
>
> Kent
>
> _______________________________________________
> Tutor maillist  -  Tutor at python.org
> http://mail.python.org/mailman/listinfo/tutor
>

--
To HIM you shall return.
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