[Tutor] List comprehensions

[linuxian iandsd]

i can't think of anything but a loop here UNLESS you take the list from its
source one element at a time, process it & then print the result.

example of this would be :

         list comes in from standard input.
         list comes from a database
         list is read from a file.

so again where the list comes from is important.

if its standard input then you program will be easy & won't use any memory i
guess.

import sys               # cgi & cgitb if going web
data = sys.stdin.readline()
<do what ever you like with data>
print data




On Wed, Apr 9, 2008 at 8:15 PM, Dinesh B Vadhia <dineshbvadhia at hotmail.com>
wrote:

>  Sorry, let's start again.
>
>  Here is a for loop operating on a list of string items:
>
> data = ["string 1", "string 2", "string 3", "string 4", "string 5",
> "string 6", "string 7", "string 8", "string 9", "string 10", "string 11"]
>
> result = ""
> for item in data:
>     result = <some operation on> item
>     print result
>
> I want to replace the for loop with another structure to improve
> performance (as the data list will contain >10,000 string items].  At each
> iteration of the for loop the result is printed (in fact, the result is sent
> from the server to a browser one result line at a time)
>
> The for loop will be called continuously and this is another reason to
> look for a potentially better structure preferably a built-in.
>
> Hope this makes sense!  Thank-you.
>
> Dinesh
>
>
>
> ----- Original Message ----- *From:* Kent Johnson <kent37 at tds.net>
> *To:* Dinesh B Vadhia <dineshbvadhia at hotmail.com>
> *Cc:* tutor at python.org
> *Sent:* Wednesday, April 09, 2008 12:40 PM
> *Subject:* Re: [Tutor] List comprehensions
>
> Dinesh B Vadhia wrote:
> > Here is a for loop operating on a list of string items:
> >
> > data = ["string 1", "string 2", "string 3", "string 4", "string 5",
> > "string 6", "string 7", "string 8", "string 9", "string 10", "string
> 11"]
> >
> > result = ""
> > for item in data:
> >     result = item + "\n"
> > print result
>
> I'm not sure what your goal is here. Do you mean to be accumulating all
> the values in data into result? Your sample code does not do that.
>
> > I want to replace the for loop with a List Comrehension (or whatever) to
>
> > improve performance (as the data list will be >10,000].  At each stage
> > of the for loop I want to print the result ie.
> >
> > [print (item + "\n")  for item in data]
> >
> > But, this doesn't work as the inclusion of the print causes an invalid
> > syntax error.
>
> You can't include a statement in a list comprehension. Anyway the time
> taken to print will swamp any advantage you get from the list comp.
>
> If you just want to print the items, a simple loop will do it:
>
> for item in data:
>    print item + '\n'
>
> Note this will double-space the output since print already adds a newline.
>
> If you want to create a string with all the items with following
> newlines, the classic way to do this is to build a list and then join
> it. To do it with the print included, try
>
> result = []
> for item in data:
>    newItem = item + '\n'
>    print newItem
>    result.append(newItem)
> result = ''.join(result)
>
> Kent
>
> _______________________________________________
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>
>
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