[Tutor] Working with lines from file and printing to another keeping sequential order

["Shantanoo Mahajan (शंत]

On 25-Apr-09, at 11:41 PM, Dan Liang wrote:

> Hi Bob and tutors,
>
> Thanks Bob for your response! currently I have the current code, but  
> it does not work:
>
> ListLines= []
> for line in open('test.txt'):
>     line = line.rstrip()
>     ListLines.append(line)
>
> for i in range(len(ListLines)):
>
>     if ListLines[i].endswith("yes") and ListLines[i 
> +1].endswith("no") and ListLines[i+1].endswith("no"):
>         print ListLines[i], ListLines[i+1], ListLines[i+2]
>     elif ListLines[i].endswith("yes") and ListLines[i 
> +1].endswith("no"):
>         print ListLines[i], ListLines[i+1]
>         elif ListLines[i].endswith("yes"):
>         print ListLines[i]
>     elif ListLines[i].endswith("no"):
>         continue
>     else:
>         break
>
> I get the following error:
> Traceback (most recent call last):
>   File "test.py", line 18, in <module>
>     if ListLines[i].endswith("yes") and ListLines[i 
> +1].endswith("no") and ListLines[i+1].endswith("no"):
> IndexError: list index out of range

You need to put check for

i <= len(ListLines)-2


or you can have a look @ try and except (exception handling in python)

> Lines in the file look like following:
>
>     word1 word2 word3 word4 yes
>     word1 word2 word3 word4 no
>     word1 word2 word3 word4 no
>     word1 word2 word3 word4 no
>     word1 word2 word3 word4 yes
>     word1 word2 word3 word4 no
>     word1 word2 word3 word4 yes
>     word1 word2 word3 word4 no
>     word1 word2 word3 word4 yes
>     word1 word2 word3 word4 yes
>     word1 word2 word3 word4 no
>     word1 word2 word3 word4 no
>     word1 word2 word3 word4 no
>     word1 word2 word3 word4 yes
>     word1 word2 word3 word4 no
>     word1 word2 word3 word4 yes
>     word1 word2 word3 word4 yes
>     word1 word2 word3 word4 no
>     word1 word2 word3 word4 no
>     word1 word2 word3 word4 no
>     word1 word2 word3 word4 no
>     word1 word2 word3 word4 yes
>
> > What do we do at steps 4ff if the line does not end in "yes" or  
> "no"?
>
> I forgot to mention that I know for sure that the file has ONLY  
> lines that end in either "yes" or "no".
>
> Any suggestions are appreciated. Also, I feel that my code is not  
> the best way of solving the problem even if the problem of list  
> indices is solved. Is my guess right?
>


Another suggestion. I would not read all data from file into array  
(memory). If your file is large, you may hit out of memory error.

regards,
shantanoo

> Thank you!
>
> -dan
>
> On Sat, Apr 25, 2009 at 10:32 AM, bob gailer <bgailer at gmail.com>  
> wrote:
> Dan Liang wrote:
> Dear Tutors,
>
>
> I have a file from which I want to extract lines that end in certain  
> strings and print to a second file. More specifically, I want to:
>
> 1) iterate over each line in the file, and if it ends in "yes",  
> print it.
> 2) move to the line following the one described in #1 above, and if  
> it ends in, "no" print it.
> 3) move to third line, and if it ends in "no", print it.
> 4) move to fourth line, and if it ends in "no" discard it, but if it  
> ends in "yes" repeat 1, 2, and 3 above.
> 5) move to fifth line, and if it ends in "no" discard it, but if it  
> ends in "yes" repeat 1, 2, 3, and 4 above, and so on.
>
> The goal is to get a ratio of 1 to 2 "yes" to "no" lines from a file  
> in such a way that keeps the order of the lines in output. An  
> abstraction away from this so that any ratio of "yes" to "no" lines  
> could be printed while keeping the order of the original lines would  
> be great.
>
> Please show us what code you have written, and in what way it fails  
> to meet your expectations.
>
> Your specification is IMHO a nice piece of pseudocode which could  
> translate to Python fairly easily!
>
> What do we do at steps 4ff if the line does not end in "yes" or "no"?
>
> If you have not written any code make a stab at it. You could start  
> by asking "how in Python does one":
> open a file?
> iterate (loop)?
> get the next line from a file?
> test for equality?
> examine the end of a string?
>
>
> -- 
> Bob Gailer
> Chapel Hill NC
> 919-636-4239
>
> _______________________________________________
> Tutor maillist  -  Tutor at python.org
> http://mail.python.org/mailman/listinfo/tutor

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