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Subject: Re: Strange behaviour with a for loop.
From: Sean Murphy <mhysnm1964@gmail.com>
In-Reply-To: <3EA05A2F-8F5D-43D3-AB9A-B3C50C4B74A6@gmail.com>
Date: Sat, 4 Jan 2014 17:32:44 +1100
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To: Mark Lawrence <breamoreboy@yahoo.co.uk>
Cc: python-list@python.org
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Hi everyone.

Worked out what I was doing wrong with the string splicing. The offset =
number was lower then the index number, so it was failing. E.G:




On 04/01/2014, at 4:54 PM, Sean Murphy <mhysnm1964@gmail.com> wrote:

> Thanks everyone.
>=20
> Mark thanks for the correction on the ':'. Since I didn't cut and =
copy, rather typed it out. Errors crept in. :-)
>=20
> another question in relation to slicing strings. If you want to get a =
single character, just using the index position will get it. If I use =
the following, shouldn't it also work? when I use Python 3.3, it didn't =
provide anything.
>=20
> a =3D "test.txt"
> print a[3]
>=20
print a[4:1] <--- index is 4 and offset is one. This is invalid.

So I suspect the offset number still starts at the beginning of the =
string and counts forward or another way to look at it you are slicing =
from element x to element y. If element y is less then element x, return =
nothing. Does this make sense?

I should have used:

print a[4:6])

to get:

t.t=20

The 2nd part of my original question still stands. I will expand upon =
this a bit more to give more context. I want to print from the beginning =
of the paragraph to the end. Each paragraph ends with "\n\n\n".=20

If I use "\n\n\n" in lines this does return true for the string. But I =
don't have a starting position and ending position. The list method =
which I mention before can be sliced by going back one element.

Any suggestion on this would be welcomed. I want to achieve this using =
standard core python objects/methods.

Sean=20
> result is:
>=20
> 't
>=20
>=20
> print a[3:1]
>=20
> Nothing is printed.=20
>=20
> print a[3:2]
>=20
>=20
> Nothing is printed.
>=20
> print a[3:-1]
>=20
> t.tx is printed.
>=20
>=20
> Why doesn't the positive number of characters to be splice return =
anything while the negative value does?
>=20
> sorry about these basic questions. I do like the splice feature within =
Python. Also what is the best method of testing for a blank string?
>=20
> end of paragraph line 1=20
>=20
>=20
> new paragraph of line 1.
>=20
>=20
> The above example text is what I want to test for. I am planning to =
either load the whole file in as a single chunk of memory using =
fp.read() or store it into an array by using fp.readlines(). The first =
option I see being useful because you can create a regular expression to =
test for multiple '\n'. While in an array (list) I would have to test =
for a blank line which I assume would be "".
>=20
> Any suggestions on this would be welcomed.
>=20
> Sean=20
>=20
>=20
>=20
> print a[
>=20
> On 04/01/2014, at 4:38 PM, Mark Lawrence <breamoreboy@yahoo.co.uk> =
wrote:
>=20
>> On 04/01/2014 04:03, Sean Murphy wrote:
>>> Hello all.
>>>=20
>>> This is a newly question. But I wish to understand why the below =
code is providing different results.
>>>=20
>>> import os, sys
>>>=20
>>>=20
>>> if len(sys.argv) > 2:
>>>  filenames =3D sys.argv[1:]
>>> else
>>>  print ("no parameters provided\n")
>>>  sys.edit()
>>>=20
>>> for filename in filenames:
>>>  print ("filename is: %s\n" %filename)
>>>=20
>>> The above code will return results like:
>>>=20
>>> filename is test.txt
>>>=20
>>> If I modify the above script slightly as shown below, I get a =
completely different result.
>>>=20
>>> if len(sys.argv) > 2:
>>>  filenames =3D sys.argv[1]
>>> else
>>>  print ("no parameters provided\n")
>>>  sys.exit()
>>>=20
>>> for filename in filenames:
>>>  print ("filename is:  %s\n" % filename)
>>>=20
>>> The result is the filename is spelled out a character at a time. The =
bit I am missing is something to do with splicing or referencing in =
Python.
>>>=20
>>> Why am I getting different results? In other languages I would have =
got the whole content of the element when using the index of the array =
(list).
>>>=20
>>>=20
>>> Sean
>>> filename is: t
>>> filename
>>>=20
>>=20
>> As you've already had answers I'd like to point out that your test =
for len(sys.argv) is wrong, else is missing a colon and sys.edit() is =
very unlikely to work :)
>>=20
>> --=20
>> My fellow Pythonistas, ask not what our language can do for you, ask =
what you can do for our language.
>>=20
>> Mark Lawrence
>>=20
>> --=20
>> https://mail.python.org/mailman/listinfo/python-list
>=20