[Tutor] Python RE uses DFA or NFA for string check?
Hello everyone,This question is just out of curiosity. I am working with this dragon book. From what I have learnt so far, RE uses either NFA or DFA to check whether the string is accepted or not. (Correct?) So what does the Python's RE module use to check the correctness of the string, NFA or DFA?-- Intercodes ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Python RE uses DFA or NFA for string check?
Intercodes wrote: Hello everyone, This question is just out of curiosity. I am working with this dragon book. From what I have learnt so far, RE uses either NFA or DFA to check whether the string is accepted or not. (Correct?) So what does the Python's RE module use to check the correctness of the string, NFA or DFA? You could look at the source. A little digging shows that REs are parsed by sre_parse.parse() which is in Python24\Lib\sre_parse.py on my computer. Kent ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Python RE uses DFA or NFA for string check?
[Intercodes] This question is just out of curiosity. I am working with this dragon book. From what I have learnt so far, RE uses either NFA or DFA to check whether the string is accepted or not. (Correct?) In the world of computer science regular expressions, yes. But the things _called_ regular expressions in programming languages are generally richer than those. For example, almost all regexp implementations support backreferences, and backreferences allow recognizing languages that computer-science regexps cannot. For example, ^(a*)b+\1$ recognizes strings that begin and end with the same number of a's, separated by one or more b's. It's the same number part that's beyond a pure regexp's abilities. So what does the Python's RE module use to check the correctness of the string, NFA or DFA? Neither, but it's much closer to NFA than to DFA. Most regexp implementations in most languages supporting such a thing are implemented via backtracking search. Jeffrey Friedl's Mastering Regular Expressions is more useful than the dragon book if you want insight into how most programming-language regexp implementations actually work: http://www.oreilly.com/catalog/regex/ To increase confusion ;-), Friedl calls backtracking search NFA in that book. ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Python RE uses DFA or NFA for string check?
Thanks Mr.Tim. That was helpful :)On 1/10/06, Tim Peters [EMAIL PROTECTED] wrote: [Intercodes] This question is just out of curiosity. I am working with this dragon book. From what I have learnt so far, RE uses either NFA or DFA to check whether the string is accepted or not. (Correct?) In the world of computer science regular expressions, yes.But thethings _called_ regular expressions in programming languages aregenerally richer than those.For example, almost all regexp implementations support backreferences, and backreferences allowrecognizing languages that computer-science regexps cannot.Forexample,^(a*)b+\1$recognizes strings that begin and end with the same number of a's, separated by one or more b's.It's the same number part that'sbeyond a pure regexp's abilities. So what does the Python's RE module use to check the correctness of the string, NFA or DFA? Neither, but it's much closer to NFA than to DFA.Most regexpimplementations in most languages supporting such a thing areimplemented via backtracking search.Jeffrey Friedl's MasteringRegular Expressions is more useful than the dragon book if you want insight into how most programming-language regexp implementationsactually work:http://www.oreilly.com/catalog/regex/To increase confusion ;-), Friedl calls backtracking search NFA in that book. -- Intercodes ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
[Tutor] How can I make a python script go directory by directory and excecute on files of choice
Dear group, I have Excel files that are arranged according to transctions under various name/directories. I found out that all these Excel files are not real OLE based files and some of them are really tab delim files with .XLS appended to the end of file name. I got fooled and started using pyExcelator module. Now I want to write a script that can go to each directory from base directory. Say: my dir path is like this: /home/srini/data/sales - jan05/(1-31).xls - feb05/(1-27).xls - mar05/(1-31).xls How can I ask my script residing in data directory to go to sales and scan all directories and report if .xls files are really Excel files or text files with .xls extension. could any one please help me here. thank you. __ Yahoo! DSL Something to write home about. Just $16.99/mo. or less. dsl.yahoo.com ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
[Tutor] need help with syntax
hello all I am new to python and this list has been helpfull so far
I am trying to convert binary file to ascii
here is the format spec
steps = int 4
value = int 4
time = float 4 * steps
so in the python terminal terminal i convert it like this
import struct
import string
f = file('binary_file','rb')
line = f.readline()
L = tuple(line)
then to get the steps value i do
s = L[:4]
s
('\x00', '\x00', '\x00', '\x06')
a = string.join( s , '')
steps = struct.unpack (l , a)
steps
(6,)
and for value
v = L[4:8]
v
('\x00', '\x00', '\x00', '\x08')
b = string.join( v , '')
value = struct.unpack (l , b)
value
(8,)
for the time value this is where I need help
I know steps = 6 so
4*6 + 8
32
t = L[8:32]
t
('\x00', '\x00', '\x00', '\x00', '=', '\x08', '\x88', '\x89', '=', '\x88', '\x88
', '\x89', '=', '\xcc', '\xcc', '\xcd', '', '\x08', '\x88', '\x89', '', '*', '
\xaa', '\xab')
c = string.join(t , '')
time = struct.unpack (6f, c)
time
(0.0, 0.03335071802139, 0.06670143604279, 0.1000149011612, 0.133
4028720856, 0.166716337204)
so I was happy to figure this much out
what I need is a way to do somthing like this
time = struct.unpack(steps,c)
but this does not work because steps is not an integer
any help would be great thanks
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Re: [Tutor] need help with syntax
On 11/01/06, bill nieuwendorp [EMAIL PROTECTED] wrote:
Hi Bill,
Some comments ---
import struct
import string
f = file('binary_file','rb')
line = f.readline()
L = tuple(line)
You can do slicing (things like L[:4]) on strings as well as on lists
and tuples. So there is probably no need for you to convert to a
tuple --- just work with the lines directly.
s = L[:4]
s
('\x00', '\x00', '\x00', '\x06')
a = string.join( s , '')
You can also do:
a = ''.join(s)
and this is considered better python these days.
what I need is a way to do somthing like this
time = struct.unpack(steps,c)
but this does not work because steps is not an integer
Sounds like you want string substitutions.. You can read about them
here: http://python.org/doc/2.4.2/lib/typesseq-strings.html
For instance:
steps = 6
%s % steps
'6'
HTH!
--
John.
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Re: [Tutor] need help with syntax
Hi John thanks for the tips
I had a bit of a typo in my first post
time = struct.unpack(steps,c)
should read somthing more like
time = struct.unpack(steps+f,c)
adding the f to tell structs it is in float format
the string substitution
seems like it would work but now I cant figure out how I would add the
f on the end
also this still requires me to reassign 6 to steps
isnt this already done here ?
s = L[:4]
s
('\x00', '\x00', '\x00', '\x06')
a = string.join( s , '')
steps = struct.unpack (l , a)
steps
(6,)
what I would like to do in the end is take the format formula.
steps = int 4
value = int 4
time = float 4 * steps
and have a python script that will spit out the ascii contents,
without me having to assign 6 to steps as that number will change from
file to file.
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[Tutor] Fwd: need help with syntax
oops, forward to list.
-- Forwarded message --
From: Liam Clarke [EMAIL PROTECTED]
Date: Jan 11, 2006 4:18 PM
Subject: Re: [Tutor] need help with syntax
To: bill nieuwendorp [EMAIL PROTECTED]
On 1/11/06, bill nieuwendorp [EMAIL PROTECTED] wrote:
hello all I am new to python and this list has been helpfull so far
I am trying to convert binary file to ascii
here is the format spec
steps = int 4
value = int 4
time = float 4 * steps
so in the python terminal terminal i convert it like this
import struct
import string
f = file('binary_file','rb')
line = f.readline()
L = tuple(line)
Try this -
import struct
f = file('binary_file','rb')
toUnpack = f.read(4)
steps = struct.unpack(i,toUnpack)[0]
floatBytes = steps * 4
furtherUnpack = f.read(4 + floatBytes) #The first 4 for value
pattern = i%df % steps
data = struct.unpack(pattern, furtherUnpack)
value = data[0]
time = data[1:]
Or probably simpler to do
f = file('binary_file','rb')
toUnpack = f.read(8)
(steps,value) = struct.unpack(2i,toUnpack)
floatBytes = steps * 4
furtherUnpack = f.read(floatBytes)
pattern = %df % steps
time = struct.unpack(pattern, furtherUnpack)
You could then call f.readline() to advance to the next line if
needed, although binary data tends not to be line break delimited as
such...
But I found that using f.read(num_of_bytes) is the best way to do
this, especially when the data you're reading is unknown but specified
at a known offset.
You may also want to have a look at f.seek() and f.tell(), and if the
%d stuff is unfamiliar you may want to google - 'format string
inurl:python.org'
Good luck, it's a bit tricky at times*, but it works, I built a simple
iPod database parser using f.read() and string substitution for the
struct module patterns.
Regards,
Liam Clarke
*Like when your brilliant plan to have objects generate patterns for
the data they're working with by checking type founders thanks to
shorts.
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Re: [Tutor] need help with syntax
On 11/01/06, bill nieuwendorp [EMAIL PROTECTED] wrote:
time = struct.unpack(steps+f,c)
adding the f to tell structs it is in float format
the string substitution
seems like it would work but now I cant figure out how I would add the
f on the end
Did you read up on string substitution? You can do a few tricks with
it, but in its simplest form, it just replaces '%s' with the argument.
eg:
'foo%sbar' % 'seven' == 'foosevenbar'
Or even:
'x%sy%sz' % ('foo', 'bar') == 'xfooybarz'
So, in your case, try something like '%s+f'.
also this still requires me to reassign 6 to steps
isnt this already done here ?
s = L[:4]
s
('\x00', '\x00', '\x00', '\x06')
a = string.join( s , '')
steps = struct.unpack (l , a)
steps
(6,)
Yes, although in this case steps is a 1-element tuple. So, steps[0]
will give you what you want :-)
--
John.
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Re: [Tutor] How can I make a python script go directory by directory and excecute on files of choice
Hi Srinivas - For walking a directory, you can use os.walk() or os.path.walk(), but I prefer the path module here - http://www.jorendorff.com/articles/python/path/. As for determining if a file is really an .XLS format file or a tab delimited file with .xls on the end, go to www.wotsit.org, have a look at the .XLS specifications; there'll probably be a sequence of identifying bytes you can read to confirm that it's an XLS. Regards, Liam Clarke On 1/11/06, Srinivas Iyyer [EMAIL PROTECTED] wrote: Dear group, I have Excel files that are arranged according to transctions under various name/directories. I found out that all these Excel files are not real OLE based files and some of them are really tab delim files with .XLS appended to the end of file name. I got fooled and started using pyExcelator module. Now I want to write a script that can go to each directory from base directory. Say: my dir path is like this: /home/srini/data/sales - jan05/(1-31).xls - feb05/(1-27).xls - mar05/(1-31).xls How can I ask my script residing in data directory to go to sales and scan all directories and report if .xls files are really Excel files or text files with .xls extension. could any one please help me here. thank you. __ Yahoo! DSL – Something to write home about. Just $16.99/mo. or less. dsl.yahoo.com ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
[Tutor] declaring list in python
Hello list I want to declare a list of a specific size as global to some nested function like so def foo (n): A[] (of size n) def foo1 ... The only way I can think to declare the list is to use dummy values: A = [0] * n A = [] * n doesn't work. [] * n = [] I'd prefer not to use dummy values I have no use for. Is there any way? Thanks Logesh Pillay ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] declaring list in python
Logesh Pillay said unto the world upon 10/01/06 11:28 PM: Hello list I want to declare a list of a specific size as global to some nested function like so Hi Logesh, what problem are you trying to solve by doing this? Knowing that will help generate more useful answers, I suspect. def foo (n): A[] (of size n) def foo1 ... The only way I can think to declare the list is to use dummy values: A = [0] * n A = [] * n doesn't work. [] * n = [] A = [None] * n would be a better way to created an n-placed list of dummy values in Python, I think. I'd prefer not to use dummy values I have no use for. Is there any way? This is why knowing your problem would be helpful. Built-in Python data structures don't have size limitations that are declared when an instance of the data structure is created. (Python's not C.) There is no way (that I know of) to create an n-placed list save creating a list with n objects. So, I think you will have to either give up on not employing dummy values or give up on creating a list of a fixed length. You could subclass list to create a class with a max. and/or min. length, but I think knowing more about what you want to do would be helpful before getting into that :-) Best, Brian vdB ___ Tutor maillist - Tutor@python.org http://mail.python.org/mailman/listinfo/tutor
