efficiently splitting up strings based on substrings

7stud bbxx789_05ss at yahoo.com
Sun Sep 6 09:14:52 CEST 2009


On Sep 5, 5:29 pm, per <perfr... at gmail.com> wrote:
> On Sep 5, 7:07 pm, "Rhodri James" <rho... at wildebst.demon.co.uk> wrote:
>
>
>
> > On Sat, 05 Sep 2009 23:54:08 +0100, per <perfr... at gmail.com> wrote:
> > > On Sep 5, 6:42 pm, "Rhodri James" <rho... at wildebst.demon.co.uk> wrote:
> > >> On Sat, 05 Sep 2009 22:54:41 +0100, per <perfr... at gmail.com> wrote:
> > >> > I'm trying to efficiently "split" strings based on what substrings
> > >> > they are made up of.
> > >> > i have a set of strings that are comprised of known substrings.
> > >> > For example, a, b, and c are substrings that are not identical to each
> > >> > other, e.g.:
> > >> > a = "0" * 5
> > >> > b = "1" * 5
> > >> > c = "2" * 5
>
> > >> > Then my_string might be:
>
> > >> > my_string = a + b + c
>
> > >> > i am looking for an efficient way to solve the following problem.
> > >> > suppose i have a short
> > >> > string x that is a substring of my_string.  I want to "split" the
> > >> > string x into blocks based on
> > >> > what substrings (i.e. a, b, or c) chunks of s fall into.
>
> > >> > to illustrate this, suppose x = "00111". Then I can detect where x
> > >> > starts in my_string
> > >> > using my_string.find(x).  But I don't know how to partition x into
> > >> > blocks depending
> > >> > on the substrings.  What I want to get out in this case is: "00",
> > >> > "111".  If x were "001111122",
> > >> > I'd want to get out "00","11111", "22".
>
> > >> > is there an easy way to do this?  i can't simply split x on a, b, or c
> > >> > because these might
> > >> > not be contained in x.  I want to avoid doing something inefficient
> > >> > like looking at all substrings
> > >> > of my_string etc.
>
> > >> > i wouldn't mind using regular expressions for this but i cannot think
> > >> > of an easy regular
> > >> > expression for this problem.  I looked at the string module in the
> > >> > library but did not see
> > >> > anything that seemd related but i might have missed it.
>
> > >> I'm not sure I understand your question exactly.  You seem to imply
> > >> that the order of the substrings of x is consistent.  If that's the
> > >> case, this ought to help:
>
> > >> >>> import re
> > >> >>> x = "001111122"
> > >> >>> m = re.match(r"(0*)(1*)(2*)", x)
> > >> >>> m.groups()
>
> > >> ('00', '11111', '22')>>> y = "00111"
> > >> >>> m = re.match(r"(0*)(1*)(2*)", y)
> > >> >>> m.groups()
>
> > >> ('00', '111', '')
>
> > >> You'll have to filter out the empty groups for yourself, but that's
> > >> no great problem.
>
> > > The order of the substrings is consistent but what if it's not 0, 1, 2
> > > but a more complicated string? e.g.
>
> > > a = 1030405, b = 1babcf, c = fUUIUP
>
> > > then the substring x might be 4051ba, in which case using a regexp
> > > with (1*) will not work since both a and b substrings begin with the
> > > character 1.
>
> > Right.  This looks approximately nothing like what I thought your
> > problem was.  Would I be right in thinking that you want to match
> > substrings of your potential "substrings" against the string x?
>
> > I'm sufficiently confused that I think I'd like to see what your
> > use case actually is before I make more of a fool of myself.
>
> > --
> > Rhodri James *-* Wildebeest Herder to the Masses
>
> it's exactly the same problem, except there are no constraints on the
> strings.  so the problem is, like you say, matching the substrings
> against the string x. in other words, finding out where x "aligns" to
> the ordered substrings abc, and then determine what chunk of x belongs
> to a, what chunk belongs to b, and what chunk belongs to c.
>
> so in the example i gave above, the substrings are: a = 1030405, b =
> 1babcf, c = fUUIUP, so abc = 10304051babcffUUIUP
>
> given a substring like 4051ba, i'd want to split it into the chunks a,
> b, and c. in this case, i'd want the result to be: ["405", "1ba"] --
> i.e. "405" is the chunk of x that belongs to a, and "1ba" the chunk
> that belongs to be. in this case, there are no chunks of c.  if x
> instead were "4051babcffUU", the right output is: ["405", "1babcf",
> "fUU"], which are the corresponding chunks of a, b, and c that make up
> x respectively.
>
> i'm not sure how to approach this. any ideas/tips would be greatly
> appreciated. thanks again.


a = "1030405"
b = "1babcf"
c = "fUUIUP"
abc = "10304051babcffUUIUP"
data = "4051babcffU"

data_start = abc.find(data)
b_start = abc.find(b) - data_start
c_start = abc.find(c) - data_start

print data[:b_start]
print data[b_start:c_start]
print data[c_start:]

--output:--
405
1babcf
fU




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