[Tutor] design question -- nested loops considered harmful?

[Liam Clarke]

Oh terrible, terrible, the above multi-find is bad. It should be as below.

index_dict={}

for item in item_flags:

 foundIndice=[]
 findIndex=0
 startIndex=0

 while findIndex ! = -1:
         findIndex=string2FindIn.find(item, startIndex)
         findIndex += len(item)
         foundIndice.append(findIndex)
         startIndex=findIndex
 

 del foundIndice[len(foundIndice)-1] 
 index_dict[item]=foundIndice

So, it starts from zero in the string, searches for the next
occurrence of your search string, records the index of that
occurrence, and searches again, this time starting from the previous
index + the length of the search string, records the next index and so
on.

String.find() returns -1 for string not found, so it searches until it
finds no more occurrences. Due to the way I've written my loop, the
last value of foundIndice is always going to be -1, so
del[foundIndice[len(foundIndice)-1] just deletes the last item.

And then, you can do a multiple search and slice in a similar method

data_dict={}
for item in item_flags:
         sliceList=[]
         for itemIndex in index_dict[item]:
              endOfSlice = string2FindIn.find('\n', itemIndex)
              slice=string2FindIn(itemIndex:endOfSlice)
              sliceList.append(slice)
  
         data_dict[item]=sliceList


et voila.

I hope this tangent helps in some small way.

I don't see any harm in a couple of nested loops, particularly when
you're a n00b, like me.
One day list comprehensions will stop being so damn scary, but until
then, I loop like buggery.



On Tue, 30 Nov 2004 16:29:54 +1300, Liam Clarke <cyresse at gmail.com> wrote:
> Oh dear, Gmail is terrible to email code in.
> 
> 
> 
> 
> On Tue, 30 Nov 2004 16:27:35 +1300, Liam Clarke <cyresse at gmail.com> wrote:
> > Hi Brian,
> >
> > Just curious -
> >
> >
> >
> > >date_flag = '[Date]'
> > > email_flag = '[Email]'
> > > item_flags = [date_flag, email_flag]
> >
> > >def parse_file(list_of_lines):
> > >      data_dict = {}
> > >      for line in list_of_lines:
> > >          for item in item_flags:
> > >              if line.startswith(item):
> > >                  data_dict[item] = line[len(item):]
> > >                  break
> > >      return data_dict
> >
> > So, you're thingmajig scans each line for either date_flag or
> > email_flag, and then slices the rest of the line and saves it as a
> > dict. And, I guess you're looking for one date_flag line or one
> > email_flag...
> >
> > Assuming that there would only be one occurrence of each one in a file -
> >
> > x=file('Brian's source file') 'r')
> > a=x.readlines() #How big is it? If it's a huge file, this may not be the best
> > x.close()
> > a="".join(a) #Turns a list into a string, the "" is the joiner, i.e.
> >                  # a=["Hello","World"] would become "HelloWorld"
> > whereas a="?M?".join(a)
> >                  #would become "Hello?M?World"
> >                  #Just had a thought - a=str(a) would do exactly the
> > same as a="".join(a)
> >                  #wouldn't it?
> > item_flags=["[Date]","[Email]"]
> >
> > def parsefile(fileString):
> >     data_dict={}
> >      for item in item_flags:
> >           tagIndex=fileString.find(item) #Finds lowest occurrence of
> > item. Index is would be
> >                                                     #first letter of
> > item_flag[item]
> >           newLineIndex=fileString.find('\n', tagIndex) #Finds next
> > newline after item_flag[item]
> >           data_dict[item]=fileString[tagIndex+len(item):newLineIndex]
> >
> > The slice at the end should slice from the char after the ']' of item,
> > to the char before the next '\n' which is the end of the line.
> >
> > If there are multiple occurrences, all you have to do is -
> >
> > for item in item_flags:
> >
> >   foundIndice=[]
> >   findIndex=0
> >   startIndex=0
> >
> >   while findIndex ! = -1:
> >           findIndex=string2FindIn.find(item, startIndex)
> >           foundIndice.append(findIndex)
> >
> >   del foundIndice[len(foundIndice)-1] #Delete last item, as .find
> > returns "-1" for string not
> >                                                     #found, and this
> > will always be appended at end.
> >    data_dict[item]=foundIndice
> >
> > Of course, this is entirely subjective on personal style.
> >
> >
> >
> >
> > On Mon, 29 Nov 2004 21:26:37 -0500, Brian van den Broek
> > <bvande at po-box.mcgill.ca> wrote:
> > > Hi all,
> > >
> > > in a recent post in the "comapring lists" thread, Danny Yoo wrote:
> > >
> > > > whenever I see nested loops
> > > > like this, I get nervous.  *grin*
> > >
> > > This got me thinking about general design issues. In various programs I
> > >   have made much use of nested loops in order to parse data files. I've
> > > done this in cases where I am interested in pulling out some data which
> > > is identified by a delimiter. Below is a minimal example of the sort of
> > > thing I have been doing:
> > >
> > > date_flag = '[Date]'
> > > email_flag = '[Email]'
> > > item_flags = [date_flag, email_flag]
> > >
> > > def parse_file(list_of_lines):
> > >      data_dict = {}
> > >      for line in list_of_lines:
> > >          for item in item_flags:
> > >              if line.startswith(item):
> > >                  data_dict[item] = line[len(item):]
> > >                  break
> > >      return data_dict
> > >
> > > In this particular toy case, the "for i in item_flags" isn't too much
> > > help, as I've only listed two delimiters. But I often have a good many
> > > more, and thus thought the nested loop much better than a long list of
> > > if-tests. Since the logic in the "for item in item_flags:" loop is quite
> > > small, it never occurred to be to move it into its own function.
> > >
> > > I think I see that the nervous-making aspect of nested loops comes from
> > > concern about clarity of control flow. (Is there some other worry I'm
> > > missing?) But is my sort of case one which shows a rule of thumb isn't a
> > > rigid law? Is there a much better design for my task that I've missed?
> > > Do more experience folk doubt my wisdom in taking the embedded loop to
> > > be too short to bother factoring out?
> > >
> > > Thanks for any input. Best to all,
> > >
> > > Brian vdB
> > >
> > > _______________________________________________
> > > Tutor maillist  -  Tutor at python.org
> > > http://mail.python.org/mailman/listinfo/tutor
> > >
> >
> >
> > --
> > 'There is only one basic human right, and that is to do as you damn well please.
> > And with it comes the only bsi  c human duty, to take the consequences.
> >
> 
> --
> 'There is only one basic human right, and that is to do as you damn well please.
> And with it comes the only basic human duty, to take the consequences.
> 


-- 
'There is only one basic human right, and that is to do as you damn well please.
And with it comes the only basic human duty, to take the consequences.