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

[Liam Clarke]

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.