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

[Liam Clarke]

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
> 


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