CODEWITHSUNDEEP
pythonpython-2.7
Baywatch
Baywatch

Reputation: 423

How can I retrieve the previous item of an ordered key/pair collection?

One of the most frustrating things about learning a new language is, well that you don't know how to do anything. I want to perform, what should be a simple task, but I am struggling to implement it.

I want to keep track of items that I have already traversed, along with their location. I ant to be able to look back into the collection when I find an item, see if it's already been seen, and if so, what was it's location (line number). I then want to look at the last item found before the current one and review it's name and location.

I am parsing some unstructured text, and sometimes I match on unintended parts of word sections.

Take the following:

'Item 1', 150
'Item 2', 340
'Item 3', 794
'Item 4', 1205
'Item 5', 1869
'Item 2', 3412  <-- I've seen 2, So I want to inspect the item before it (5, 1869)

My idea is to test the distance between 2 and 5 and make a determination on if it's noise. In this scenario, I would want to drop (Item 2, 3412) because 2 should come before 5 AND line 3412 is such a long distance away from the previous 2 (line 340), and there is also sequential items between the "last seen" item and this one.

Of course, if anyone has a better idea, I am all for that as well.

I have no idea how to walk a collection in python. I'm not even sure what type of collection I should be using. I seem to be favoring lists of paired tuples at the moment, but that's probably just me being silly.

Any guidance is appreciated.

for line_num, line in enumerate(all_lines):
# matching requires back-tracking - we will always be at least 1 line behind loop  
    if line_num < 1: continue  
        blob = ''.join(all_lines[line_num : line_num + _blob_length_])  
        # evaluate text aginst match expressions
        matches = self.match_patterns_sb(blob) if is_sb_edition else self.match_patterns(blob)  
        #iterate each pattern and test if match was successful
        for pattern in matches.iterkeys():
            if matches[pattern] and line_num >= last_line_matched + 1: #Try not to rematch
                if pattern == last_matched_pattern and line_num < (last_line_matched + 2) :continue  
                #store match info in a local tuple nested within a higher level list
                if not '(continued)' in blob.lower() and not '( continued )' in blob.lower():  
                    print '{0}  -  {1}'.format(pattern, line_num)
                    '''
                    At this point I want to look into last_seen, and
                    1) Get the last seen item that matches this one ('Item 2')
                    2) Get the last item added into last_seen
                    3) do some calculations 
                     '''
                    last_seen[pattern] = line_num
                    if pattern in dict(section_items).keys():
                        test = dict(section_items)
                        existing_line = test[pattern]
                        print '{0} exists with LINE NUMBER {1}'.format(pattern, existing_line)  
                    section_items.append( (pattern, line_num) )
                    # track last match
                    last_line_matched = line_num
                    last_matched_pattern = pattern
                # order and normalize the item matches
                fixed_list = OrderedDict(self.sorted_nicely(section_items, itemgetter(0))).items()

Upvotes: 0

Views: 97

Answers (1)

roippi
roippi

Reputation: 25974

Accumulate things as you go using a dict, checking if you've seen it before each time.

sequence = [('item 1',150),('item 2',340),('item 3',794),('item 4',1205,),('item 5',1869),('item 2',3412)]
d = {}

for i,tup in enumerate(sequence):
    item,val = tup
    if d.get(item):
        print("I've seen {} before, it was {} at index {}".format(item,*d.get(item)))
    d[item] = (val, i)

#I've seen item 2 before, it was 340 at index 1

d will always have the last time you've seen item, or None.

If you need to keep track of all the times you've seen item in the past, move up to a defaultdict to accumulate (item, i) tuples into a list for you.

Upvotes: 3

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