Python find and keep patterns in a list and replace others-CodePudding

I am trying to write a code to extract patterns from a each list in a list of lists. I search for patterns with specified lengths, such as 'B-' followed by 'I-'. For example, I want to keep patterns with lengths of two and replace others with a specified string from the following list:

list = ['O', 'B-', 'I-', 'I-', 'O', 'B-', 'I-', 'B-']

Expected output should be as follows:

expected_list_2 = ['O', 'O', 'O', 'O', 'O', 'B-', 'I-', 'O']

As can be seen only length of two pattern 'B-', 'I-' is kept and others changed with 'O' label.

If I want to keep patterns with lengths of three, the output should be as follows:

expected_list_3 = ['O', 'B-', 'I-', 'I-', 'O', 'O', 'O', 'O']

Considering each elements of my list of lists includes such kind of lists and I try to implement this task for each list, I asked for, is there any efficient or tricky way to do this instead of defining some if-else conditions and looping over the each element?

CodePudding user response：

This solution should (please test with more relevant cases before deploying to production) find all positions of pattern 'B-', n-1 x 'I-' in a list. I extended the example list1 to cover more cases, like pattern at start and end of the list and consecutive patterns.

list1 = ['B-', 'I-', 'I-', 'O', 'B-', 'I-', 'I-', 'B-', 'I-', 'B-', 'I-', 'O', 'B-', 'I-', 'I-']
#n = 2:                                            ^^^^^^^^^   ^^^^^^^^^
#n = 3:  ^^^^^^^^^^^^^^^^        ^^^^^^^^^^^^^^^                                ^^^^^^^^^^^^^^^

def find_pattern(list1, n):
    pattern = ['B-']   ['I-'] * (n-1)
    first = pattern[0]
    
    # find starting indices of matching patterns
    idx = [e for e, i in enumerate(list1[:-n 1 or None])
             if i == first                   # optimization for long pattern
            and list1[e:e n] == pattern
            and list1[e n:e n 1] != ['I-']]

    # insert pattern at those indices
    res = ['O'] * len(list1)
    for i in idx:
        res[i:i n] = pattern 
    return res

print(find_pattern(list1, 2))
print(find_pattern(list1, 3))

Output

['O', 'O', 'O', 'O', 'O', 'O', 'O', 'B-', 'I-', 'B-', 'I-', 'O', 'O', 'O', 'O']
['B-', 'I-', 'I-', 'O', 'B-', 'I-', 'I-', 'O', 'O', 'O', 'O', 'O', 'B-', 'I-', 'I-']