CODEWITHSUNDEEP
pythonpandasnlp
benwagner
benwagner

Reputation: 25

How to sequence and pad text in Dataframe column

I wanted to transform a dataset or create a new one that takes a dataset column with labels as input which automatically has sequences of strings according to a pre-defined length (and pads if necessary). The example below should demonstrate what I mean.

I was able to manually create a new dataframe based on ngrams. This is obviously computationally expensive and creates many columns with repetitive words. 

                                               text  labels
0  from dbl visual com david b lewis subject comp...       5
1  from johan blade stack urc tue nl johan wevers...      11
2  from mzhao magnus acs ohio state edu min zhao ...       6
3  from lhawkins annie wellesley edu r lee hawkin...      14
4  from seanmcd ac dal ca subject powerpc ruminat...       4

for example for sequence length 4 into something like this:

   text                                      labels
0  from dbl visual com                            5
1  david b lewis subject                          5
2  comp windows x frequently                      5
3  asked questions <PAD> <PAD>                    5
4  from johan blade stack                        11
5  urc tue nl johan                              11
6  wevers subject re <PAD>                       11
7  from mzhao magnus acs                          6
8  ohio state edu min                             6
9  zhao subject composite <PAD>                   6

As explained I was able to create a new dataframe based on ngrams. I could theoretically delete every n-rows afterwards again.

    df = pd.read_csv('data.csv')
    longform = pd.DataFrame(columns=['text', 'labels'])

    for idx, content in df.iterrows():
        name_words = (i.lower() for i in content[0].split())
        ngramlis = list(ngrams(name_words,20))
        longform = longform.append(
            [{'words': ng, 'labels': content[1]} for ng in ngramlis],
            ignore_index=True
        )
    longform['text_new'] = longform['words'].apply(', '.join)
    longform['text_new'] = longform['text_new'].str.replace(',', '')

This is really bad code which is why I am quite confident that someone might come up with a better solutions.

Thanks in advance!

Upvotes: 0

Views: 617

Answers (1)

Chris
Chris

Reputation: 29742

Use pandas.DataFrame.explode.

Divide the words into evenly sized chunks (and padded) then :

def chunks(l, n):
    """Yield successive n-sized chunks from l.
    Source: https://stackoverflow.com/questions/312443/how-do-you-split-a-list-into-evenly-sized-chunks
    """
    n_items = len(l)
    if n_items % n:
        n_pads = n - n_items % n
    else:
        n_pads = 0
    l = l + ['<PAD>' for _ in range(n_pads)] 
    for i in range(0, len(l), n):
        yield l[i:i + n]

df['text'] = df['text'].str.split().apply(lambda x: list(chunks(x, 4)))
df = df.explode('text').reset_index(drop=True)
df['text'] = df['text'].apply(' '.join)
print(df)

Output:

                             text  labels
0             from dbl visual com       5
1           david b lewis subject       5
2          comp <PAD> <PAD> <PAD>       5
3          from johan blade stack      11
4                urc tue nl johan      11
5        wevers <PAD> <PAD> <PAD>      11
6           from mzhao magnus acs       6
7              ohio state edu min       6
8          zhao <PAD> <PAD> <PAD>       6
9   from lhawkins annie wellesley      14
10               edu r lee hawkin      14
11            from seanmcd ac dal       4
12     ca subject powerpc ruminat       4

Upvotes: 1

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