Inserting list values from a list to another in a specific order in Python

Question

I was trying to insert list values from one list to another, but in a specific order, where dates[0] entered text[1], dates[1] entered text[3] and so on.

dates=['21/11/2044', '31/12/2018', '23/9/3000', '25/12/2007']

text=['What are dates? ', ', is an example.\n', ', is another format as
well.\n', ', also exists, but is a bit ludicrous\n', ', are examples but more commonly used']

I tried this method:

for j in range(len(text)):
  for i in range(len(dates)):
   text.insert(int((j*2)+1), dates[i])

This was the result, which was incorrect:

['What are dates? ', '25/12/2007', '23/9/3000', '25/12/2007', '23/9/3000',
'25/12/2007', '23/9/3000', '25/12/2007', '23/9/3000', '25/12/2007',
'23/9/3000', '31/12/2018', '21/11/2044', '31/12/2018', '21/11/2044',
'31/12/2018', '21/11/2044', '31/12/2018', '21/11/2044', '31/12/2018',
'21/11/2044', ', is an example.\n', ', is another format as well.\n', ',
also exists, but is a bit ludicrous\n', ', are examples but more commonly used']

I was trying to get back a list that reads like:

['What are dates? ','21/11/2044', 'is an example.\n','31/12/2018', ', is
another format as well.\n','23/9/3000', ', also exists, but is a bit
ludicrous\n', '25/12/2007',', are examples but more commonly used']

Is there a way to insert dates[i] into text[2*j+1] in the way I wanted? Should I even use a for loop, or is there another way without listing everything in dates as well?

Peter Mortensen · Accepted Answer · 2018-01-07 18:00:14Z

4

A simpler way to achieve this is using itertools.zip_longest in Python 3.x (or izip_longest in Python 2.x) as:

>>> from itertools import zip_longest # for Python 3.x

>>> # For Python 2.x
>>> # from itertools import izip_longest

>>> dates=['21/11/2044', '31/12/2018', '23/9/3000', '25/12/2007']
>>> text=['What are dates? ', ', is an example.\n', ', is another format as well.\n', ', also exists, but is a bit ludicrous\n', ', are examples but more commonly used']

>>> [w for x in zip_longest(text, dates, fillvalue='') for w in x if w]
['What are dates? ', '21/11/2044', ', is an example.\n', '31/12/2018', ', is another format as well.\n', '23/9/3000', ', also exists, but is a bit ludicrous\n', '25/12/2007', ', are examples but more commonly used']

The issue with your code is that you have nested for loops, and that's why for each index of j, all values of dates are getting added.

edited Jan 7, 2018 at 18:00

Peter Mortensen

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answered Jan 7, 2018 at 14:11

Moinuddin Quadri

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2 Comments

Alden Tan Over a year ago

Thank you for the reply, the code works as intended. However, I was doing a project in Al Sweigart's book, where I used regular expressions to find dates of different formats, i.e. 5/14/2000, 2016/5/16, replace them in a standard format, D/M/Y. The book hinted using the regex.sub() function to find and replace these unstandardised date formats, and I used :'foregex=re.compile(r'\d{1,4}/\d{1,2}/\d{1,4}') for i in range(len(dates)): print(foregex.sub(dates[i], text)) , which didn't work. Is there any way to use the info from previous chapters (currently chap. 7) to get my intended list?

Moinuddin Quadri Over a year ago

You'll have to write a very smart regex in order to identify the difference between " 5/14/2000" and "2016/5/16" because Python doesn't know what is the year, and what is the month. For Python all are strings. Simpler way to do it is using dateutil library as used here: stackoverflow.com/a/470303/2063361 .However if you want to regex only, you should create a separate question, and the community, I am noot that good in regex :)

djhoese · Accepted Answer · 2018-01-07 14:12:06Z

1

For your specific example of wanting to fit the dates in every other element you can use zip:

parts = zip(['a', 'b', 'c', 'd'], ['d1', 'd2', 'd3'])
text = [x for y in parts for x in y]
# ['a', 'd1', 'b', 'd2', 'c', 'd3']

You may need to use itertools.izip_longest and/or handle unequal lengths between the list or you'll see results like the above where 'd' was left off the end. The second line is ugly list comprehension magic to flatten a list of lists.

edited Jan 7, 2018 at 14:12

answered Jan 7, 2018 at 14:10

djhoese

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2 Comments

Moinuddin Quadri Over a year ago

zip will skip the last element from one the list here

djhoese Over a year ago

Yep, that's why I mentioned that that would happen. I upvoted your answer since you took the time to do the full answer instead of my shortcut.

Aaron Klein · Accepted Answer · 2018-01-07 14:13:49Z

0

The double for loop is the problem here; just using a single for loop, as in

for i in range(len(dates)):
    text.insert(int((i*2)+1), dates[i])

should do the trick.

However, if you are planning on the second list ending up as a string, it might be simpler to use .format, as in

text = 'Date with one format is {} and date with another format is {}.'.format(*dates)

which will give you

'Date with one format is 21/11/2044 and date with another format is 31/12/2018.'

answered Jan 7, 2018 at 14:13

Aaron Klein

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Comments

habet · Accepted Answer · 2018-01-07 14:24:33Z

0

Maybe it is not the most elegant version, but this works for example:

from itertools import zip_longest


dates=['21/11/2044', '31/12/2018', '23/9/3000', '25/12/2007']

text=['What are dates? ', ', is an example.\n', ', is another format as well.\n', ', also exists, but is a bit ludicrous\n', ', are examples but more commonly used']


l3 = list()
for i, j in zip_longest(text, dates, fillvalue=None):
    if i is not None:
        l3.append(i)
    if j is not None:
        l3.append(j)

print(l3)

zip_longest from the standard itertools module zips 2 lists of uneven length together and fills the missing values with "fillvalue". If you just discard that fillvalue, you are good to go.

answered Jan 7, 2018 at 14:24

habet

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Comments

Adi219 · Accepted Answer · 2018-01-07 14:47:18Z

0

A simple solution which is pretty easy to understand

counter = 0
while counter < len(dates):
    text.insert(counter + counter + 1, dates[counter])
    counter += 1

Essentially, .insert() is used to just append to a list in a specific position.

answered Jan 7, 2018 at 14:47

Adi219

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Comments

Peter Mortensen · Accepted Answer · 2018-01-07 17:58:46Z

0

You can use:

result = [ ]
for x in enumerate(dates, text):
   result.append(dates[x]).append(text[x])

edited Jan 7, 2018 at 17:58

Peter Mortensen

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answered Jan 7, 2018 at 14:10

Vasu Pal

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