Appending certain elements from a list of list into another list

Question

I have a nested list as shown below:

L = [['James', 'Bob', '23', '47'], ['Earl', 'Carl', '16', '43'], ['Milly', 'Wendy', '1', '21']]

I want to take the names from each of them and put them into a new list called 'newList' without the numbers.

I tried the following code:

for i in L[0][0]:
    newList.append(i)
print(newList)

However, this just printed the first name and each letter was seperated by a comma. I want the new list to look like this:

newList = ['James', 'Bob', 'Earl', 'Carl', 'Milly', 'Wendy']

Possible duplicate of [Explain Python's slice notation](http://stackoverflow.com/questions/509211/explain-pythons-slice-notation) — user2390182, May 09 '17 at 09:46

score 3 · Answer 1 · answered May 09 '17 at 09:44

3

Just use a nested list comprehension:

In [61]: [i for sub in L for i in sub[:2]]
Out[61]: ['James', 'Bob', 'Earl', 'Carl', 'Milly', 'Wendy']

answered May 09 '17 at 09:44

Mazdak

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score 1 · Answer 2 · answered May 09 '17 at 09:44

1

You can use isdigit() to filter out names

res = [j for i in L for j in i if not j.isdigit()]
['James', 'Bob', 'Earl', 'Carl', 'Milly', 'Wendy']

answered May 09 '17 at 09:44

akash karothiya

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score 1 · Answer 3 · answered May 09 '17 at 09:46

Do it in two steps.

First, grab the first two elements of each list:

>>> names_nested = [sub[:2] for sub in L]
>>> names_nested
[['James', 'Bob'], ['Earl', 'Carl'], ['Milly', 'Wendy']]

Then unnest this list:

>>> names = [name for sub in names_nested for name in sub]
>>> names
['James', 'Bob', 'Earl', 'Carl', 'Milly', 'Wendy']

... or do it more comfortably with itertools.chain:

>>> from itertools import chain
>>> list(chain(*(sub[:2] for sub in L)))
['James', 'Bob', 'Earl', 'Carl', 'Milly', 'Wendy']

... or itertools.chain.from_iterable to get rid of the unpacking star and one set of parentheses.

>>> list(chain.from_iterable(sub[:2] for sub in L))
['James', 'Bob', 'Earl', 'Carl', 'Milly', 'Wendy']

itzMEonTV · Accepted Answer · 2017-05-09T10:19:50.130

1

You need to iterate over elements in list L

for i in L:
    newList.append(i[0])
    newList.append(i[1])
    #or simply newList.extend(i[:2])
print(newList)

EDIT

for i in L:
    if i[0] not in newList:
        newList.append(i[0])
    if i[1] not in newList:
        newList.append(i[1])
    #or simply newList.extend(i[:2])
print(newList)

edited May 09 '17 at 10:19

answered May 09 '17 at 09:47

itzMEonTV

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Thank you! How would I alter this so that if I had a nested list with repeated names it would only print the name once in the new list? – Billy Whales May 09 '17 at 10:02
Here `i[0]` is the first element, `i[1]` is the second element.You can check whether equal or not before `append` – itzMEonTV May 09 '17 at 10:06
I tried putting `if i[0] != i[1]` after the for statement and before the appending statements. This however just produced the same result as before. – Billy Whales May 09 '17 at 10:12
I dont undertand repeating names, is that in same list or in diferent list? – itzMEonTV May 09 '17 at 10:15
In a different list, for example: `[['James', 'Alan'], ['Alan', 'James']]` It would only print `['James', 'Alan']` – Billy Whales May 09 '17 at 10:17

score 0 · Answer 5 · answered May 09 '17 at 09:44

If the problem is simply the first two elements from each sublist you can do:

from itertools import chain
result = chain.from_iterable(sublist[:2] for sublist in L)

This will produce a chain object, you can use list to evaluate it:

list(result)
# ['James', 'Bob', 'Earl', 'Carl', 'Milly', 'Wendy']

score 0 · Answer 6 · answered May 09 '17 at 09:53

0

Using numpy to slice the list and flatten the results to a 1D list:

import numpy as np
np.array(L)[:,0:2].flatten().tolist()
Out[349]: ['James', 'Bob', 'Earl', 'Carl', 'Milly', 'Wendy']

answered May 09 '17 at 09:53

Allen Qin

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score 0 · Answer 7 · answered May 09 '17 at 09:58

0

Use the isdigit method to see if a str is a digit:

[j for l in L for j in l if not j.isdigit()]

Output:

['James', 'Bob', 'Earl', 'Carl', 'Milly', 'Wendy']

answered May 09 '17 at 09:58

elena

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Appending certain elements from a list of list into another list

7 Answers7