Flatten a list of lists containing single strings to a list of ints

Question

i have list with lists of strings looks like

allyears
#[['1916'], ['1919'], ['1922'], ['1912'], ['1924'], ['1920']]

i need to have output like this:

#[1916, 1919, 1922, 1912, 1924, 1920]

have been try this:

for i in range(0, len(allyears)): 
    allyears[i] = int(allyears[i])

but i have error

>>> TypeError: int() argument must be a string, a bytes-like object or a number, not 'list'

You are looking for this: https://stackoverflow.com/questions/952914/how-to-make-a-flat-list-out-of-list-of-lists — Tomás Denis Reyes Sánchez, Apr 03 '20 at 08:55
`int(allyears[i][0])`, or simply this: `[int(item[0]) for item in allyears]` — Jongware, Apr 03 '20 at 08:57

score 9 · Accepted Answer · answered Apr 03 '20 at 09:04

9

You can simply do this:

allyears = [int(i[0]) for i in allyears]

Because all the elements in your allyears is a list which has ony one element, so I get it by i[0]

The error is because ypu can't convert a list to an int

answered Apr 03 '20 at 09:04

Binh

1,143
6
8

1

If there is only one item in nested list then one can also do it without indexing: `[int(*item) for item in allyears]` – Aivar Paalberg Apr 21 '20 at 12:35

score 6 · Answer 2 · answered Apr 03 '20 at 08:58

You're very close, you just need to take the first (and only) element of allyears[i] before doing the int conversion:

for i in range(0, len(allyears)): 
    allyears[i] = int(allyears[i][0])

Alternatively, you could do this in one line using a list comprehension:

allyears = [int(l[0]) for l in allyears]

kederrac · Answer 3 · 2020-04-03T09:26:24.210

if you want a simple for loop you also may use enumerate built-in function and unpack your inner list:

for i, [n] in  enumerate(allyears): 
    allyears[i] = int(n)

you can use a list comprehension:

allyears = [int(e) for l in allyears for e in l]

also, you can use itertools.chain

from itertools import chain

list(map(int, chain.from_iterable(allyears)))

output:

[1916, 1919, 1922, 1912, 1924, 1920]

the last 2 solutions will work also if in your inner lists you have more than one element

score 2 · Answer 4 · answered Apr 03 '20 at 08:59

2

You need to access individual elements of the list instead of lists itself.

Try:

for i in range(0, len(allyears)): 
    allyears[i] = int(allyears[i][0])

answered Apr 03 '20 at 08:59

Hamza Khurshid

765
7
18

score 1 · Answer 5 · answered Apr 03 '20 at 08:57

1

Try:

allyears = [['1916'], ['1919'], ['1922'], ['1912'], ['1924'], ['1920']]

out = []
for i in range(0, len(allyears)): 
    out.append(int(allyears[i][0]))

print (out)

answered Apr 03 '20 at 08:57

m8factorial

188
6

score 0 · Answer 6 · answered Apr 03 '20 at 18:38

You tried this:

for i in range(0, len(allyears)): 
    allyears[i] = int(allyears[i])

and got the error

TypeError: int() argument must be a string, a bytes-like object or a number, not 'list'

The next thing you should have tried is:

for i in range(0, len(allyears)): 
    print(allyears[i])

And you would have seen that you are looping through these values:

['1916']
['1919']
['1922']

etc.

Those cannot be converted to int using int(), because those are lists. You should get the only element of those lists using the [0] index like this:

>>> for i in range(0, len(allyears)): 
...    print(allyears[i][0])
1916
1919
1922...

And now you know how to correct your original code:

for i in range(0, len(allyears)): 
    allyears[i] = int(allyears[i][0])

The more Pythonic solution to your problem would use list comprehension:

allyears = [int(i[0]) for i in allyears]

Flatten a list of lists containing single strings to a list of ints

6 Answers6