Get ALL items in a python list?

Question

it's very easy to get the number of items in a list, len(list), but say I had a matrix like: [[1,2,3],[1,2,3]] Is there a pythonic way to return 6? Or do I have to iterate.

use: `l = [[1,2,3],[1,2,3]]` `sum(map(len, l))` to get the 6, but for you an item is a number, but an item for a list is just an item that can be another list, number, floating, object, etc. — eyllanesc, Sep 29 '18 at 23:11
The operation you're looking for is "flatten" the list. Then take the length of the flattened list — eddiewould, Sep 29 '18 at 23:12
https://stackoverflow.com/questions/2158395/flatten-an-irregular-list-of-lists — eddiewould, Sep 29 '18 at 23:17
Whenever you're doing anything fancy with list, the answer is *always* "just use numpy". — o11c, Sep 29 '18 at 23:18

Brown Bear · Accepted Answer · 2018-09-29T23:16:39.470

9

You can use chain

from itertools import chain
l = [[1,2,3],[1,2,3]]
len(list(chain(*l))) # give you 6

the expression list(chain(*l)) give you flat list: [1, 2, 3, 1, 2, 3]

edited Sep 29 '18 at 23:16

answered Sep 29 '18 at 23:12

Brown Bear

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Would this work if the Matrix was 3 dimensional as well ex `[[[1,2,3]]]` – Sep 30 '18 at 00:49

score 1 · Answer 2 · edited Sep 29 '18 at 23:19

1

Make the matrix numpy array like this

mat = np.array([[1,2,3],[1,2,3]])

Make the array 1D like this

arr = mat.ravel()

Print length

print(len(arr))

edited Sep 29 '18 at 23:19

eyllanesc

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answered Sep 29 '18 at 23:17

Sreeram TP

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@eyllanesc thanks for the edit. I am now from a mobile device. I was unable to format the code from here. – Sreeram TP Sep 29 '18 at 23:20

score 1 · Answer 3 · answered Sep 29 '18 at 23:19

1

l = [[1,2,3],[1,2,3]]    
len([item for innerlist in l for item in innerlist])

gives you 6

answered Sep 29 '18 at 23:19

Khalil Al Hooti

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score 0 · Answer 4 · answered Mar 11 '21 at 06:54

You just need to flatten list. Numpy is the best option. Still if you want, you can use simple if/else to flatten and return length of list. Example,

list_1 = [1, 2, 3, 'ID45785', False, '', 2.85, [1, 2, 'ID85639', True, 1.8], (e for e in range(589, 591))]




def to_flatten3(my_list, primitives=(bool, str, int, float)):
    flatten = []
    for item in my_list:
        if isinstance(item, primitives):
            flatten.append(item)
        else:
            flatten.extend(item)
    return len(flatten)
   
print(to_flatten3(list_1))

14

[Program finished]

Get ALL items in a python list?

4 Answers4