How to convert array of array into set in Python 3?

Question

What I have as input:

list: [[2, 4], [2, 6], [2, 8]]

What I want as output:

set: 2,4,6,8

What I'm currently doing(not working):

def convert(input_list):
    all_nums = set([arr[0], arr[1]] for arr in input_list)
    return all_nums

I know I can manually iterate through parent array and add contents of child array to the set like this:

def convert(input_list):
    all_nums = set()
    for inner_list in input_list:
        all_nums.add(inner_list[0])
        all_nums.add(inner_list[1])
    return all_nums

How to do do that in pythonic way?
In second approach can I do something like all_nums.add(inner_list[0], inner_list[1])?

Where are your 6 and 8 coming from in your expected set? Do you just want a set of all numbers in the flattened input list? In this case `{1, 2, 3, 4}`? — Engineero, Aug 10 '18 at 15:40
@user3483203 In second approach can I do something like all_nums.add(inner_list[0], inner_list[1])? — noobie, Aug 10 '18 at 15:47

wizzwizz4 · Accepted Answer · 2018-08-10T16:23:50.360

6

Simply:

my_list = [[2, 4], [2, 6], [2, 8]]
my_set = {e for l in my_list for e in l}

This is using a "set comprehension", which is a squashed down version of:

my_list = [[2, 4], [2, 6], [2, 8]]
my_set = set()
for l in my_list:
    for e in l:
        my_set.add(e)

Alternatively, you could do:

my_list = [[2, 4], [2, 6], [2, 8]]
my_set = set()
for l in my_list:
    my_set.update(l)

(Variable names shamelessly stolen from modesitt.)

edited Aug 10 '18 at 16:23

answered Aug 10 '18 at 15:39

wizzwizz4

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In second approach can I do something like all_nums.add(inner_list[0], inner_list[1])? – noobie Aug 10 '18 at 15:45
You need double brackets so you're adding a tuple (immutable, hashable list), but yes. Alternatively, you could do `my_set = {(l[0], l[1]) for l in my_list}` or `my_set = {tuple(l) for l in my_list}` or `my_set = {(x, y) for x, y in my_list}`. – wizzwizz4 Aug 10 '18 at 15:48
in second approach doing `all_nums.add((inner_list[0], inner_list[1]))` gives `{(2, 8), (2, 6), (2, 4)}` but **not** {2, 4, 6, 8} – noobie Aug 10 '18 at 16:07
I thought that was what you wanted... – wizzwizz4 Aug 10 '18 at 16:18
is there a way I can get {2, 4, 6, 8} without adding them separately inside for-loop (see the second approach)? – noobie Aug 10 '18 at 16:21
@noobie Use `my_set.update(inner_list)`. – wizzwizz4 Aug 10 '18 at 16:22

score 2 · Answer 2 · answered Aug 10 '18 at 15:36

2

One approach using itertools.chain and set

Ex:

from itertools import chain
l = [[1, 2], [1, 3], [1, 4]]
print(set(chain.from_iterable(l)))

Output:

set([1, 2, 3, 4])

chain to flatten the list

answered Aug 10 '18 at 15:36

Rakesh

81,458
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score 1 · Answer 3 · answered Aug 10 '18 at 15:40

1

You can try this:

from itertools import chain

l = [[1, 2], [1, 3], [1, 4]]
l = list(chain.from_iterable(l))
set_l = set(l)

answered Aug 10 '18 at 15:40

brandonwang

1,603
10
17

1

There's no need for the intermediate list, by the way. – wizzwizz4 Aug 10 '18 at 15:43

score 0 · Answer 4 · answered Aug 10 '18 at 15:40

0

I would do the following

set_ = {e for l in my_list for e in l}

answered Aug 10 '18 at 15:40

modesitt

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Can I steal your variable names? There better than my `x` and `y` (`e`lement and `l`ist, I assume yours are) but my answer has 2 upvotes. – wizzwizz4 Aug 10 '18 at 15:43
of course! you won that race condition ;) @wizzwizz4 – modesitt Aug 10 '18 at 15:44

score 0 · Answer 5 · answered Aug 10 '18 at 15:41

0

You can use set comprehension:

l = [[2, 4], [2, 6], [2, 8]]
print({i for s in l for i in s})

This outputs:

{8, 2, 4, 6}

answered Aug 10 '18 at 15:41

blhsing

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How to convert array of array into set in Python 3?

5 Answers5