python how to create many-to-many of lists inside one list

Question

I have variable like this :

message = "hello world"

and I want to put each 2 letters inside one list like this:

list = [['h', 'e'], ['l', 'l'], ...]

I have tried this method:

message = "hello world"
x,test = 0,[[]] * len(message)
for i in message:
   if len(test[x]) >= 2:
      x += 1
      test[x].append(i)
   else:
      test[x].append(i)

but the result was adding hello world for every list.

Refer [this](https://stackoverflow.com/a/240205/14429185). – nobleknight Jun 08 '21 at 06:33 — nobleknight, Jun 08 '21 at 06:33

score 1 · Accepted Answer · answered Jun 07 '21 at 22:55

The problem here is that your outer list contains a reference to a single inner list, just repeated. You can see what I mean by taking your resulting test and reassigning the value of one of the elements:

>>> test[1][0] = 9999
[[9999, 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd'],
 [9999, 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd'],
 [9999, 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd'],
 [9999, 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd'],
 [9999, 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd'],
 [9999, 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd'],
 [9999, 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd'],
 [9999, 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd'],
 [9999, 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd'],
 [9999, 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd'],
 [9999, 'e', 'l', 'l', 'o', ' ', 'w', 'o', 'r', 'l', 'd']]

So even though x is incrementing, you're still just appending to a single list object because your test variable is a list of repeated references to the same object.

You can get around this by using a comprehension to initialize your test variable:

test = [[] for _ in range(len(message))]

You can also use zip and slicing to get what you want in a single line of code:

[[*z] for z in zip(s[0::2], s[1::2])]

python how to create many-to-many of lists inside one list

1 Answers1