Hi pythonists! This is my first post there... I faced with a question when I needed to create a list with a number of empty lists. I found two approaches:
1. ls = [[] for _ in range(9)]
2. ls = [[]] * 9
And after I printed them and their types I got the same results:
a = [[]] * 9
b = [[] for _ in range(9)]
print(a)
print(b)
print(type(a))
print(type(b))
results:
[[], [], [], [], [], [], [], [], []]
[[], [], [], [], [], [], [], [], []]
<class 'list'>
<class 'list'>
However when I was solving
this task and checked the solution (available by the link too) from leetcode these lists worked in different ways - only with the second I got right result. What is the difference between them?
you should post your code (not original) using
bbcode tags.
You may have introduced a typo.
(Jun-12-2024, 11:16 AM)Larz60+ Wrote: [ -> ]you should post your code (not original) using bbcode tags.
You may have introduced a typo.
This below is the solution which is not working. If uncomment 3 commented lines and replace three lines above with them it works.
class Solution(object):
def isValidSudoku(self, board):
rows = [[]] * 9
columns = [[]] * 9
box = [[]] * 9
# rows = [[] for _ in range(9)]
# columns = [[] for _ in range(9)]
# box = [[] for _ in range(9)]
for i in range(9):
for j in range(9):
num = board[i][j]
if num != ".":
box_index = (i // 3) * 3 + (j // 3)
if num in rows[i] or num in columns[j] or num in box[box_index]:
return False
rows[i].append(num)
columns[j].append(num)
box[box_index].append(num)
return True
It looks like a problem similar to
this one
a = [[]] * 9
b = [[] for _ in range(9)]
a == b
True
Pedroski55 this do not show the problem here.
a is the wrong solution,even if is it equals to
b when test.
Test with
is.
>>> a is b
False
So
a and
b will behave different,as
a every sublist point to same memory object.
>>> a[0].append(9)
>>> a
[[9], [9], [9], [9], [9], [9], [9], [9], [9]]
>>>
>>> b[0].append(9)
>>> b
[[9], [], [], [], [], [], [], [], []]
Ataman for the sudok solution use this way to create the lists
[[] for _ in range(9)]
Now each sublist will be a separate object in memory.
class Solution(object):
def valid_sudoku(self, board):
rows = [[] for _ in range(9)]
columns = [[] for _ in range(9)]
box = [[] for _ in range(9)]
for i in range(9):
for j in range(9):
num = board[i][j]
if num != ".":
box_index = (i // 3) * 3 + (j // 3)
if num in rows[i] or num in columns[j] or num in box[box_index]:
return False
rows[i].append(num)
columns[j].append(num)
box[box_index].append(num)
return True
snippsat, Gribouillis, thank you guys for the detailed explanation!