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Creating 2D array without Numpy - landlord1984 - Jan-27-2017
I want to create a 2D array and assign one particular element.
The second way below works.
But the first way doesn't.
I am curious to know why the first way does not work.
Is there any way to create a zero 2D array without numpy and without loop?
The first way is:
n=10 Grid=[[0]*n]*n Grid[1][1]=1
Quote:[0, 1, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 1, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 1, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 1, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 1, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 1, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 1, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 1, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 1, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 1, 0, 0, 0, 0, 0, 0, 0, 0]
The second way is:
n=10 Grid = [0] * n for i in range(n): Grid[i] = [0] * n Grid[1][1]=1
Quote:[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 1, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
[0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
RE: Creating 2D array without Numpy - ichabod801 - Jan-27-2017
The first way doesn't work because [[0] * n] creates a mutable list of zeros once. Then when the second *n copies the list, it copies references to first list, not the list itself. So you have a list of references, not a list of lists.
The second way a new [0] * n is created each time through the loop. That way there is no copying being done, so you end up with an actual list of lists. I often do the second way with a list comprehension:
[[0] * n for row in n]
RE: Creating 2D array without Numpy - landlord1984 - Jan-27-2017
(Jan-27-2017, 09:43 AM)ichabod801 Wrote: The first way doesn't work because [[0] * n] creates a mutable list of zeros once. Then when the second *n copies the list, it copies references to first list, not the list itself. So you have a list of references, not a list of lists. The second way a new [0] * n is created each time through the loop. That way there is no copying being done, so you end up with an actual list of lists. I often do the second way with a list comprehension:[[0] * n for row in n]
So there is no way to do this by one line code unless to use numpy?
L
RE: Creating 2D array without Numpy - ichabod801 - Jan-28-2017
(Jan-27-2017, 05:29 PM)landlord1984 Wrote: So there is no way to do this by one line code unless to use numpy?
The list comprehension I showed you does it in one line of code.