Generating a List with A sublists and B elements in each sublist

Question

for some reason

a=4
b=3

sublists=[]
p=0
while p<b:
    sublists.append(0)
    p+=1
list=[]
p=0
while p<a:
    list.append(sublists)
    p+=1

is not the same as:

list=[[0,0,0], [0,0,0], [0,0,0], [0,0,0]]

although they print the same, but they don't work the same way... Can someone clarify if there is a difference between the lists, and which is it?

Answer 1

In the first example, you're appending the same sublist four times to list (which is an unfortunate name because it also shadows the built-in name, but that's not pertinent to the problem here). So if you modify one of the sublists, you modify all of them.

In the second example, you create a list with four identical but distinct sublists.

At any rate, that code looks more like a C program than a Python script - you should read a Python tutorial to start learning the language as it's meant to be used.

The correct way to create an a * b -sized "matrix" with entries predefined to 0 is:

>>> a = 4
>>> b = 3
>>> matrix = [[0] * b for _ in range(a)]
>>> matrix
[[0, 0, 0], [0, 0, 0], [0, 0, 0], [0, 0, 0]]

Answer 2

they are working same if youo simple print the id of each sublists

print map(id, list) #will print the id of all sublist
sublists[1] = 2
print sublists, list, 

output:-

[0, 0, 0]
[3053888556L, 3053888556L, 3053888556L, 3053888556L]
[0, 2, 0] [[0, 2, 0], [0, 2, 0], [0, 2, 0], [0, 2, 0]]
>>>