When I do an exercise, I want to generate a m*n list , and all the elements are 0. I find an interesting question:
# first way
a = [[0]*n]*m
a[0][0]=1
# then I find a[1][0]=1,a[2][0]=1....
# second way
b = [[0 for _ in range(n)] for _ in range(m)]
b[0][0]=1
# in this way, I get the normal effect
I am confused about the first way, what is the * means for list?
The * operator essentially takes the old list, and recreates it n times. The new items are the same (the same references) as the old ones, but the list if n times as long.
The original list is not modified, where simply appending new copies to the end would modify it.
Eg:
[1,2,3] * 3 = [1,2,3,1,2,3,1,2,3]
[[1],[2]] * 3 = [[1],[2],[1],[2],[1],[2]]
>>> myfunc = lambda: None
>>> a = [myfunc]
>>> a
[<function <lambda> at 0x03193468>]
>>> b = a * 2
>>> b
[<function <lambda> at 0x03193468>, <function <lambda> at 0x03193468>]
>>> a
[<function <lambda> at 0x03193468>]
>>> b is a
False
>>> a is b
False
Achieving the 'same' thing with append:
>>> X = [1,2]
>>> a = X.copy()
>>> n = 2
>>> b = a
>>> for i in range(n-1):
for j in X:
b.append(j)
>>> b
[1,2,1,2]
>>> a
[1,2,1,2]
>>> X
[1,2]
>>> a is b
True
>>> b is a
True
