How to create nested lists in python?
I know you can create easily nested lists in python like this:
[[1,2],[3,4]]
But how to create a 3x3x3 matrix of zeroes?
[[[0] * 3 for i in range(0, 3)] for j in range (0,3)]
or
[[[0]*3]*3开发者_JS百科]*3
Doesn't seem right. There is no way to create it just passing a list of dimensions to a method? Ex:
CreateArray([3,3,3])
In case a matrix is actually what you are looking for, consider the numpy package.
http://docs.scipy.org/doc/numpy/reference/generated/numpy.zeros.html#numpy.zeros
This will give you a 3x3x3 array of zeros:
numpy.zeros((3,3,3))
You also benefit from the convenience features of a module built for scientific computing.
List comprehensions are just syntactic sugar for adding expressiveness to list initialization; in your case, I would not use them at all, and go for a simple nested loop.
On a completely different level: do you think the n-dimensional array of NumPy could be a better approach?
Although you can use lists to implement multi-dimensional matrices, I think they are not the best tool for that goal.
NumPy addresses this problem
Link
>>> a = array( [2,3,4] )
>>> a
array([2, 3, 4])
>>> type(a)
<type 'numpy.ndarray'>
But if you want to use the Python native lists as a matrix the following helper methods can become handy:
import copy
def Create(dimensions, item):
for dimension in dimensions:
item = map(copy.copy, [item] * dimension)
return item
def Get(matrix, position):
for index in position:
matrix = matrix[index]
return matrix
def Set(matrix, position, value):
for index in position[:-1]:
matrix = matrix[index]
matrix[position[-1]] = value
Or use the nest function defined here, combined with repeat(0) from the itertools module:
nest(itertools.repeat(0),[3,3,3])
Just nest the multiplication syntax:
[[[0] * 3] * 3] * 3
It's therefore simple to express this operation using folds
def zeros(dimensions):
return reduce(lambda x, d: [x] * d, [0] + dimensions)
Or if you want to avoid reference replication, so altering one item won't affect any other you should instead use copies:
import copy
def zeros(dimensions):
item = 0
for dimension in dimensions:
item = map(copy.copy, [item] * dimension)
return item
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