CODEWITHSUNDEEP
pythonarraysboolean
user6461129
user6461129

Reputation:

Comprehension to instantiate a boolean 2D array?

I'm new to Python, thus the question,

So I'm trying to instantiate a 2D array/list with all false of the size str +1 rows and pattern +1 cols.

Here's my code,

memo = []

for i in range(0, len(str) + 1):
    memo[i] = [[False] for j in range(len(pattern) + 1)]

Now I've two questions,

Is there a more pythonic way of doing this in 1 line? Also if I just create the list and dont initialize it with anything, what is there in each grid(java equivalent of non-initialization meaning initalized with false)?

Upvotes: 3

Views: 6837

Answers (5)

martineau
martineau

Reputation: 123393

This is the shortest list comprehension I can think of:

memo = [[False] * (len(pattern)+1) for i in range(len(str)+1)]

If you create the list and don't initialize it, it will be empty.

One alternative to work around having to do that would be to represent the 2D array as a defaultdict of defaultdicts. This would essentially make it be a "sparse" array in the sense that it would only have entries where they were assigned values or whose value was referenced—also known as "lazy-initialization". Even though data in it was stored this way, its contents could be indexed in a manner similar to that of a list of lists.
(i.e. using memo[i][j])

Here's what I mean:

from collections import defaultdict

memo = defaultdict(lambda: defaultdict(bool))
memo[1][1] = True
print(memo[2][4])  # -> False

This would result in a dictionary containing only the values assigned or referenced so far:

{
    1: {
        1: True
    }, 
    2: {
        4: False
    }
}

Upvotes: 0

kezzos
kezzos

Reputation: 3221

A short hand way would be to write:

ncols = 3  # len(string1) + 1
nrows = 4  # len(pattern1) + 1
memo = nrows * [ncols*[False]] 

>>> [[False, False, False], [False, False, False], [False, False, False], [False, False, False]]

In this case the second part [ncols*[False]] makes one of the inner lists

Upvotes: 1

Keith Hughitt
Keith Hughitt

Reputation: 4960

Depending on how big the 2d list is and what you plan to do with it, you might also consider using a numpy ndarray to store the data:

import numpy as np
memo = np.full((len(str) + 1, len(pattern) + 1), False, dtype=bool)

# example
> np.full((3,2), False, dtype=bool)
> 
array([[False, False],
       [False, False],
       [False, False]], dtype=bool)

Upvotes: 1

Netwave
Netwave

Reputation: 42678

Using itertools.repeat aproach, fully loaded in a generator:

memo = itertools.repeat(itertools.repeat(False, xrange(len(pattern) + 1)), xrange(len(str) + 1))

Upvotes: 0

DeepSpace
DeepSpace

Reputation: 81594

A one-liner would be
memo = [[False for j in range(len(pattern) + 1)] for i in range(len(str) + 1)].

As a side-note, keep in mind that using str as a variable name should be avoided as it shadows the built-in str type.

if I just create the list and dont initialize it with anything, what is there in each grid(java equivalent of non-initialization meaning initalized with false)?

Nothing, it is simply empty.

Python lists store references to other objects. If you don't insert any reference to the list, the list doesn't contain anything.

Upvotes: 4

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