Writing list comprehensions dependent on previous values?-CodePudding

I've seen this question before, but it only deals with recursions that are linear in nature. I'm looking for something more general.

Suppose I have the following code

n = 10
num_bits = [0]
for i in range(n):
    nums_bits.append(num_bits[i>>1] i%2)

This code will compute num_bits, a 11 element array of value with num_bits[i] representing the number of bits it takes to represent i.

Is it possible to write this as a list comprehension? Something like this doesn't work

num_bits = [0]*11
num_bits = [num_bits[i>>1] i%2 for i in range(11)]

since the comprehension doesn't update the value of num_bits in the middle of evaluation. Is there a canonical way to do something like this, besides a for loop?

P.S. I'm aware there are other ways to solve this problem: I'm just using it as a vehicle to understand Python's features better.

Edit: To summarize, I'd like to know what the proper way of generating lists of values that are dependent on previous values is. For a simpler example, consider the Fibonacci Numbers

fibonacci = [0,1]
for i in range(10):
    fibonacci.append(fibonacci[-1] fibonacci[-2])

Is there a way to generate these numbers in a comprehension? If not, what tools are there for this other than for loops (or are for/while loops my only option)?

CodePudding user response：

Given it is not a piece of code I'd recommend, for the reasons discussed in the comments above and in the other answer, this comprehension should be faster than the for loop:

fibonacci = [0,1]
deque((fibonacci.append(fibonacci[-1] fibonacci[-2]) for _ in range(10)), maxlen=0)

as it fills the list consuming the generator and discarding the result (an empty queue, it's the fastest recommended way to consume an iterator)

It produces:

>>> fibonacci
[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

CodePudding user response：

No.

There is no nice way to do this with a list comprehension, and that is not what they’re for. The purpose of list comprehensions is to offer a more readable alternative to maps and filters, and this isn’t that, so it’s not possible to do this in a sensible way.