Python列表理解

Question

I am learning python3 list comprehensions. I understand how to format a list comprehension: [equation, for loop, if statement for filtering], but I cannot figure out how to condense three lines of code into a single equation for the 'equation' part.

I am taking a number and adding it to itself and then taking the result and adding it to itself and so on to create a sequence of numbers in the list.

I can accomplish this by declaring x = 1 and then looping the following:

y = x + x

x = y

Can anybody help me to turn this into a single-lined equation and if possible, resources that I might study to help me with this in the future?

Answer 1

Your algorithm is equivalent to multiplying by powers of 2:

x = 3
res = [x * 2**i for i in range(10)]

# [3, 6, 12, 24, 48, 96, 192, 384, 768, 1536]

To see why this is the case, note you are multiplying your starting number by 2 in each iteration of your for loop:

x = 3
res = [x]
for _ in range(9):
    y = x + x
    x = y
    res.append(y)

print(res)

# [3, 6, 12, 24, 48, 96, 192, 384, 768, 1536]

As @timgeb mentions, you can't refer to elements of your list comprehension as you go along, as they are not available until the comprehension is complete.