当N = 1时，为什么我得到一个空列表而不是[0]

Question

I am working on list comprehension

[0 if N<2 else val*2 for val in range(0,N//2)]

when N<2 I expect out put to be [0] but I am getting []

Answer 1

If N = 1 , then N//2 = 0 . So your for loop is not executing, since it would be a range from 0 to 0. You can do [val * 2 for val in range(0, N // 2)] if N >= 2 else [0]

Answer 2

This is how your expression is read from the computer's perspective.

[(0 if N < 2 else val * 2) for val in range(0, N // 2)]

So the for bit happens first. And if N is less than 2 , then range(0, N // 2) is empty, so the 0 if N < 2 else val * 2 bit never actually executes.

In your case, it would make more sense to simply have two different cases.

if N < 2:
  return [0]
else:
  return [val * 2 for val in range(0, N // 2)]

Or, if you want it in one line,

[0] if N < 2 else [val * 2 for val in range(0, N // 2)]

Answer 3

1//2 is 0, so you have range(0, 0) which is an empty list, hence the map part of the list comprehension isn't executed at all.

If you want to get [0] for N<2 you probably want to do it like this:

[val*2 for val in range(0,N//2)] if N<2 else [0]

Answer 4

it works for me. try the following:

N = 3
a = [0 if N<2 else val*2 for val in range(0,N//2)]
#output: [0]

Answer 5

Or do list indexing:

print([[val * 2 for val in range(0, N // 2)],[0]][N<2])

Or dict.get :

print({0:[val * 2 for val in range(0, N // 2)],1:[0]}.get(N<2))

Both reproduce the expected result.