How to return a list of numbers of the power of 2?

Question

def problem(n):
myList = []
for j in range(0, n):
    number = 2 ** j
    myList.append(number)
return myList

I want this code to return the powers of 2 based upon the first n powers of 2. So for example, if I enter in 4, I want it to return [2,4,6,8,16]. Right now, the code returns [1,2,4,8] if I enter in 4. I think it's my range that's messing up the code.

Answer 1

just use range(1,n+1) and it should all work out. range is a little confusing for some because it does not include the endpoint. So, range(3) returns the list [0,1,2] and range(1,3) returns [1,2].

---

As a side note, simple loops of the form:

out = []
for x in ...:
   out.append(...)

should typically be replaced by list comprehensions:

out = [ 2**j for j in range(1,n+1) ]

Answer 2

Yet another simple and efficient solution (every step in O(1)) without the power operator:

def problem(n):
    return [1 << i for i in range(n)]

The 1 << i operations is a bitwise operation which translates to 2 ^ i for i integer positive.

https://en.wikipedia.org/wiki/Arithmetic_shift