如何在Python 3中为每个for循环迭代将step变量加倍？

Question

I'm new to Python, and I'm playing around with recursive functions just for practice.

I made the following algorithm which takes a number as x and halves it until it is equal to 1. n is the number of times x has been halved.

def binary(x, n = 0):
    print(n,":",x)
    x = x // 2
    n += 1
    if x > 0:
       binary(x, n)
       return x
    return x

I'm trying to make a loop that will call binary() with multiple values for x. I want my step to be doubled each iteration. I have it working with a while loop like the one below.

i = 1
while i < 1000000000:
    print("when x is", i, ":")
    binary(i)
    i += i

For some reason though, I can't seem to achieve the same thing with a For loop. Here's what I have now.

for i in range(1,1000):
    print("when x is", i, ":")
    binary(i)
    i += i

In the code above, i += i does not seem to effect the i in my header. I know that range() takes a third parameter called step, but I've tried this:

for i in range(1,1000, i += i):
    # statements

This gives me a name error, and says "i is not defined". Most of my experience with programming is in JavaScript and C#. In both of those languages I wouldn't of had any trouble doing this.

How would I get this to work in a For loop using Python 3?

Answer 1

The third parameter of range is indeed step. But you should use it with a computed value like:

for i in range(1,1000,2):
    #statements

The reason why your i += i didn't worked is because, under the hood, the for-loop is executing something similar to i = next(...) at the end of an iteration, overiding your last increment.

Edit

You can achieve the desired effect using a generator, but it kinda kills the "trying to avoid while-loops" thing:

def myrange(start, stop):
    i = start
    while i < stop:
        yield i
        i += i

for i in myrange(1, 1000):
    print(i)

Anyway, while-loops are perfectly valid constructs and I'd personnally go with one in this case. Do not forget that the for-loop has a rather different semantic in python than in both languages you're used to. So trying to use a for-loop because you are able to do so with javascript seems like a bad idea if all what you need is a while-loop.

Answer 2

range can step by a fixed amount, but not a variable amount. Use a while-loop to increment i by i :

i += i

---

You could replace the while-loop with an iterator, such as:

import itertools as IT
for i in (2**i for i in IT.count()):
    if i >= 1000000000: break
    print("when x is", i, ":")
    binary(i)

but I don't think this has any advantage over a simple while-loop .

Answer 3

If all you're doing is doubling i , then why not just raise it to the power?

for p in range(int(1000000000**0.5)):
    print(binary(2**p)