Python：如何在数组中以“ n”为单位选择性地切换布尔元素？

Question

I have a string of 50 Christmas lights in a single line, and when they are first plugged in, all of the lights are off. The lights are controlled by a single button, where every time the button is pressed, some of the lights flip their state (ie if they are off, they change to on; if they are on, they change to off). The selection of lights which change depends on how many times the button has been pressed so far:

• On the 1st press, the 1st, 2nd, 3rd, 4th, …, 50th lights change state
• On the 2nd press, the 2nd, 4th, 6th, …, lights change state;
• On the 3rd press, the 3rd, 6th, 9th, …, lights change state;
• etc.
• On the nth press, the lights corresponding to all multiples of n change state.

How do I find out what lights are on after any given number of "n" presses?

I tried defining a boolean array to represent the lights. I wrote the following function:

lightsarray = [False] * 50 ### All lights are off at start ###

def pressbutton(n):
    count = 0

    for count in range(0, n):   
        lightsarray[::int(n)] = [not y for y in lightsarray[::int(n)]]
        count = count + 1
        return lightsarray

Currently, the output for n = 1 is True, True, True, True, True, True...

The output for n = 2 is True, False, True, False, True, False...

So far so good. But for n = 3, I'm expecting True, False, False, False, True, True, True...

But I'm getting True, False, False, True, False, False...

I think I'm going wrong in assigning/getting the state of the array. How do I correct this? Or is there a better approach?

--EDIT-- Thanks for the responses everyone. I implemented the changes and I'm getting the output you've suggested, but I'm not sure whether this matches the expected state of the lights. Keeping in mind that False = OFF and True = ON, let's look at the first 5 lights. This is what is expected:

For 0 presses, everything is off:

False, False, False, False, False...

For 1 press, lights at every multiple of 1 are flipped:

True, True, True, True, True...

For 2 presses, the lights at every multiple of 2 are flipped:

True, False, True, False, True...

Note that the second and the fourth light were toggled, otherwise the rest remained unchanged.

For 3 presses, the lights at every multiple of 3 are flipped:

True, False, False, False, True...

However after making the changes, for instance, 2 presses gives False, True... not True, False... and 3 presses gives True, True, False, False, False... (there are two Trues in the beginning.)

How do I correct this? I hope that made sense.

Answer 1

Your main problem is that you are using count in multiple ways in the same function. In essence, for count in range uses the same variable as count=0 , and count = count + 1 is modifying the loop variable.

The other problem is that you are modifying the global lightsarray every time you run the function, so your output will depend on how you previously rand the function.

Finally, if your pressbutton function is supposed to be showing the result of pressing the button n times, as it appears to be doing, you should be changing different lights every loop iteration, instead of flipping every n th light every loop iteration.

You should just be doing

def pressbutton(n):
    lightsarray = [False] * 50 ### All lights are off at start ###
    for count in range(0, n):
        lightsarray[::count] = [not y for y in lightsarray[::count]]
    return lightsarray

Answer 2

If I understand the progression I would say that for each step k, we select first the kth element and then move to the 2kth element, 3k, 4k...

This can be achieved using xrange (I prefer xrange to range since it doesn't fillup memory) wich take a step argument.

here's an example of such progressions

for k in xrange(1, n + 1):
    print range(k, n, k)

That should do it, here's the output, looks similar to your example:

[1, 2, 3, 4, 5, 6, 7, 8, 9]
[2, 4, 6, 8]
[3, 6, 9]
[4, 8]
[5]
[6]
[7]
[8]
[9]
[]

Notince that you don't have to go to n + 1, you can stop at n :)

Answer 3

You switch the wrong lights; you are using n , not count , the loop counter, so you are always switching the wrong lights.

You need to start counting at 1, and count up to n (so range to `n + 1).

Your indentation is wrong; you return too early . Unindent the return statement to not be part of the loop.

I'd also make the lightsarray value part of the function itself, don't use a global here, as it'll be mutated each time you call the function, making it harder to test the function itself.

Last but not least, there is no need to increment count manually, that's what the for loop is already doing for you:

def pressbutton(n):
    lightsarray = [False] * 50 ### All lights are off at start ###
    for count in range(1, n + 1):   
        lightsarray[::count] = [not y for y in lightsarray[::count]]
    return lightsarray

Now this works as expected:

>>> pressbutton(1)
[True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True, True]
>>> pressbutton(2)
[False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True]
>>> pressbutton(3)
[True, True, False, False, False, True, True, True, False, False, False, True, True, True, False, False, False, True, True, True, False, False, False, True, True, True, False, False, False, True, True, True, False, False, False, True, True, True, False, False, False, True, True, True, False, False, False, True, True, True]