Need help with coding a function in Python

[MRAB]

On 2016-10-31 22:09, devers.meetthebadger.jason at gmail.com wrote:
> http://imgur.com/a/rfGhK#iVLQKSW
>
> How do I code a function that returns a list of the first n elements of the sequence defined in the link? I have no idea!!!!!
>
> So far this is my best shot at it (the problem with it is that the n that i'm subtracting or adding in the if/else part does not represent the element's position, but just the n that I am plugging into the function):
>
> def reca(n):
>     rlist=[0]
>     while len(rlist) < n:
>         if (rlist[-1]-n) > 0 and (rlist[-1]-n) not in rlist:
>             rlist.append(rlist[-1]-n)
>
>         else:
>             rlist.append(rlist[-1]+n)
>
>     return(rlist)
>
When you're calculating a[n], what's n?

Well:

When you're calculating a[1], you already have a[0], or 1 element.

When you're calculating a[2], you already have a[0] and a[1], or 2 elements.

So, in general, when you're calculating a[n], you already have a[0] ... 
a[n - 1], or n elements.

The value of n is the length of the list so far.