[Tutor] Alternating patterns

[Kent Johnson]

kevin parks wrote:
> I have a set that i iterate over... but each time through it i would 
> like to alternate between the original set and a variation of the set 
> that has one of the members of the set altered (by + or - 1)
> 
> So if my original set is:
> 
> [0, 2, 4, 5, 7, 9, 11]
> 
> I would use that the first pass but on the second pass i might like  
> the third member (4,) to become 3, (-1) resulting in : [0, 2, 3, 5, 7, 
> 9, 11]
> 
> But then back again to the original  on the next pass (+1 back to 4,): 
> [0, 2, 4, 5, 7, 9, 11]
> 
> and then back: [0, 2, 3, 5, 7, 9, 11] again, etc.

> How can one make such alternating patterns?

itertools.cycle() will repeat a sequence indefinitely:
In [2]: from itertools import cycle

In [3]: i=cycle([1,2])

In [5]: for j in range(6):
    ...:     print i.next()
    ...:
    ...:
1
2
1
2
1
2

For non-repeating sequences I would look at writing a generator function 
for the sequences.

Kent