CODEWITHSUNDEEP
pythonlist
Daniel Tan
Daniel Tan

Reputation: 181

How to traverse a list until it reaches the last element, then traverse it backwards until it reaches the 1st element, then repeat?

How to traverse a list until it reaches the last element, then traverse it backwards until it reaches the 1st element, then repeat?

In other wards, how can iterate through a list specifically in a pattern that bounces off the 1st and last element?

e.g For a list of length=3
time=1, index=0
time=2, index=1
time=3, index=2
time=4, index=1
time=5, index=0
time=6, index=1
time=7, index=2

Upvotes: 0

Views: 136

Answers (3)

Daniel Tan
Daniel Tan

Reputation: 181

I also found that the index can also be calculated using

index = (time-1)%(2*length-2)

Upvotes: 0

hilberts_drinking_problem
hilberts_drinking_problem

Reputation: 11602

One way using itertools.cycle:

from itertools import cycle

lst = [0, 1, 2]
res1 = cycle(lst + lst[-2:0:-1])


In [143]: [next(q.res1) for _ in range(10)]
Out[143]: [0, 1, 2, 1, 0, 1, 2, 1, 0, 1]

Thanks to @Chris_Rands for pointing out that the list comprehension can be replaced nicely with list(itertools.islice(res1, 10)).

Upvotes: 2

Moberg
Moberg

Reputation: 5503

If you actually start with a list you can use slice and reversed to bounce it once like this:

def bounce(a_list):
    yield from a_list
    yield from reversed(a_list[1:-1])

for x in bounce([1, 2, 3, 4]):
    print(x)

Repeat the bouncyness for added fun.

for x in itertools.cycle(bounce([0, 1, 2])):
    print(x)

Upvotes: 3

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