CODEWITHSUNDEEP
python
neilli1992
neilli1992

Reputation: 15

How to cycle a variable in a given range in python

is there an easy way in python to cycle a variable in a given range? For example: Given a range(), I want a variable goes like this: 0 1 2 3 2 1 0 1 2 3... till some conditions are satisfied.

Upvotes: 0

Views: 2533

Answers (6)

Bakuriu
Bakuriu

Reputation: 101929

You want to cycle the sequence 0, 1, ..., n, n-1, ..., 1. You can easily build this sequence using chain

from itertools import chain, cycle

def make_base_sequence(n):
    base = range(n+1)                   # 0, ..., n
    rev_base = reversed(range(1, n))    # n-1, ..., 1
    return chain(base, rev_base)        # 0, ..., n, n-1, ..., 1

for x in cycle(make_base_sequence(5)):
    print(x)

Sample run:

In [2]: from itertools import chain, cycle
   ...: 
   ...: def make_base_sequence(n):
   ...:     base = range(n+1)
   ...:     rev_base = reversed(range(1, n))
   ...:     return chain(base, rev_base)
   ...: 
   ...: for i, x in enumerate(cycle(make_base_sequence(5))):
   ...:     print(x, end=' ')
   ...:     if i > 20:
   ...:         break
   ...:     
0 1 2 3 4 5 4 3 2 1 0 1 2 3 4 5 4 3 2 1 0 1

Upvotes: 1

Inbar Rose
Inbar Rose

Reputation: 43447

import itertools

def f(cycle_range, condition_func):
    sequence = range(cycle_range) + range(cycle_range)[-2:0:-1]
    cycle_generator = itertools.cycle(sequence)
    while not condition_func():
        yield next(cycle_generator)

def condition_func():
    """Checks some condition"""

Essentially, you just want to loop and constantly check the condition. And each time yeild the next item from the cycle. Now, admittedly there are better ways to check a condition than a function call, but this is just an example.

Upvotes: 0

Hackaholic
Hackaholic

Reputation: 19733

you need itertools.cycle:

demo:

>>> import itertools
>>> count = 0
>>> for x in itertools.cycle(range(3)): 
...     if count == 10:
...         break
...     print x,
...     count += 1
... 
0 1 2 0 1 2 0 1 2 0

Upvotes: 0

dragon2fly
dragon2fly

Reputation: 2419

import time

def cycle(range_):
    num = -1
    current = 0
    a=time.time()
    while 1:
        print current
        if current in (0, range_):
            num*=-1
        current += num
        if time.time() - a > 0.002:
            break

cycle(3)

output:

0 1 2 3 2 1 0 1 2 3 2 1 0 1 2 3 2 1 0 1 2 3 2 1 0 1 2 3 2

Upvotes: 0

agastalver
agastalver

Reputation: 1056

itertools.cycle is a good start. Else you can program it yourself:

cycle = [0,1,2,3,2,1]
i = 0
while some_condition:
    value = cycle[i]
    i = (i+1) % len(cycle)
    #do stuff

Upvotes: 0

John Zwinck
John Zwinck

Reputation: 249133

You want itertools.cycle(), see here:

https://docs.python.org/2/library/itertools.html#itertools.cycle

Upvotes: 0

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