问题
I want to loop a list over a cycle. ex: I have three elements in the array L = [1,2,3] I want to get the output as
L[0],L[1]
L[1],L[2]
L[2],L[0]
Is there a easy way get the slightly different output
L[0],L[1]
L[1],L[2]
L[0],L[2]
回答1:
Using modulus operator
>>> a = [1,2,3]
>>> for x in range(10):
print a[x % len(a)]
Using itertools.cycle
>>> iterator = cycle(a)
>>> for _ in range(10):
print next(iterator)
As for your output, you could just do this.
>>> for x in range(10):
print '{0}, {1}'.format(a[x % len(a)], a[(x+1) % len(a)])
>>> 1, 2
>>> 2, 3
>>> 3, 1
... etc etc
回答2:
You can just use increasing index, and use modulo (remainder of division)
myList = [1,2,3]
for i in xrange(len(myList)):
myTuple = (myList[i],myList[(i+1)%len(myList)])
print myTuple
will produce:
(1,2)
(2,3)
(3,1)
回答3:
You could try something like
L = [1,2,3]
length = len(L)
for i in xrange(length):
print L[i % length], L[(i+1) % length]
Output
1 2
2 3
3 1
This way, you can do something like xrange(10) and still have it cycle:
1 2
2 3
3 1
1 2
2 3
3 1
1 2
2 3
3 1
1 2
回答4:
l = [0,1,2,3]
for i in xrange(0, len(l)):
print l[i], l[(i+1) % len(l)]
0 1
1 2
2 3
3 0
回答5:
There is a recipe in the itertools documentation that you can use:
import itertools
def pairwise(iterable):
a, b = itertools.tee(iterable)
next(b, None)
return itertools.izip(a, b)
Call this with itertools.cycle(L) and you're good to go:
L = [1, 2, 3]
for pair in pairwise(itertools.cycle(L)):
print pair
# (1, 2)
# (2, 3)
# (3, 1)
# (1, 2)
# ...
回答6:
Did you mean combinations? http://en.wikipedia.org/wiki/Combination
from itertools import combinations
comb = []
for c in combinations([1, 2, 3], 2):
print comb.append(c)
For sorting you can use then
sorted(comb, key=lambda x: x[1])
Output:
[(1, 2), (1, 3), (2, 3)]
来源：https://stackoverflow.com/questions/18648824/construct-a-circular-loop-in-python