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-rw-r--r--Doc/library/itertools.rst40
1 files changed, 30 insertions, 10 deletions
diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst
index 098972d..9da51aa 100644
--- a/Doc/library/itertools.rst
+++ b/Doc/library/itertools.rst
@@ -102,26 +102,24 @@ loops that truncate the stream.
Each result tuple is ordered to match the input order. So, every
combination is a subsequence of the input *iterable*.
- Example: ``combinations(range(4), 3) --> (0,1,2), (0,1,3), (0,2,3), (1,2,3)``
-
Equivalent to::
def combinations(iterable, r):
+ 'combinations(range(4), 3) --> (0,1,2) (0,1,3) (0,2,3) (1,2,3)'
pool = tuple(iterable)
n = len(pool)
- assert 0 <= r <= n
- vec = range(r)
- yield tuple(pool[i] for i in vec)
+ indices = range(r)
+ yield tuple(pool[i] for i in indices)
while 1:
for i in reversed(range(r)):
- if vec[i] != i + n - r:
+ if indices[i] != i + n - r:
break
else:
return
- vec[i] += 1
+ indices[i] += 1
for j in range(i+1, r):
- vec[j] = vec[j-1] + 1
- yield tuple(pool[i] for i in vec)
+ indices[j] = indices[j-1] + 1
+ yield tuple(pool[i] for i in indices)
.. versionadded:: 2.6
@@ -356,7 +354,29 @@ loops that truncate the stream.
value. So if the input elements are unique, there will be no repeat
values in each permutation.
- Example: ``permutations(range(3),2) --> (1,2) (1,3) (2,1) (2,3) (3,1) (3,2)``
+ Equivalent to::
+
+ def permutations(iterable, r=None):
+ 'permutations(range(3), 2) --> (0,1) (0,2) (1,0) (1,2) (2,0) (2,1)'
+ pool = tuple(iterable)
+ n = len(pool)
+ r = n if r is None else r
+ indices = range(n)
+ cycles = range(n-r+1, n+1)[::-1]
+ yield tuple(pool[i] for i in indices[:r])
+ while n:
+ for i in reversed(range(r)):
+ cycles[i] -= 1
+ if cycles[i] == 0:
+ indices[i:] = indices[i+1:] + indices[i:i+1]
+ cycles[i] = n - i
+ else:
+ j = cycles[i]
+ indices[i], indices[-j] = indices[-j], indices[i]
+ yield tuple(pool[i] for i in indices[:r])
+ break
+ else:
+ return
.. versionadded:: 2.6