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| -rw-r--r-- | Doc/library/itertools.rst | 40 |
1 files changed, 30 insertions, 10 deletions
diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst index d940632..f26d3bb 100644 --- a/Doc/library/itertools.rst +++ b/Doc/library/itertools.rst @@ -104,26 +104,24 @@ 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 @@ -369,7 +367,29 @@ 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[:] = 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 |