bpo-37128: Add math.perm().#13731
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serhiy-storchaka
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@serhiy-storchaka Please could you open a new b.p.o. issue for |
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Doc/library/math.rst
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| .. function:: perm(n, k) | ||
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| Return the number of ways to choose *k* items from *n* items without repetition. |
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The "with order" part should be mentioned. Perhaps s/the number/the distinct number/.
FWIW, here is the wording from the itertools docs:
permutations() | p[, r] | r-length tuples, all possible orderings, no repeated elements
combinations() | p, r | r-length tuples, in sorted order, no repeated elements
Modules/mathmodule.c
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| static PyObject * | ||
| math_comb_impl(PyObject *module, PyObject *n, PyObject *k) | ||
| /*[clinic end generated code: output=bd2cec8d854f3493 input=2f336ac9ec8242f9]*/ | ||
| perm_comb(PyObject *n, PyObject *k, int comb) |
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ISTM that combining the code for comb() and perm() doesn't save us much. I would rather they be decoupled at outset so as to not preclude possible optimizations (there are a number of ways to compute combinations that do early cancellation to keep the intermediate products from exploding in size).
mdickinson
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LGTM; one request for a docstring tweak.
Doc/library/math.rst
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| .. function:: perm(n, k) | ||
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| Return the distinct number of ways to choose *k* items from *n* items |
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"distinct number of ways" doesn't quite make sense. "number of distinct ways"? Or just lose the "distinct" word altogether.
| if (n == NULL) { | ||
| return NULL; | ||
| } | ||
| if (!PyLong_CheckExact(n)) { |
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I'm looking forward to the day when we can remove this kind of block as unnecessary (perhaps in 3.9). I thought I'd persuaded myself that everything does eventually get converted to plain ints, but now that I look again it's not clear, and I agree that making this check up front is a good idea.
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https://bugs.python.org/issue37128