[Python-checkins] Add polynomial_from_roots() to the itertools recipes (GH-96102) (#96155)
rhettinger
webhook-mailer at python.org
Sun Aug 21 10:47:54 EDT 2022
https://github.com/python/cpython/commit/21c46f359b8c0ef3c63b01afb81053c591434b60
commit: 21c46f359b8c0ef3c63b01afb81053c591434b60
branch: 3.11
author: Miss Islington (bot) <31488909+miss-islington at users.noreply.github.com>
committer: rhettinger <rhettinger at users.noreply.github.com>
date: 2022-08-21T09:47:45-05:00
summary:
Add polynomial_from_roots() to the itertools recipes (GH-96102) (#96155)
files:
M Doc/library/itertools.rst
diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst
index 416c4eca5eb..da550e6fa25 100644
--- a/Doc/library/itertools.rst
+++ b/Doc/library/itertools.rst
@@ -800,6 +800,18 @@ which incur interpreter overhead.
window.append(x)
yield sum(map(operator.mul, kernel, window))
+ def polynomial_from_roots(roots):
+ """Compute a polynomial's coefficients from its roots.
+
+ (x - 5) (x + 4) (x - 3) expands to: x³ -4x² -17x + 60
+ """
+ # polynomial_from_roots([5, -4, 3]) --> [1, -4, -17, 60]
+ roots = list(map(operator.neg, roots))
+ return [
+ sum(map(math.prod, combinations(roots, k)))
+ for k in range(len(roots) + 1)
+ ]
+
def flatten(list_of_lists):
"Flatten one level of nesting"
return chain.from_iterable(list_of_lists)
@@ -1137,6 +1149,13 @@ which incur interpreter overhead.
>>> list(convolve(data, [1, -2, 1]))
[20, 0, -36, 24, -20, 20, -20, -4, 16]
+ >>> polynomial_from_roots([5, -4, 3])
+ [1, -4, -17, 60]
+ >>> factored = lambda x: (x - 5) * (x + 4) * (x - 3)
+ >>> expanded = lambda x: x**3 -4*x**2 -17*x + 60
+ >>> all(factored(x) == expanded(x) for x in range(-10, 11))
+ True
+
>>> list(flatten([('a', 'b'), (), ('c', 'd', 'e'), ('f',), ('g', 'h', 'i')]))
['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i']
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