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[3.12] Misc improvements to the itertools docs (GH-104916) #104917

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May 25, 2023
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[3.12] Misc improvements to the itertools docs (GH-104916) #104917

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31 changes: 24 additions & 7 deletions Doc/library/itertools.rst
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Original file line number Diff line number Diff line change
Expand Up @@ -147,10 +147,10 @@ loops that truncate the stream.
>>> list(accumulate(data, max)) # running maximum
[3, 4, 6, 6, 6, 9, 9, 9, 9, 9]

# Amortize a 5% loan of 1000 with 4 annual payments of 90
>>> cashflows = [1000, -90, -90, -90, -90]
>>> list(accumulate(cashflows, lambda bal, pmt: bal*1.05 + pmt))
[1000, 960.0, 918.0, 873.9000000000001, 827.5950000000001]
# Amortize a 5% loan of 1000 with 10 annual payments of 90
>>> account_update = lambda bal, pmt: round(bal * 1.05) + pmt
>>> list(accumulate(repeat(-90, 10), account_update, initial=1_000))
[1000, 960, 918, 874, 828, 779, 728, 674, 618, 559, 497]

See :func:`functools.reduce` for a similar function that returns only the
final accumulated value.
Expand Down Expand Up @@ -951,7 +951,10 @@ which incur interpreter overhead.
nexts = cycle(islice(nexts, num_active))

def partition(pred, iterable):
"Use a predicate to partition entries into false entries and true entries"
"""Partition entries into false entries and true entries.

If *pred* is slow, consider wrapping it with functools.lru_cache().
"""
# partition(is_odd, range(10)) --> 0 2 4 6 8 and 1 3 5 7 9
t1, t2 = tee(iterable)
return filterfalse(pred, t1), filter(pred, t2)
Expand Down Expand Up @@ -1031,7 +1034,7 @@ The following recipes have a more mathematical flavor:
return chain.from_iterable(combinations(s, r) for r in range(len(s)+1))

def sieve(n):
"Primes less than n"
"Primes less than n."
# sieve(30) --> 2 3 5 7 11 13 17 19 23 29
data = bytearray((0, 1)) * (n // 2)
data[:3] = 0, 0, 0
Expand Down Expand Up @@ -1068,7 +1071,7 @@ The following recipes have a more mathematical flavor:

def matmul(m1, m2):
"Multiply two matrices."
# matmul([(7, 5), (3, 5)], [[2, 5], [7, 9]]) --> (49, 80), (41, 60)
# matmul([(7, 5), (3, 5)], [(2, 5), (7, 9)]) --> (49, 80), (41, 60)
n = len(m2[0])
return batched(starmap(math.sumprod, product(m1, transpose(m2))), n)

Expand Down Expand Up @@ -1109,6 +1112,17 @@ The following recipes have a more mathematical flavor:
powers = map(pow, repeat(x), reversed(range(n)))
return math.sumprod(coefficients, powers)

def polynomial_derivative(coefficients):
"""Compute the first derivative of a polynomial.

f(x) = x³ -4x² -17x + 60
f'(x) = 3x² -8x -17
"""
# polynomial_derivative([1, -4, -17, 60]) -> [3, -8, -17]
n = len(coefficients)
powers = reversed(range(1, n))
return list(map(operator.mul, coefficients, powers))

def nth_combination(iterable, r, index):
"Equivalent to list(combinations(iterable, r))[index]"
pool = tuple(iterable)
Expand Down Expand Up @@ -1297,6 +1311,9 @@ The following recipes have a more mathematical flavor:
>>> all(factored(x) == expanded(x) for x in range(-10, 11))
True

>>> polynomial_derivative([1, -4, -17, 60])
[3, -8, -17]

>>> list(iter_index('AABCADEAF', 'A'))
[0, 1, 4, 7]
>>> list(iter_index('AABCADEAF', 'B'))
Expand Down