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Steven D'Aprano wrote: > On Wed, 15 Aug 2018 05:34:06 -0700, tomusatov wrote: > >> I am not terribly familiar with Python, but am currently authoring an >> integer sequence for www.oeis.org and was wondering if anyone in the >> community could help me with authoring a Python program that outputs, >> "Composite numbers that are one less than a composite number." > > > Do you have a function to test for primality? For now, I'll assume you do. > > > def is_prime(n): > # Returns True if n is prime, False otherwise. > # implementation left as an exercise > > > # 0 (and 1) are neither prime nor composite; skip them. > # 2 and 3 are prime; start at the first composite, 4 > i = 4 > for j in range(5, 1001): > if not is_prime(j): > print(i) > i = j I think that are "any numbers that are one less than a composite number" while the OP wants only composite numbers that are one less than a composite number". In code from itertools import count, islice def cntaoltacn(): prev = None for i in count(2): if is_prime(i): prev = None else: if prev is not None: yield prev prev = i print(list(islice(cntaoltacn(), 20))) # [8, 9, 14, 15, 20, 21, 24, 25, 26, 27, 32, 33, 34, 35, 38, 39, 44, 45, 48, 49] > > > The above will stop at 999. To go forever, use this instead: > > > > from itertools import count > i = 4 > for j in count(5): > if not is_prime(j): > print(i) > i = j > > > > Alternatively, if you have a function which efficiently returns primes > one at a time, you can do this: > > > n = 4 # start at the first composite > for p in primes(5): # primes starting at 5 > print(list(range(n, p-1)) > n = p + 1 > > > > This ought to print out lists of composites, starting with: > > [] > [] > [8, 9] > [] > [14, 15] > > > etc. Take care though: I have not tested this code. > > >

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