flebber wrote: > In repsonse to this question: Write a program that prints the first 100 > members of the sequence 2, -3, 4, -5, 6, -7, 8. > > This is my solution it works but ugly. > > series = range(2,100) > # answer = [(x,(y* -1)) for x, y in series[::2]] > # print(answer) > answer = [] > for item in series: > if item % 2 != 0: > answer.append(item * -1) > else: > answer.append(item) > > print(answer)
Hm, that's the only first 98 members. > > I know I should be better off doing this with map but cannot get it to > work. I understand also that map returns a generator so this solution > should only working in python2(correct me please if I am wrong). > > In [6]: map? > Type: builtin_function_or_method > String Form:<built-in function map> > Namespace: Python builtin > Docstring: > map(function, sequence[, sequence, ...]) -> list > > Just getting something wrong > list(map((lambda x: x * -1 if (x%2 != 0)), series)) Here's another way to look at the problem -- start with an infinite series and then slice it: >>> from itertools import * >>> list(islice(count(2), 10)) [2, 3, 4, 5, 6, 7, 8, 9, 10, 11] Factor out the slicing to focus on the important stuff: >>> def first_ten(items): ... return list(islice(items, 10)) ... >>> first_ten(count(2)) Now add the alternating sign: >>> first_ten(cycle([1, -1])) [1, -1, 1, -1, 1, -1, 1, -1, 1, -1] Combine the two: >>> from operator import mul >>> first_ten(imap(mul, cycle([1, -1]), count(2))) [2, -3, 4, -5, 6, -7, 8, -9, 10, -11] It is important that you use itertools.imap() in Python 2 because the map() builtin would try to build the infinite list... Finally put the slicing back into the expression if you like: >>> list(islice(imap(mul, cycle([1, -1]), count(2)), 10)) [2, -3, 4, -5, 6, -7, 8, -9, 10, -11] -- https://mail.python.org/mailman/listinfo/python-list
