Help with map python 2
Peter Otten
__peter__ at web.de
Sun Jan 4 11:33:14 EST 2015
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]
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