Path: csiph.com!usenet.pasdenom.info!news.etla.org!news.stack.nl!newsfeed.xs4all.nl!newsfeed4.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail
Return-Path: <python-python-list@m.gmane.org>
X-Original-To: python-list@python.org
Delivered-To: python-list@mail.python.org
X-Spam-Status: OK 0.000
X-Spam-Evidence: '*H*': 1.00; '*S*': 0.00; 'else:': 0.03; 'operator': 0.03; 'list...': 0.07; 'string': 0.09; '-1)': 0.09; '[1,': 0.09; 'builtin': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'subject:Help': 0.11; 'python': 0.11; 'def': 0.12; '-1,': 0.16; '[2,': 0.16; 'itertools': 0.16; 'received:80.91.229.3': 0.16; 'received:dip0.t-ipconnect.de': 0.16; 'received:plane.gmane.org': 0.16; 'received:t-ipconnect.de': 0.16; 'sequence,': 0.16; 'wrong).': 0.16; 'subject:python': 0.16; 'wrote:': 0.18; '>>>': 0.22; 'import': 0.22; 'header:User-Agent:1': 0.23; 'form:': 0.24; 'header:X-Complaints-To:1': 0.27; 'work.': 0.31; 'getting': 0.31; 'factor': 0.31; 'prints': 0.31; 'question:': 0.31; 'another': 0.32; 'problem': 0.35; 'subject:with': 0.35; 'something': 0.35; 'but': 0.35; 'add': 0.35; 'sequence': 0.36; 'doing': 0.36; 'should': 0.36; 'members.': 0.37; 'wrong': 0.37; 'list': 0.37; 'to:addr:python-list': 0.38; 'to:addr:python.org': 0.39; 'received:org': 0.40; 'expression': 0.60; 'first': 0.61; 'back': 0.62; 'map': 0.64; 'finally': 0.65; 'series': 0.66; '100': 0.79; 'repsonse': 0.84; 'stuff:': 0.84
X-Injected-Via-Gmane: http://gmane.org/
To: python-list@python.org
From: Peter Otten <__peter__@web.de>
Subject: Re: Help with map python 2
Date: Sun, 04 Jan 2015 17:33:14 +0100
Organization: None
References: <60e62263-6cd1-48b9-acbc-f1a0b947b8c9@googlegroups.com>
Mime-Version: 1.0
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 7Bit
X-Gmane-NNTP-Posting-Host: p57bda966.dip0.t-ipconnect.de
User-Agent: KNode/4.13.3
X-BeenThere: python-list@python.org
X-Mailman-Version: 2.1.15
Precedence: list
List-Id: General discussion list for the Python programming language <python-list.python.org>
List-Unsubscribe: <https://mail.python.org/mailman/options/python-list>, <mailto:python-list-request@python.org?subject=unsubscribe>
List-Archive: <http://mail.python.org/pipermail/python-list/>
List-Post: <mailto:python-list@python.org>
List-Help: <mailto:python-list-request@python.org?subject=help>
List-Subscribe: <https://mail.python.org/mailman/listinfo/python-list>, <mailto:python-list-request@python.org?subject=subscribe>
Newsgroups: comp.lang.python
Message-ID: <mailman.17374.1420389211.18130.python-list@python.org>
Lines: 71
NNTP-Posting-Host: 2001:888:2000:d::a6
X-Trace: 1420389211 news.xs4all.nl 2849 [2001:888:2000:d::a6]:56543
X-Complaints-To: abuse@xs4all.nl
Xref: csiph.com comp.lang.python:83208

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]