Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!aioe.org!feeder.news-service.com!xlned.com!feeder5.xlned.com!newsfeed.xs4all.nl!newsfeed6.news.xs4all.nl!xs4all!newsgate.cistron.nl!newsgate.news.xs4all.nl!post.news.xs4all.nl!not-for-mail Return-Path: 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; 'example:': 0.03; 'subject:using': 0.04; 'memory.': 0.05; 'not?': 0.09; 'received:80.91': 0.09; 'received:80.91.229': 0.09; 'received:80.91.229.12': 0.09; 'received:gmane.org': 0.09; 'received:list': 0.09; 'received:lo.gmane.org': 0.09; 'underlying': 0.09; 'pm,': 0.10; '>>>': 0.12; 'wrote:': 0.14; '(0,': 0.16; '(1,': 0.16; '(2,': 0.16; 'enigma': 0.16; 'kern': 0.16; 'left-hand': 0.16; 'numpy': 0.16; 'propagate': 0.16; 'received:austin.res.rr.com': 0.16; 'subject:arrays': 0.16; 'subject:numpy': 0.16; 'header:In-Reply-To:1': 0.21; 'trying': 0.23; 'mask': 0.23; 'structured': 0.23; 'values': 0.25; 'object': 0.26; 'interpret': 0.29; 'array': 0.30; 'changes': 0.30; 'yet': 0.32; 'expression': 0.32; 'header:X-Complaints-To:1': 0.32; 'does': 0.33; 'to:addr:python-list': 0.33; 'actually': 0.33; 'header:User-Agent:1': 0.35; 'assignment': 0.35; 'list:': 0.36; 'received:rr.com': 0.36; 'skip:. 10': 0.36; 'change': 0.37; 'received:org': 0.38; 'could': 0.38; 'hello,': 0.38; 'url:org': 0.38; 'mailing': 0.38; 'though': 0.38; 'subject:: ': 0.38; 'some': 0.38; 'attempt': 0.39; 'header:Mime-Version:1': 0.39; 'to:addr:python.org': 0.39; 'final': 0.60; 'back': 0.63; 'world': 0.63; 'our': 0.63; 'view': 0.66; 'believe': 0.66; 'eco': 0.91 X-Injected-Via-Gmane: http://gmane.org/ To: python-list@python.org From: Robert Kern Subject: Re: using masks and numpy record arrays Date: Wed, 25 May 2011 16:22:50 -0500 Organization: The Church of Last Thursday References: Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Gmane-NNTP-Posting-Host: cpe-70-113-40-77.austin.res.rr.com User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.2.17) Gecko/20110414 Thunderbird/3.1.10 In-Reply-To: X-BeenThere: python-list@python.org X-Mailman-Version: 2.1.12 Precedence: list List-Id: General discussion list for the Python programming language List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Newsgroups: comp.lang.python Message-ID: Lines: 47 NNTP-Posting-Host: 82.94.164.166 X-Trace: 1306358584 news.xs4all.nl 49046 [::ffff:82.94.164.166]:43965 X-Complaints-To: abuse@xs4all.nl Xref: x330-a1.tempe.blueboxinc.net comp.lang.python:6265 On 5/25/11 3:27 PM, Catherine Moroney wrote: > Hello, > > I am trying to work with a structured array and a mask, and am encountering some > problems. You will want to ask numpy questions on the numpy mailing list: http://www.scipy.org/Mailing_Lists > For example: > > >>> xtype = numpy.dtype([("n", numpy.int32), ("x", numpy.float32)]) > >>> a = numpy.zeros((4), dtype=xtype) > >>> b = numpy.arange(0,4) > >>> a2 = numpy.zeros((4), dtype=xtype) > >>> mask = numpy.where(b%2 == 0) > >>> a2[:]["n"] += b ! this changes the values of a2 > >>> a[mask]["n"] += b[mask] ! this does not change the values of a > >>> a2 > array([(0, 0.0), (1, 0.0), (2, 0.0), (3, 0.0)], > dtype=[('n', ' >>> a > array([(0, 0.0), (0, 0.0), (0, 0.0), (0, 0.0)], > dtype=[('n', ' > Why do the values of a2 get updated, and yet the values of a do not? Only the final [] on the left-hand side of the assignment actually turns into a .__setitem__() call to the object that is the result of the expression to its left. a[mask] makes a copy while a2[:] makes a view. You could do a["n"][mask] += b[mask] since a["n"] will also make a view and the .__setitem__() on it will propagate back to the original memory. -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco