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• Orbital equations, and power required to run them

  - by Adam Davis

  Due to a discussion on the SO IRC today, I'm curious about orbital mechanics, and The equations needed to solve orbital problems The computing power required to solve complex problems The question in particular is calculating when the Earth will plow into the Sun (or vice versa, depending on the frame of reference). I suspect that all the gravitational pulls within our solar system may need to be calculated, which makes me wonder what type of computer cluster is required, or can this be done on a single box? I don't have the experience to do a back of the napkin test here, but perhaps you do? Also, much thx to Gortok for the original inspiration (see comments).

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• What are some exciting, fun, and educational Computer Science activities for students?

  - by Nixuz

  I am a volunteer for Let's Talk Science, an organization which places science graduate students into elementary school and high school classrooms to present short, fun, yet educational demonstrations or experiments related to their particular field. Physics, Chemistry, and Biology have an abundance of such demonstrations, however as a computer scientist, I have no good ideas of what I can present to these students which will demonstrate computer programming and computers in an understandable yet inspiring way in only a 1 - 3 hour presentation. So I am turning to SO for suggestions. Thanks. Presentation Requirements Length: 1 - 3 hours. Explainable in a single sitting. Captivates elementary school and high school audiences. Educational. Please Note Computer's are available at the schools. Please, indicate the suitable age range for your suggestion in your answer.

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• How can I run code in a C# class definition each time any instance of the class is deserialized?

  - by Ben

  I am trying to derive a class from ObservableCollection and I need to run just a single line of code each and every time any instance of this class is deserialized. My thought was to do this: [Serializable] public class ObservableCollection2<T> : ObservableCollection<T>, ISerializable { public ObservableCollection2() : base() { } public ObservableCollection2(SerializationInfo info, StreamingContext context) : base(info, context) { // Put additional code here. } void ISerializable.GetObjectData(SerializationInfo info, StreamingContext context) { base.GetObjectData(info, context); } } But I don't have access to those base methods related to serialization. Am I forced to re-write all of the serialization manually?

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• Apache RewriteRule .* index.php [NC,L] Not working

  - by Daok

  I am trying to redirect everything to a single page from my /website/folder/ directory. I added a simple .htaccess with this simple code (I know this would require more code in .htaccess but it's just a test). RewriteEngine on RewriteRule .* index.php [NC,L] When I try : http://127.0.0.1:8888/website/folder/fileDoesntExistMustGoIndex I have an "Object not found 404" error and the access.log display: 127.0.0.1 - - [13/Mar/2010:13:48:31 -0500] "GET /website/folder/fileDoesntExistMustGoIndexHTTP/1.1" 404 1118 I have check the httpd.conf and I see nothing that could alter this .htaccess statement. Any idea?

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• Graph diffing and versioning tool

  - by hashable

  I am working with a team that edits large DAGs represented as single files. Currently we are unable to work with multiple users concurrently modifying the DAG. Is there a tool (somewhat like the Eclipse SVN plugin) that can do do revision control on the file (manage timestamps/revision stamps) to identify incoming/outgoing/conflicting changes (Node/Link insertion/deletion/modification) and merge changes just like programmers do with source code files? The system should be able to do dependency management also. E.g. an incoming Link must not be accepted when one of the two Nodes is absent. That is, it should not "break" the existing DAG by allowing partial updates. If there is a framework to do this using generic "Node" and "Link" interfaces? Note: I am aware of Protege and its plugins. They currently do not satisfy my requirements.

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• R: Using sapply on vector of POSIXct

  - by Chris

  I have what may be a very simple question. I want to process a column of POSIXct objects from a dataframe and generate a vector of datetime strings. I tried to use the following sapply call dt <- sapply(df$datetime, function(x) format(x,"%Y-%m-%dT%H:%M:%S")) but to no avail. I keep getting the following error Error in prettyNum(.Internal(format(x, trim, digits, nsmall, width, 3L, : invalid 'trim' argument When I apply this function to a single POSIXct object from the column, I have no problem. So I'm stumped at the moment about what the problem is. Do I need to do something special with POSIXct objects?

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• MySQL locking problem

  - by teehoo

  I have a simple setup of a set of writers and a set of readers working with a MySQL ISAM table. The writers are only inserting rows while the readers are only checking for new rows. OK, so I know that I don't need a lock in this situation, since I'm not modifying existing rows. However my Writers are accessing one more table that does need a lock. I piece of information seems irrelevant except for the following limitation stated in the MySQL documentation: A session that requires locks must acquire all the locks that it needs in a single LOCK TABLES statement. While the locks thus obtained are held, the session can access only the locked tables. For example, in the following sequence of statements, an error occurs for the attempt to access t2 because it was not locked in the LOCK TABLES statement: So to access the table I want to insert rows into, I NEED to lock it, which is causing me performance problems. Any suggestions of how to get around this?

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• Select the first row in a join of two tables in one statement

  - by Oscar Cabrero

  hi i need to select only the first row from a query that joins tables A and B, on table B exist multiple records with same name. there are not identifiers in any of the two tables. i cannt change the scheme either because i do not own the DB TABLE A NAME TABLE B NAME DATA1 DATA2 Select Distinct A.NAME,B.DATA1,B.DATA2 From A Inner Join B on A.NAME = B.NAME this gives me NAME DATA1 DATA2 sameName 1 2 sameName 1 3 otherName 5 7 otherName 8 9 but i need to retrieve only one row per name NAME DATA1 DATA2 sameName 1 2 otherName 5 7 i was able to do this by adding the result into a temp table with a identity column and the select the Min Id per name. the problem here is that i require to do this in one single statement. this is a DB2 database thanks

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• Test whether a glob has any matches in bash

  - by Ken Bloom

  If I want to check for the existance of a single file, I can test for it using test -e filename or [ -e filename ]. Supposing I have a glob and I want to know whether any files exist whose names match the glob. The glob can match 0 files (in which case I need to do nothing), or it can match 1 or more files (in which case I need to do something). How can I test whether a glob has any matches.? (test -f glob* fails if the glob matches more than one file.)

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• Segmentation fault when accessing a PHP page

  - by Justin Ethier

  Sometimes when one of our Apache web servers is restarted, we experience segmentation faults when any PHP page is subsequently accessed. The following line is printed in the httpd error_log: [Wed Jun 16 10:59:33 2010] [notice] child pid 31513 exit signal Segmentation fault (11) There will be one of these lines for each PHP page that is accessed. This appears to happen randomly - the "workaround" to-date is to restart httpd, which eventually fixes the problem (almost always after a single restart). Although we only see this happen rarely, it still happens frequently enough to be of concern. So my question is, why is this happening in the first place? Is this a known bug with the version of Apache / PHP / Linux / etc that we are using? Any ideas? The environment is: Fedora 11 Apache 2.2.15 (Default settings) PHP 5.2.13 I can provide more information if that would help narrow things down, since this error message is rather generic... Any help is appreciated.

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• Supervising multiple gen_servers with same module / different arguments

  - by Justin

  Hi, I have a OTP application comprising a single supervisor supervising a small number of gen_servers. A typical child specification is as follows: {my_server, {my_server, start_link, [123]}, permanent, 5000, worker, [my_server]} No problems so far. I now want to an add extra gen_server to the supervisor structure, using the same module Module/Fn as above, but different arguments, eg {my_server_2, {my_server, start_link, [123]}, permanent, 5000, worker, [my_server_2]} I thought this would work, but no: =SUPERVISOR REPORT==== 15-Apr-2010::16:50:13 === Supervisor: {local,my_sup} Context: start_error Reason: {already_started,<0.179.0>} Offender: [{pid,undefined}, {name,my_server_2}, {mfa,{my_server,start_link,[]}}, {restart_type,permanent}, {shutdown,5000}, {child_type,worker}] Do the module arguments in the second element of each child specification need to be different ? Thanks, Justin

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• Changing <img src="XXX" />, js event when new image has finished loading?

  - by carillonator

  I have a photo gallery web page where a single <img src="XXX" /> element's src is changed (on a click) with javascript to show the next image—a poor man's ajax I guess. Works great on faster connections when the new image appears almost immediately. Even if it takes a few seconds to load, every browser I've tested it on keeps the old image in place until the new one is completely loaded. It's a little confusing waiting those few seconds on a slow connection, though, and I'm wondering if there's some javascript event that fires when the new image is done loading, allowing me to put a little working... animated gif or something up in the meantime. I know I could use AJAX for real (I'm using jQuery already), but this is such a nice and simple solution. Besides this lag, is there any other reason I should stay away from this approach to changing images? thanks.

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• Why does dojo parsing time depend on css and images availability?

  - by Kniganapolke

  I have been profiling javascript on my page that uses dojo widgets. I don't use explicit parsing - the parser runs on page load. What I noticed is that if I clear browser cache before refreshing the page, dojo parsing takes much more time than if all the files are already cached. Note that we build all the required dojo modules into a layer (a single file), so we don't lazy-load any js files. I wonder if dojo parsing process depends on images and css resources, as far as I know it only instantiates widgets and injects dom nodes. Do you have any ideas why dojo parser runs longer (2-3 times longer in my case) when the cache is cleared?

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• MPI4Py Scatter sendbuf Argument Type?

  - by Noel

  I'm having trouble with the Scatter function in the MPI4Py Python module. My assumption is that I should be able to pass it a single list for the sendbuffer. However, I'm getting a consistent error message when I do that, or indeed add the other two arguments, recvbuf and root: File "code/step3.py", line 682, in subbox_grid i = mpi_communicator.Scatter(station_range, station_data) File "Comm.pyx", line 427, in mpi4py.MPI.Comm.Scatter (src/ mpi4py_MPI.c:44993) File "message.pxi", line 321, in mpi4py.MPI._p_msg_cco.for_scatter (src/mpi4py_MPI.c:14497) File "message.pxi", line 232, in mpi4py.MPI._p_msg_cco.for_cco_send (src/mpi4py_MPI.c:13630) File "message.pxi", line 36, in mpi4py.MPI.message_simple (src/ mpi4py_MPI.c:11904) ValueError: message: expecting 2 or 3 items Here is the relevant code snipped, starting a few lines above 682 mentioned above. for station in stations #snip--do some stuff with station station_data = [] station_range = range(1,len(station)) mpi_communicator = MPI.COMM_WORLD i = mpi_communicator.Scatter(station_range, nsm) #snip--do some stuff with station[i] nsm = combine(avg, wt, dnew, nf1, nl1, wti[i], wtm, station[i].id) station_data = mpi_communicator.Gather(station_range, nsm) I've tried a number of combinations initializing station_range, but I must not be understanding the Scatter argument types properly. Does a Python/MPI guru have a clarification this?

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• Django gives "I/O operation on closed file" error when reading from a saved ImageField

  - by Rob Osborne

  I have a model with two image fields, a source image and a thumbnail. When I update the new source image, save it and then try to read the source image to crop/scale it to a thumbnail I get an "I/O operation on closed file" error from PIL. If I update the source image, don't save the source image, and then try to read the source image to crop/scale, I get an "attempting to read from closed file" error from PIL. In both cases the source image is actually saved and available in later request/response loops. If I don't crop/scale in a single request/response loop but instead upload on one page and then crop/scale in another page this all works fine. This seems to be a cached buffer being reused some how, either by PIL or by the Django file storage. Any ideas on how to make an ImageField readable after saving?

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• Read VB binary file with c# (knowing the structure)

  - by Kai

  I'd like to read a file that has been binary saved by a vb prog. the file should be read line by line, 'cause every single line represents an object with many attributes. a link looks the following: 999011011/10/1 ELW the structure of data types is: Public Type FahrzeugDAT Kennung As String * 8 Name As String * 30 Info As String * 50 StatusFzg As Integer DatumFzg As Date StatusLst As Integer DatumLst As Date TKI As Integer Folgetlg As Integer LstKurztext As String * 100 FME1 As String * 5 FME2 As String * 5 FME3 As String * 5 DME1 As String * 8 DME2 As String * 8 DME3 As String * 8 Bemerkung As String * 256 Art As Long Standort As Long End Type

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• Grails multi column indexes

  - by Kimble

  Can someone explain how to define multi column indexes in Grails? The documentation is at best sparse. This for example does not seem to work at all: http://grails.org/GORM+Index+definitions I've had some luck with this, but the results seems random at best. Definitions that works in one domain class does not when applied to another (with different names of course). http://www.grails.org/doc/1.1/guide/single.html#5.5.2.6%20Database%20Indices Some working examples and explanations would be highly appreciated!

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• Restrict access to a specific URL, running on IIS7 / ASP.NET

  - by frankadelic

  I am deploying a public ASP.NET website on an IIS7 web farm. The application runs on 3 web servers and is behind a firewall. We want to create a single page on the website that is accessible only to internal users. It is primarily used for diagnostics, trigger cache expiry, etc. /admin/somepage.aspx What is the best way to control access to this page? We need to: Prevent all external (public) users from accessing the URL. Permit specific internal users to access the page, only from certain IPs or networks. Should this access control be done at the (a) network level, (b) application level, etc.?

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• Get domain name (not subdomain) in php

  - by Cyclone

  I have a URL which can be any of the following formats: http://example.com https://example.com http://example.com/foo http://example.com/foo/bar www.example.com example.com foo.example.com www.foo.example.com foo.bar.example.com http://foo.bar.example.com/foo/bar example.net/foo/bar Essentially, I need to be able to match any normal URL. How can I extract example.com (or .net, whatever the tld happens to be. I need this to work with any TLD.) from all of these via a single regex? This is in PHP. Thanks for the help!

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• Eclipse doesn't see my new junit test

  - by morgancodes

  I'm using eclipse to run the tests in a single junit(4) test class. The tests in the class all run just fine. Then I add an additional test and run the class through the test running in ecplise again. Only the old tests are run. The new test isn't seen by eclipse. There's no error or anything, it's just as if eclipse is looking at an old version of the test. If I run the tests using maven, everything works fine. Additionally, after I run the tests in maven, ecplipse can see and run the new test correctly. Any ideas what's going on? Any ideas how to get ecplipse's test runner to see my new test cases?

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• Using javascript on an HTML table?

  - by Michael McKay

  Hi, few questions, just wondering if anyone can help? I have a table with 1 long row (1000 pixels) and one single column, how do i go about creating a method whereby when the mouse cursor is on the leftmost side of the cell, a variable, lets say X is set to 0, the further right the mouse cursor moves in the cell, the value of X increases. I know that sounds like a strange question but im working on a project where this type of functionality is desired. Is there a javascript method to create this feature? Thanks for any help.

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• Wordpress podcasting with youtube video hosting

  - by Salim Fadhley

  I'm trying to build a site based on Wordpress for a client. Eventually it will be used to publish videos which could be hosted locally, but to reduce costs it would be nicer if we could host all our videos on somewhere like YouTube or Vimeo. I'm looking for a plugin or a way of hacking an existing plugin in order to achieve this. The idea is that when we list a new item on the site, users of iTunes / iPod and any other (more) standards compliant podcatcher should be able to view the video. Ideally I'd like to get away without writing a single line of code, but that may not be possible. The best solution for me is the one which requires the least amount of work!

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• Linq2Sql Best Practices

  - by this. __curious_geek

  I'm recently migrating to Linq2Sql and all my future projects will be using Linq2Sql. Having said that, I researched a lot on how to properly plug-in Linq2Sql in application design. what to put at what layer ? How do you design your repositories and business layer services ? Should I use DTOs over Linq2Sql entities on interaction layer ? what things should I be careful about ? what problems did you face ? I did not find any rock-solid material that really talked about one single thing and everyone have their own opinions. I'm looking forward to your ideas on how to integrate/use Linq2Sql in projects. My priority is maintenance[it should be maintenable and when multiple people work on same project] and scalabilty [it should have scope of evolution]. Thanks.

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• What are the virtues of using XML comments in .NET?

  - by Michal Czardybon

  I can't understand the virtues of using XML comments. I know they can be converted into nice documentation external to the code, but the same can be achieved with the much more concise DOxygen syntax. In my opinion the XML comments are wrong, because: They obfuscate the comments and the code in general. (They are more difficult to read by humans). Less code can be viewed on a single screen, because "summary" and "/summary" take additional lines. They suggest that all method parameters have to be commented, whereas 90% of them are obvious and SHOULD be left not commented. The only problem I have with this is that my point of view seems to be in minority. Why?

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• SQL to display an event on start date, end date and any days in between.

  - by Tim

  Hello, This should be fairly simple, but I can't get my head around it. I have an event in my database with a startDate and an endDate. I need to display this event (based on the current date) on every day the event occurs. So if the event starts on the 3rd of May and finishes on the 7th of May, the SQL query must find it on every single day. How can I achieve this? SELECT * FROM events WHERE startDate ??? Thanks, Tim

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