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• how would you like computer science classes to be taught?

  - by aaa

  hello I am a graduate student now, and hopefully someday I will teach. my interests are C++, Python, embedded languages, and scientific computing. Meanwhile I daydream about how I would teach. I was not quite happy with my undergraduate university as I found many computer science classes lacking. so I would like to ask you, if you were a student, how would you like your computer science classes to be taught? I understand it is a very subjective question, but nevertheless I think it's important to know what people want. Some specific points I am interested in: should computer languages be taught explicitly, or should students be required to pick up language on their own? what is better for learning, tests, projects, some sort of take-home exam? how do you think classtime should be used? theory, introduction, explanations, etc.? do you think the group projects are important? how much about computer architecture do you want to learn in computer science class, not necessarily assembler class. should particular operating system/editor be mandated or encouraged? Thanks thank you for your comments. Question has been closed because it is a discussion question rather than Q&A. If you know appropriate website for discussions of such sort with low noise ratio, please let me know.

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• Add Keyboard Binding To Existing Emacs Mode

  - by Sean M

  I'm attempting my first modification of emacs. I recorded a little keyboard macro and had emacs spit it out as elisp, resulting in: (setq add-docstring "\C-rdef\C-n\C-a\C-m\C-p\C-i\C-u6\"\C-u3\C-b") (global-set-key "\C-c\C-d" 'add-docstring) Searching the emacs reference, though, revealed that C-c C-d is already bound in diff mode. I don't plan on using diff mode, but the future is unknowable and I'd like to not lay a trap for myself. So I'd like this keybinding to only operate in python mode, where it tries to help me add docstrings. In my /usr/share/emacs/23.whatever/list/progmodes, I found python.elc and python.el.gz. I unzipped python.el.gz and got a readable version of the elisp file. Now, though, the documentation becomes opaque to me. How can I add my key binding to the python mode, instead of globally? Is it possible, for bonus points, to apply the changes to python mode without restarting emacs or closing open files? It's the self-modifying editor, I figure there's a good chance that it's possible.

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• Optimal two variable linear regression calculation

  - by Dave Jarvis

  Problem Am looking to apply the y = mx + b equation (where m is SLOPE, b is INTERCEPT) to a data set, which is retrieved as shown in the SQL code. The values from the (MySQL) query are: SLOPE = 0.0276653965651912 INTERCEPT = -57.2338357550468 SQL Code SELECT ((sum(t.YEAR) * sum(t.AMOUNT)) - (count(1) * sum(t.YEAR * t.AMOUNT))) / (power(sum(t.YEAR), 2) - count(1) * sum(power(t.YEAR, 2))) as SLOPE, ((sum( t.YEAR ) * sum( t.YEAR * t.AMOUNT )) - (sum( t.AMOUNT ) * sum(power(t.YEAR, 2)))) / (power(sum(t.YEAR), 2) - count(1) * sum(power(t.YEAR, 2))) as INTERCEPT, FROM (SELECT D.AMOUNT, Y.YEAR FROM CITY C, STATION S, YEAR_REF Y, MONTH_REF M, DAILY D WHERE -- For a specific city ... -- C.ID = 8590 AND -- Find all the stations within a 15 unit radius ... -- SQRT( POW( C.LATITUDE - S.LATITUDE, 2 ) + POW( C.LONGITUDE - S.LONGITUDE, 2 ) ) < 15 AND -- Gather all known years for that station ... -- S.STATION_DISTRICT_ID = Y.STATION_DISTRICT_ID AND -- The data before 1900 is shaky; insufficient after 2009. -- Y.YEAR BETWEEN 1900 AND 2009 AND -- Filtered by all known months ... -- M.YEAR_REF_ID = Y.ID AND -- Whittled down by category ... -- M.CATEGORY_ID = '001' AND -- Into the valid daily climate data. -- M.ID = D.MONTH_REF_ID AND D.DAILY_FLAG_ID <> 'M' GROUP BY Y.YEAR ORDER BY Y.YEAR ) t Data The data is visualized here: Question The following results (to calculate the start and end points of the line) appear incorrect. Why are the results off by ~10 degrees (e.g., outliers skewing the data)? (1900 * 0.0276653965651912) + (-57.2338357550468) = -4.66958228 (2009 * 0.0276653965651912) + (-57.2338357550468) = -1.65405406 I would have expected the 1900 result to be around 10 (not -4.67) and the 2009 result to be around 11.50 (not -1.65). Related Sites Least absolute deviations Robust regression Thank you!

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• Efficient Clojure workflow?

  - by Alex B

  I am developing a pet project with Clojure, but wonder if I can speed up my workflow a bit. My current workflow (with Compojure) is: Start Swank with lein swank. Go to Emacs, connect with M-x slime-connect. Load all existing source files one by one. This also starts a Jetty server and an application. Write some code in REPL. When satisfied with experiments, write a full version of a construct I had in mind. Eval (C-c C-c) it. Switch REPL to namespace where this construct resides and test it. Switch to browser and reload browser tab with the affected page. Tweak the code, eval it, check in the browser. Repeat any of the above. There are a number of annoyances with it: I have to switch between Emacs and the browser (or browsers if I am testing things like templating with multiple browsers) all the time. Is there a common idiom to automate this? I used to have a JavaScript bit that reloads the page continuously, but it's of limited utility, obviously, when I have to interact with the page for more than a few seconds. My JVM instance becomes "dirty" when I experiment and write test functions. Basically namespaces become polluted, especially if I'm refactoring and moving the functions between namespaces. This can lead to symbol collisions and I need to restart Swank. Can I undef a symbol? I load all source files one by one (C-c C-k) upon restarting Swank. I suspect I'm doing it all wrong. Switching between the REPL and the file editor can be a bit irritating, especially when I have a lot of Emacs tabs open, alongside the browser(s). I'm looking for ways to improve the above points and the entire workflow in general, so I'd appreciate if you'd share yours. P. S. I have also used Vimclojure before, so Vimclojure-based workflows are welcome too.

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• How to hand-over a TCP listening socket with minimal downtime?

  - by Shtééf

  While this question is tagged EventMachine, generic BSD-socket solutions in any language are much appreciated too. Some background: I have an application listening on a TCP socket. It is started and shut down with a regular System V style init script. My problem is that it needs some time to start up before it is ready to service the TCP socket. It's not too long, perhaps only 5 seconds, but that's 5 seconds too long when a restart needs to be performed during a workday. It's also crucial that existing connections remain open and are finished normally. Reasons for a restart of the application are patches, upgrades, and the like. I unfortunately find myself in the position that, every once in a while, I need to do this kind of thing in production. The question: I'm looking for a way to do a neat hand-over of the TCP listening socket, from one process to another, and as a result get only a split second of downtime. I'd like existing connections / sockets to remain open and finish processing in the old process, while the new process starts servicing new connectinos. Is there some proven method of doing this using BSD-sockets? (Bonus points for an EventMachine solution.) Are there perhaps open-source libraries out there implementing this, that I can use as is, or use as a reference? (Again, non-Ruby and non-EventMachine solutions are appreciated too!)

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• Linear regression confidence intervals in SQL

  - by Matt Howells

  I'm using some fairly straight-forward SQL code to calculate the coefficients of regression (intercept and slope) of some (x,y) data points, using least-squares. This gives me a nice best-fit line through the data. However we would like to be able to see the 95% and 5% confidence intervals for the line of best-fit (the curves below). What these mean is that the true line has 95% probability of being below the upper curve and 95% probability of being above the lower curve. How can I calculate these curves? I have already read wikipedia etc. and done some googling but I haven't found understandable mathematical equations to be able to calculate this. Edit: here is the essence of what I have right now. --sample data create table #lr (x real not null, y real not null) insert into #lr values (0,1) insert into #lr values (4,9) insert into #lr values (2,5) insert into #lr values (3,7) declare @slope real declare @intercept real --calculate slope and intercept select @slope = ((count(*) * sum(x*y)) - (sum(x)*sum(y)))/ ((count(*) * sum(Power(x,2)))-Power(Sum(x),2)), @intercept = avg(y) - ((count(*) * sum(x*y)) - (sum(x)*sum(y)))/ ((count(*) * sum(Power(x,2)))-Power(Sum(x),2)) * avg(x) from #lr Thank you in advance.

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• Several Objective-C objects become Invalid for no reason, sometimes.

  - by farnsworth

  - (void)loadLocations { NSString *url = @"<URL to a text file>"; NSStringEncoding enc = NSUTF8StringEncoding; NSString *locationString = [[NSString alloc] initWithContentsOfURL:[NSURL URLWithString:url] usedEncoding:&enc error:nil]; NSArray *lines = [locationString componentsSeparatedByString:@"\n"]; for (int i=0; i<[lines count]; i++) { NSString *line = [lines objectAtIndex:i]; NSArray *components = [line componentsSeparatedByString:@", "]; Restaurant *res = [byID objectForKey:[components objectAtIndex:0]]; if (res) { NSString *resAddress = [components objectAtIndex:3]; NSArray *loc = [NSArray arrayWithObjects:[components objectAtIndex:1], [components objectAtIndex:2]]; [res.locationCoords setObject:loc forKey:resAddress]; } else { NSLog([[components objectAtIndex:0] stringByAppendingString:@" res id not found."]); } } } There are a few weird things happening here. First, at the two lines where the NSArray lines is used, this message is printed to the console- *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '*** -[NSCFDictionary count]: method sent to an uninitialized mutable dictionary object' which is strange since lines is definitely not an NSMutableDictionary, definitely is initialized, and because the app doesn't crash. Also, at random points in the loop, all of the variables that the debugger can see will become Invalid. Local variables, property variables, everything. Then after a couple lines they will go back to their original values. setObject:forKey never has an effect on res.locationCoords, which is an NSMutableDictionary. I'm sure that res, res.locationCoords, and byId are initialized. I also tried adding a retain or copy to lines, same thing. I'm sure there's a basic memory management principle I'm missing here but I'm at a loss.

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• No resource type specified (at 'id' with value '@+id\st')

  - by Refaat

  I'm new at android programming, I'm now trying to make some buttons, I configured these buttons using the following code: The MainActivity class : public class MainActivity extends Activity { /** Called when the activity is first created. */ Button st,nd,center; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); st = (Button)findViewById(R.id.st); st = (Button)findViewById(R.id.center); st = (Button)findViewById(R.id.nd); } } and the XML Layout: <?xml version="1.0" encoding="utf-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:layout_width="fill_parent" android:layout_height="fill_parent" android:background="@drawable/background" > <TextView android:layout_width="fill_parent" android:layout_height="wrap_content" android:text="Hello from animation!!" /> <button android:layout_width="100dp" android:layout_height="50dp" android:text="1st half" android:id="@+id\st" /> // and the other two points defined the same way </LinearLayout> And i got that syntax error: error: Error: No resource type specified (at 'id' with value '@+id\st'). // and the same error with the other two buttons HINT: The R class is imported and accessible from the MainActivity class but it can't read R.id.

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• Dynamic expression tree how to

  - by Savvas Sopiadis

  Hello everybody! Implemented a generic repository with several Methods. One of those is this: public IEnumerable<T> Find(Expression<Func<T, bool>> where) { return _objectSet.Where(where); } Given to be it is easy to call this like this: Expression<Func<Culture, bool>> whereClause = c => c.CultureId > 4 ; return cultureRepository.Find(whereClause).AsQueryable(); But now i see (realize) that this kind of quering is "limiting only to one criteria". What i would like to do is this: in the above example c is of type Culture. Culture has several properties like CultureId, Name, Displayname,... How would i express the following: CultureId 4 and Name.contains('de') and in another execution Name.contains('us') and Displayname.contains('ca') and .... Those queries should be created dynamically. I had a look in Expression trees (as i thought this to be a solution to my problem - btw i never used them before) but i cannot find anything which points to my requirement. How can this be costructed? Thanks in advance

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• ruby/datamapper: Refactor class methods to module

  - by DeSchleib

  Hello, i've the following code and tried the whole day to refactor the class methods to a sperate module to share the functionality with all of my model classes. Code (http://pastie.org/974847): class Merchant include DataMapper::Resource property :id, Serial [...] class << self @allowed_properties = [:id,:vendor_id, :identifier] alias_method :old_get, :get def get *args [...] end def first_or_create_or_update attr_hash [...] end end end I'd like to archive something like: class Merchant include DataMapper::Resource include MyClassFunctions [...] end module MyClassFunctions def get [...] def first_or_create_or_update[...] end => Merchant.allowed_properties = [:id] => Merchant.get( :id=> 1 ) But unfortunately, my ruby skills are to bad. I read a lot of stuff (e.g. here) and now i'm even more confused. I stumbled over the following two points: alias_method will fail, because it will dynamically defined in the DataMapper::Resource module. How to get a class method allowed_properties due including a module? What's the ruby way to go? Many thanks in advance.

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• Understanding Symbols In Ruby

  - by Kezzer

  Despite reading this article, I'm still confused as to the representation of the data in memory when it comes to using symbols. If a symbol, two of them contained in different objects, exist in the same memory location, then how is it that they contain different values? I'd have expected the same memory location to contain the same value. As a quote from the link: Unlike strings, symbols of the same name are initialized and exist in memory only once during a session of ruby I just don't understand how it manages to differentiate the values contained in the same memory location. EDIT So let's consider the example: patient1 = { :ruby => "red" } patient2 = { :ruby => "programming" } patient1.each_key {|key| puts key.object_id.to_s} 3918094 patient2.each_key {|key| puts key.object_id.to_s} 3918094 patient1 and patient2 are both hashes, that's fine. :ruby however is a symbol. If we were to output the following: patient1.each_key {|key| puts key.to_s} Then what will be output? "red", or "programming"? FURTHER EDIT I'm still really quite confused. I'm thinking a symbol is a pointer to a value. Let's forget hashes for a second. The questions I have are; can you assign a value to a symbol? Is a symbol just a pointer to a variable with a value in it? If symbols are global, does that mean a symbol always points to one thing?

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• Does android maps support ground overlay?

  - by wmh1108

  I have a weather image I would like to use as an overlay in the google maps android api. I would like to achieve the same result that I get from using GroundOverlay in KML files, such as <GroundOverlay> <name>myimage</name> <Icon> <href>myimage.png</href> <viewBoundScale>0.75</viewBoundScale> </Icon> <LatLonBox> <north>75.6088</north> <south>5.0121</south> <east>182.2805</east> <west>120.6795</west> </LatLonBox> </GroundOverlay> The above will ensure that the 4 corners of my image stay anchored to the 4 lat/long points listed, regardless of scrolling, zooming etc.. Is there a way to accomplish this using the google api/maps provided for android?

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• Creating Mobile Cross-Platform Scripting Solution

  - by Aplomb

  I am having the dream to design a Mobile cross-platform scripting solution to achieve Developer only need to code once by scripting language(it's possible be Javascript or others need further investigation), then the solution will be able to generate the installation files for multiple mobile platforms like J2me, Android, Symbian, BlackBerry, Palm, Windows Mobile/Phone, iPhone, etc. Using the scripting language, developer can code with unified platform API. And other extension frameworks under scripting language, like, 1. 2D UI framework Most probably using for Mobile Applications, the UI style will compete with native UI framework and no longer every application in particular platform looks similar. 2. 3D UI framework Provide the platforms who has 3D capability can represent their Mobile Application with more fashion 3D style which will be much more abstractive. 3. Variable game engine Developers can can use it for easily and high-quality build games. 4. etc This Mobile solution is providing three points, 1. Cross-Platform 2. Scripting 3. Middleware So guys, what do you think about this idea, is it good for Developers? is it profitable? where is the better direction for this idea?

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• Button Onclick event (which is in codbehind) doesn't get triggered in MVC 2

  - by rksprst

  I had an MVC 1.0 web application that was in VS 2008; I just upgraded the project to VS 2010 which automatically upgraded MVC to 2.0. I have a bunch of viewpages have codebehind files that were manually added. The project worked fine before the upgrade, but now the onclick even't don't get triggered. I.e. I have an asp:button with an onclick event that points to a method in the codebehind. When you click the button, the onclick event doesn't get triggered. In fact, when you look at the Page variable, IsPostBack is false. This is really bizarre and I'm wondering if anyone know what happened and how to fix it. I'm thinking it has something to do with the changes in MVC 2.0; but I'm not sure. Any help is really appreciated, I've been trying to figure this out for a while. (deleting the codebehinds and moving that to the controller is not really an option since there is so many pages, moving back to vs 2008 is a last resort as I want to make use of some of the VS 2010 features like performance testing.)

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• Distributing a bundle of files across an extranet

  - by John Zwinck

  I want to be able to distribute bundles of files, about 500 MB per bundle, to all machines on a corporate "extranet" (which is basically a few LANs connected using various private mechanisms, including leased lines and VPN). The total number of hosts is roughly 100, and the goal is to get a copy of the bundle from one host onto all the other hosts reliably, quickly, and efficiently. One important issue is that some hosts are grouped together on single fast LANs in which case the network I/O should be done once from one group to the next and then within each group between all the peers. This is as opposed to a strict central server system where multiple hosts might each fetch the same bundle over a slow link, rather than once via the slow link and then between each other quickly. A new bundle will be produced every few days, and occasionally old bundles will be deleted (but that problem can be solved separately). The machines in question happen to run recent Linuxes, but bonus points will go to solutions which are at least somewhat cross-platform (in which case the bundle might differ per platform but maybe the same mechanism can be used). That's pretty much it. I'm not opposed to writing some code to handle this, but it would be preferable if it were one of bash, Python, Ruby, Lua, C, or C++.

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• Help with Assembly/SSE Multiplication

  - by Brett

  I've been trying to figure out how to gain some improvement in my code at a very crucial couple lines: float x = a*b; float y = c*d; float z = e*f; float w = g*h; all a, b, c... are floats. I decided to look into using SSE, but can't seem to find any improvement, in fact it turns out to be twice as slow. My SSE code is: Vector4 abcd, efgh, result; abcd = [float a, float b, float c, float d]; efgh = [float e, float f, float g, float h]; _asm { movups xmm1, abcd movups xmm2, efgh mulps xmm1, xmm2 movups result, xmm1 } I also attempted using standard inline assembly, but it doesn't appear that I can pack the register with the four floating points like I can with SSE. Any comments, or help would be greatly appreciated, I mainly need to understand why my calculations using SSE are slower than the serial C++ code? I'm compiling in Visual Studio 2005, on a Windows XP, using a Pentium 4 with HT if that provides any additional information to assit. Thanks in advance!

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• Grails , how do I get an object NOT to save

  - by user350325

  Hello I am new to grails and trying to create a form which allows a user to change the email address associated with his/her account for a site I am creating. It asks for the user for their current password and also for the new email address they want to use. If the user enters the wrong password or an invalid email address then it should reject them with an appropriate error message. Now the email validation can be done through constraints in grails, but the password change has to match their current password. I have implemented this check as a method on a service class. See code below: def saveEmail = { def client = ClientUser.get(session.clientUserID) client.email = params.email if(clientUserService.checkPassword(session.clientUserID , params.password) ==false) { flash.message = "Incorrect Password" client.discard() redirect(action:'changeEmail') } else if(!client.validate()) { flash.message = "Invalid Email Address" redirect(action:'changeEmail') } else { client.save(); session.clientUserID = null; flash.message = "Your email address has been changed, please login again" redirect(controller: 'clientLogin' , action:'index') } } Now what I noticed that was odd was that if I entered an invalid email then it would not save the changes (as expected) BUT if I entered the wrong password and a valid email then it would save the changes and even write them back into the database even though it would give the correct "invalid password" error message. I was puzzled so set break points in all the if/else if/else blocks and found that it was hitting the first if statement as expected and not hitting the others , so it would never come accross a call to the save() method, yet it was saved anyway. After a little research I came accross documentation for the discard() method which you can see used in the code above. So I added this but still no avail. I even tried using discard then reloading the client object from the DB again but still no dice. This is very frustrating and I would be grateful for any help, since I think that this should surely not be a complicated requirement!

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• How to set ReceiveBufferSize for UDPClient? or Does it make sense to set? C#

  - by Jack

  Hello all. I am implementing a UDP data transfer thing. I have several questions about UDP buffer. I am using UDPClient to do the UDP send / receive. and my broadband bandwidth is 150KB/s (bytes/s, not bps). I send out a 500B datagram out to 27 hosts 27 hosts send back 10KB datagram back if they receive. So, I should receive 27 responses, right? however, I only get averagely 8 - 12 instead. I then tried to reduce the size of the response down to 500B, yes, I receive all. A thought of mine is that if all 27 hosts send back 10KB response at almost same time, the incoming traffic will be 270KB/s (likely), that exceeds my incoming bandwidth so loss happens. Am I right? But I think even if the incoming traffic exceeds the bandwidth, is the Windows supposed to put the datagram in the buffer and wait for receive? I then suspect that maybe the ReceiveBufferSize of my UdpClient is too small? by default, it is 8092B?? I don't know whether I am all right at these points. Please give me some help.

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• mysql/algorithm: Weighting an average to accentuate differences from the mean

  - by Sai Emrys

  This is for a new feature on http://cssfingerprint.com (see /about for general info). The feature looks up the sites you've visited in a database of site demographics, and tries to guess what your demographic stats are based on that. All my demgraphics are in 0..1 probability format, not ratios or absolute numbers or the like. Essentially, you have a large number of data points that each tend you towards their own demographics. However, just taking the average is poor, because it means that by adding in a lot of generic data, the number goes down. For example, suppose you've visited sites S0..S50. All except S0 are 48% female; S0 is 100% male. If I'm guessing your gender, I want to have a value close to 100%, not just the 49% that a straight average would give. Also, consider that most demographics (i.e. everything other than gender) does not have the average at 50%. For example, the average probability of having kids 0-17 is ~37%. The more a given site's demographics are different from this average (e.g. maybe it's a site for parents, or for child-free people), the more it should count in my guess of your status. What's the best way to calculate this? For extra credit: what's the best way to calculate this, that is also cheap & easy to do in mysql?

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• Change the colour of ablines on ggplot

  - by Sarah

  Using this data I am fitting a plot: p <- ggplot(dat, aes(x=log(Explan), y=Response)) + geom_point(aes(group=Area, colour=Area))+ geom_abline(slope=-0.062712, intercept=0.165886)+ geom_abline(slope= -0.052300, intercept=-0.038691)+ scale_x_continuous("log(Mass) (g)")+ theme(axis.title.y=element_text(size=rel(1.2),vjust=0.2), axis.title.x=element_text(size=rel(1.2),vjust=0.2), axis.text.x=element_text(size=rel(1.3)), axis.text.y=element_text(size=rel(1.3)), text = element_text(size=13)) + scale_colour_brewer(palette="Set1") The two ablines represent the phylogenetically adjusted relationships for each Area trend. I am wondering, is it possible to get the ablines in the same colour palette as their appropriate area data? The first specified is for Area A, the second for Area B. I used: g <- ggplot_build(p) to find out that the first colour is #E41A1C and the second is #377EB8, however when I try to use aes within the +geom_abline command to specify these colours i.e. p <- ggplot(dat, aes(x=log(Explan), y=Response)) + geom_point(aes(group=Area, colour=Area))+ geom_abline(slope=-0.062712, intercept=0.165886,aes(colour='#E41A1C'))+ geom_abline(slope= -0.052300, intercept=-0.038691,aes(colour=#377EB8))+ scale_x_continuous("log(Mass) (g)")+ theme(axis.title.y=element_text(size=rel(1.2),vjust=0.2), axis.title.x=element_text(size=rel(1.2),vjust=0.2), axis.text.x=element_text(size=rel(1.3)), axis.text.y=element_text(size=rel(1.3)), text = element_text(size=13)) + scale_colour_brewer(palette="Set1") It changes the colour of the points and adds to the legend, which I don't want to do. Any advice would be much appreciated!

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• Why does adding Crossover to my Genetic Algorithm give me worse results?

  - by MahlerFive

  I have implemented a Genetic Algorithm to solve the Traveling Salesman Problem (TSP). When I use only mutation, I find better solutions than when I add in crossover. I know that normal crossover methods do not work for TSP, so I implemented both the Ordered Crossover and the PMX Crossover methods, and both suffer from bad results. Here are the other parameters I'm using: Mutation: Single Swap Mutation or Inverted Subsequence Mutation (as described by Tiendil here) with mutation rates tested between 1% and 25%. Selection: Roulette Wheel Selection Fitness function: 1 / distance of tour Population size: Tested 100, 200, 500, I also run the GA 5 times so that I have a variety of starting populations. Stop Condition: 2500 generations With the same dataset of 26 points, I usually get results of about 500-600 distance using purely mutation with high mutation rates. When adding crossover my results are usually in the 800 distance range. The other confusing thing is that I have also implemented a very simple Hill-Climbing algorithm to solve the problem and when I run that 1000 times (faster than running the GA 5 times) I get results around 410-450 distance, and I would expect to get better results using a GA. Any ideas as to why my GA performing worse when I add crossover? And why is it performing much worse than a simple Hill-Climb algorithm which should get stuck on local maxima as it has no way of exploring once it finds a local max?

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• Should i enforce realloc check if the new block size is smaller than the initial ?

  - by nomemory

  Can realloc fail in this case ? int *a = NULL; a = calloc(100, sizeof(*a)); printf("1.ptr: %d \n", a); a = realloc(a, 50 * sizeof(*a)); printf("2.ptr: %d \n", a); if(a == NULL){ printf("Is it possible?"); } return (0); } The output in my case is: 1.ptr: 4072560 2.ptr: 4072560 So 'a' points to the same adress. So should i enforce realloc check ? Later edit: Using MinGW compiler under Windows XP. Is the behaviour similar with gcc on Linux ? Later edit 2: Is it OK to check this way ? int *a = NULL, *b = NULL; a = calloc(100, sizeof(*a)); b = realloc(a, 50 * sizeof(*a)); if(b == NULL){ return a; } a = b; return a;

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• Accelerometer gravity components

  - by Dvd

  Hi, I know this question is definitely solved somewhere many times already, please enlighten me if you know of their existence, thanks. Quick rundown: I want to compute from a 3 axis accelerometer the gravity component on each of these 3 axes. I have used 2 axes free body diagrams to work out the accelerometer's gravity component in the world X-Z, Y-Z and X-Y axes. But the solution seems slightly off, it's acceptable for extreme cases when only 1 accelerometer axis is exposed to gravity, but for a pitch and roll of both 45 degrees, the combined total magnitude is greater than gravity (obtained by Xa^2+Ya^2+Za^2=g^2; Xa, Ya and Za are accelerometer readings in its X, Y and Z axis). More detail: The device is a Nexus One, and have a magnetic field sensor for azimuth, pitch and roll in addition to the 3-axis accelerometer. In the world's axis (with Z in the same direction as gravity, and either X or Y points to the north pole, don't think this matters much?), I assumed my device has a pitch (P) on the Y-Z axis, and a roll (R) on the X-Z axis. With that I used simple trig to get: Sin(R)=Ax/Gxz Cos(R)=Az/Gxz Tan(R)=Ax/Az There is another set for pitch, P. Now I defined gravity to have 3 components in the world's axis, a Gxz that is measurable only in the X-Z axis, a Gyz for Y-Z, and a Gxy for X-Y axis. Gxz^2+Gyz^2+Gxy^2=2*G^2 the 2G is because gravity is effectively included twice in this definition. Oh and the X-Y axis produce something more exotic... I'll explain if required later. From these equations I obtained a formula for Az, and removed the tan operations because I don't know how to handle tan90 calculations (it's infinity?). So my question is, anyone know whether I did this right/wrong or able to point me to the right direction? Thanks! Dvd

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• Enumerate all paths in a weighted graph from A to B where path length is between C1 and C2

  - by awmross

  Given two points A and B in a weighted graph, find all paths from A to B where the length of the path is between C1 and C2. Ideally, each vertex should only be visited once, although this is not a hard requirement. I supose I could use a heuristic to sort the results of the algorithm to weed out "silly" paths (e.g. a path that just visits the same two nodes over and over again) I can think of simple brute force algorithms, but are there any more sophisticed algorithms that will make this more efficient? I can imagine as the graph grows this could become expensive. In the application I am developing, A & B are actually the same point (i.e. the path must return to the start), if that makes any difference. Note that this is an engineering problem, not a computer science problem, so I can use an algorithm that is fast but not necessarily 100% accurate. i.e. it is ok if it returns most of the possible paths, or if most of the paths returned are within the given length range.

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• The uncatchable exception, pt 2

  - by chaiguy

  Ok I've done some testing and I've reduced the problem to something very simple: i. Create a method in a new class that throws an exception: public class Class1 { public void CallMe() { string blah = null; blah.ToLower(); } } ii. Create a MethodInfo that points to this method somewhere else: Type class1 = typeof( Class1 ); Class1 obj = new Class1(); MethodInfo method = class1.GetMethod( "CallMe" ); iii. Wrap a call to Invoke() in a try/catch block: try { method.Invoke( obj, null ); // exception is not being caught! } catch { } iv. Run the program without the debugger (works fine). v. Now run the program with the debugger. The debugger will halt the program when the exception occurs, even though it's wrapped in a catch handler that tries to ignore it. (Even if you put a breakpoint in the catch block it will halt before it reaches it!) In fact, the exception is happening when you run it without the debugger too. In a simple test project it's getting ignored at some other level, but if your app has any kind of global exception handling, it will get triggered there as well. This is causing me a real headache because it keeps triggering my app's crash-handler, not to mention the pain it is to attempt to debug.

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