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• While trying to set up Django on Windows: AttributeError: 'Settings' object has no attribute 'DATABA

  - by user326370

  I'm following these instructions in order to set up Django on Windows. I have installed Python 2.6, PostgreSQL 8.4, Psycopg 2.0.14 for Python 2.6 and the latest version of Django from SVN. I'm now following these instructions to run a test project (copied from the page linked to above): C:\Documents and Settings\John>cd C:\ C:\>mkdir django C:\>cd django C:\django>django-admin.py startproject testproject C:\django>cd testproject C:\django\testproject>python manage.py runserver When I run the last line, this is the output: Validating models... Unhandled exception in thread started by <function inner_run at 0x01ECB930> Traceback (most recent call last): File "J:\Python26\lib\site-packages\django\core\management\commands\runserver.py", line 48, in inn er_run self.validate(display_num_errors=True) File "J:\Python26\lib\site-packages\django\core\management\base.py", line 249, in validate num_errors = get_validation_errors(s, app) File "J:\Python26\lib\site-packages\django\core\management\validation.py", line 22, in get_validat ion_errors from django.db import models, connection File "J:\Python26\lib\site-packages\django\db\__init__.py", line 14, in <module> if not settings.DATABASES: File "J:\Python26\lib\site-packages\django\utils\functional.py", line 273, in __getattr__ return getattr(self._wrapped, name) AttributeError: 'Settings' object has no attribute 'DATABASES' Did I forget to do something with the database? Any help will be appreciated. Thank you!

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• Redirect to another action in an interceptor in struts 2

  - by user292662

  I am currently in the process of learning Struts 2 and I am currently building a simple application where unverified users are redirected to a login form. I have a login form and action functional which takes the users credentials, verifies them and stores a User object in the session however I am now trying to prevent access to pages before the login has taken place and I am trying to do this with an interceptor. My problem is that I have written an interceptor that checks whether the User object has been saved in the session but if it has not I want to redirect to the login page and can't find any way of doing this without bypassing struts and using the HttpServletResponse.sendRedirect method Configuration: <package name="mypackage" extends="struts-default" namespace="/admin"> <interceptors> <interceptor name="login" class="my.LoginInterceptor" /> </interceptors> <default-interceptor-ref name="login"/> <action name="login" class="my.LoginAction"> <result name="input">/admin/login.jsp</result> <result name="success" type="redirect">/admin</result> </action> <action name="private" class="my.PrivateAction"> <result>/admin/private.jsp</result> </action> </package> The interceptor code: @Override public String intercept(ActionInvocation inv) throws Exception { Map<String, Object> session = inv.getInvocationContext().getSession(); Object user = session.get("user"); if(user == null) { // redirect to the 'login' action here } else { return inv.invoke(); } }

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• How can I use "Dependency Injection" in simple php functions, and should I bother?

  - by Tchalvak

  I hear people talking about dependency injection and the benefit of it all the time, but I don't really understand it. I'm wondering if it's a solution to the "I pass database connections as arguments all the time" problem. I tried reading wikipedia's entry on it, but the example is written in Java so I don't solidly understand the difference it is trying to make clear. ( http://en.wikipedia.org/wiki/Dependency_injection ). I read this dependency-injection-in-php article ( http://www.potstuck.com/2009/01/08/php-dependency-injection/ ), and it seems like the objective is to not pass dependencies to an object directly, but to cordon off the creation of an object along with the creation of it's dependencies. I'm not sure how to apply that in a using php functions context, though. Additionally, is the following Dependency Injection, and should I bother trying to do dependency injection in a functional context? Version 1: (the kind of code that I create, but don't like, every day) function get_data_from_database($database_connection){ $data = $database_connection->query('blah'); return $data; } Version 2: (don't have to pass a database connection, but perhaps not dependency injection?) function get_database_connection(){ static $db_connection; if($db_connection){ return $db_connection; } else { // create db_connection ... } } function get_data_from_database(){ $conn = get_database_connection(); $data = $conn->query('blah'); return $data; } $data = get_data_from_database(); Version 3: (the creation of the "object"/data is separate, and the database code is still, so perhaps this would count as dependency injection?) function factory_of_data_set(){ static $db_connection; $data_set = null; $db_connection = get_database_connection(); $data_set = $db_connection->query('blah'); return $data_set; } $data = factory_of_data_set(); Anyone have a good resource or just insight that makes the method and benefit -crystal- clear?

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• Translate XSD element names to English

  - by Coov

  I have an XML Schema Definition file .XSD who's elements are in Spanish. I have an XML data file that is also in Spanish that matches the schema definition. I created a dataset from the xsd using xsd.exe and I'm loading the XML into the dataset. I want to translate the element names to English. I can think of two options off the top of my head. Either scrape the XSD & XML files and translate the elements prior to generating the dataset with esd.exe, or iterate the dataset after I have loaded it with the xml, and translate my objects. I do have a written document that provides the English names for every element name in Spanish. The problem is there are hundreds of elements and I was trying to avoid coding that manually. Getting precise translations is not that important, it just needs to be readable by an English speaking person. Here is an example of what an element may look like "Apellidos": <xs:element name="Apellidos" type="xs:string"/> that I will translate to "SirName": <xs:element name="SirName" type="xs:string"/> I'm looking for ideas and or opinions on a quick way to do this. It's a one time deal so I'm not working about it scaling or being functional for things other than this single xml file. I'll be taking this dataset and writing out a flat file for English speaking users to read.

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• Translating CURL to FLEX HTTPRequests

  - by Joshua

  I am trying to convert from some CURL code to FLEX/ActionScript. Since I am 100% ignorant about CURL and 50% ignorant about Flex and 90% ignorant on HTTP in general... I'm having some significant difficulty. The following CURL code is from http://code.google.com/p/ga-api-http-samples/source/browse/trunk/src/v2/accountFeed.sh I have every reason to believe that it's working correctly. USER_EMAIL="[email protected]" #Insert your Google Account email here USER_PASS="secretpass" #Insert your password here googleAuth="$(curl https://www.google.com/accounts/ClientLogin -s \ -d Email=$USER_EMAIL \ -d Passwd=$USER_PASS \ -d accountType=GOOGLE \ -d source=curl-accountFeed-v2 \ -d service=analytics \ | awk /Auth=.*/)" feedUri="https://www.google.com/analytics/feeds/accounts/default\ ?prettyprint=true" curl $feedUri --silent \ --header "Authorization: GoogleLogin $googleAuth" \ --header "GData-Version: 2" The following is my abortive attempt to translate the above CURL to AS3 var request:URLRequest=new URLRequest("https://www.google.com/analytics/feeds/accounts/default"); request.method=URLRequestMethod.POST; var GoogleAuth:String="$(curl https://www.google.com/accounts/ClientLogin -s " + "-d [email protected] " + "-d Passwd=secretpass " + "-d accountType=GOOGLE " + "-d source=curl-accountFeed-v2" + "-d service=analytics " + "| awk /Auth=.*/)"; request.requestHeaders.push(new URLRequestHeader("Authorization", "GoogleLogin " + GoogleAuth)); request.requestHeaders.push(new URLRequestHeader("GData-Version", "2")); var loader:URLLoader=new URLLoader(); loader.dataFormat=URLLoaderDataFormat.BINARY; loader.addEventListener(Event.COMPLETE, GACompleteHandler); loader.addEventListener(IOErrorEvent.IO_ERROR, GAErrorHandler); loader.addEventListener(SecurityErrorEvent.SECURITY_ERROR, GAErrorHandler); loader.load(request); This probably provides you all with a good laugh, and that's okay, but if you can find any pity on me, please let me know what I'm missing. I readily admit functional ineptitude, therefore letting me know how stupid I am is optional.

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• Using ARIMA to model and forecast stock prices using user-friendly stats program

  - by Brian

  Hi people, Can anyone please offer some insight into this for me? I'm coming from a functional magnetic resonance imaging research background where I analyzed a lot of time series data, and I'd like to analyze the time series of stock prices (or returns) by: 1) modeling a successful stock in a particular market sector and then cross-correlating the time series of this historically successful stock with that of other newer stocks to look for significant relationships; 2) model a stock's price time series and use forecasting (e.g., exponential smoothing) to predict future values of it. I'd like to use non-linear modeling methods (ARIMA and ARCH) to do this. Several questions: How often do ARIMA and ARCH modeling methods (given that the individual who implements them does so accurately) actually fit the stock time series data they target, and what is the optimal fit I can expect? Is the extent to which this model fits the data commensurate with the extent to which it predicts this stock time series' future values? Rather than randomly selecting stocks to compare or model, if profit is my goal, what is an efficient approach, if any, to selecting the stocks I'm going to analyze? Which stats program is the most user-friendly for this? Any thoughts on this would be great and would go a long way for me. Thanks, Brian

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• Database design for a media server containing movies, music, tv and everything in between?

  - by user364114

  In the near future I am attempting to design a media server as a personal project. MY first consideration to get the project underway is architecture, it will certainly be web based but more specifically than that I am looking for suggestions on the database design. So far I am considering something like the following, where I am using [] to represent a table, the first text is the table name to give an idea of purpose and the items within {} would be fields of the table. Also not, fid is functional id referencing some other table. [Item {id, value/name, description, link, type}] - this could be any entity, single song or whole music album, game, movie - almost see this as a recursive relation, ie. a song is an item but an album that song is part of is also an item or for example a tv season is an item, with multiple items being tv episodes [Type {id, fid, mime type, etc}] - file type specific information - could identify how code handles streaming/sending this item to a user [Location {id, fid, path to file?}] [Users {id, username, email, password, ...? }] - user account information [UAC {id, fid, acess level}] - i almost feel its more flexible to seperate access control permissions form the user accounts themselves [ItemLog {id, fid, fid2, timestamp}] - fid for user id, and fid2 for item id - this way we know what user access what when [UserLog {id, fid, timestamp}] -both are logs for access, whether login or last item access [Quota {id, fid, cap}] - some sort of way to throttle users from queing up the entire site and letting it download ... Suggestions or comments are welcome as the hope is that this project will be a open source project once some code is laid out.

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• What am i doing wrong

  - by Erik Sapir

  I have the following code. I need B class to have a min priority queue of AToTime objects. AToTime have operator, and yet i receive error telling me than there is no operator matching the operands... #include <queue> #include <functional> using namespace std; class B{ //public functions public: B(); virtual ~B(); //private members private: log4cxx::LoggerPtr m_logger; class AToTime { //public functions public: AToTime(const ACE_Time_Value& time, const APtr a) : m_time(time), m_a(a){} bool operator >(const AToTime& other) { return m_time > other.m_time; } //public members - no point using any private members here public: ACE_Time_Value m_time; APtr m_a; }; priority_queue<AToTime, vector<AToTime>, greater<AToTime> > m_myMinHeap; };

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• django+mod_wsgi on virtualenv not working

  - by jwesonga

  I've just finished setting up a django app on virtualenv, deployment went smoothly using a fabric script, but now the .wsgi is not working, I've tried every variation on the internet but no luck. My .wsgi file is: import os import sys import django.core.handlers.wsgi # put the Django project on sys.path root_path = os.path.abspath(os.path.dirname(__file__) + '../') sys.path.insert(0, os.path.join(root_path, 'kcdf')) sys.path.insert(0, root_path) os.environ['DJANGO_SETTINGS_MODULE'] = 'kcdf.settings' application = django.core.handlers.wsgi.WSGIHandler() I keep getting the same error: [Sun Apr 18 12:44:30 2010] [error] [client 41.215.123.159] mod_wsgi (pid=16938): Exception occurred processing WSGI script '/home/kcdfweb/webapps/kcdf.web/releases/current/kcdf/apache/kcdf.wsgi'. [Sun Apr 18 12:44:30 2010] [error] [client 41.215.123.159] Traceback (most recent call last): [Sun Apr 18 12:44:30 2010] [error] [client 41.215.123.159] File "/usr/local/lib/python2.6/dist-packages/django/core/handlers/wsgi.py", line 230, in __call__ [Sun Apr 18 12:44:30 2010] [error] [client 41.215.123.159] self.load_middleware() [Sun Apr 18 12:44:30 2010] [error] [client 41.215.123.159] File "/usr/local/lib/python2.6/dist-packages/django/core/handlers/base.py", line 33, in load_middleware [Sun Apr 18 12:44:30 2010] [error] [client 41.215.123.159] for middleware_path in settings.MIDDLEWARE_CLASSES: [Sun Apr 18 12:44:30 2010] [error] [client 41.215.123.159] File "/usr/local/lib/python2.6/dist-packages/django/utils/functional.py", line 269, in __getattr__ [Sun Apr 18 12:44:30 2010] [error] [client 41.215.123.159] self._setup() [Sun Apr 18 12:44:30 2010] [error] [client 41.215.123.159] File "/usr/local/lib/python2.6/dist-packages/django/conf/__init__.py", line 40, in _setup [Sun Apr 18 12:44:30 2010] [error] [client 41.215.123.159] self._wrapped = Settings(settings_module) [Sun Apr 18 12:44:30 2010] [error] [client 41.215.123.159] File "/usr/local/lib/python2.6/dist-packages/django/conf/__init__.py", line 75, in __init__ [Sun Apr 18 12:44:30 2010] [error] [client 41.215.123.159] raise ImportError, "Could not import settings '%s' (Is it on sys.path? Does it have syntax errors?): %s" % (self.SETTINGS_MODULE, e) [Sun Apr 18 12:44:30 2010] [error] [client 41.215.123.159] ImportError: Could not import settings 'kcdf.settings' (Is it on sys.path? Does it have syntax errors?): No module named kcdf.settings my virtual environment is on /home/user/webapps/kcdfweb my app is /home/user/webapps/kcdf.web/releases/current/project_name my wsgi file home/user/webapps/kcdf.web/releases/current/project_name/apache/project_name.wsgi

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• Building a formset dynamically

  - by vorpyg

  I initially wrote code to build a form dynamically, based on data from the DB, similar to what I described in my previous SO post. As SO user Daniel Roseman points out, he would use a formset for this, and now I've come to the realization that he must be completely right. :) My approach works, basically, but I can't seem to get validation across the entire form to be working properly (I believe it's possible, but it's getting quite complex, and there has to be a smarter way of doing it = Formsets!). So now my question is: How can I build a formset dynamically? Not in an AJAX way, I want each form's label to be populated with an FK value (team) from the DB. As I have a need for passing parameters to the form, I've used this technique from a previous SO post. With the former approach, my view code is (form code in previous link): def render_form(request): teams = Team.objects.filter(game=game) form_collection = [] for team in teams: f = SuggestionForm(request.POST or None, team=team, user=request.user) form_collection.append(f) Now I want to do something like: def render_form(request): teams = Team.objects.filter(game=game) from django.utils.functional import curry from django.forms.formsets import formset_factory formset = formset_factory(SuggestionForm) for team in teams: formset.form.append(staticmethod(curry(SuggestionForm, request.POST or None, team=team, user=request.user))) But the append bit doesn't work. What's the proper way of doing this? Thanks!

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• How to compile ocaml to native code

  - by Indra Ginanjar

  i'm really interested learning ocaml, it fast (they said it could be compiled to native code) and it's functional. So i tried to code something easy like enabling mysql event scheduler. #load "unix.cma";; #directory "+mysql";; #load "mysql.cma";; let db = Mysql.quick_connect ~user:"username" ~password:"userpassword" ~database:"databasename"();; let sql = Printf.sprintf "SET GLOBAL EVENT_SCHEDULER=1;" in (Mysql.exec db sql);; It work fine on ocaml interpreter, but when i was trying to compile it to native (i'm using ubuntu karmic), neither of these command worked ocamlopt -o mysqleventon mysqleventon.ml unix.cmxa mysql.cmxa ocamlopt -o mysqleventon mysqleventon.ml unix.cma mysql.cma i also tried ocamlc -c mysqleventon.ml unix.cma mysql.cma all of them resulting same message File "mysqleventon.ml", line 1, characters 0-1: Error: Syntax error Then i tried to remove the "# load", so the code goes like this let db = Mysql.quick_connect ~user:"username" ~password:"userpassword" ~database:"databasename"();; let sql = Printf.sprintf "SET GLOBAL EVENT_SCHEDULER=1;" in (Mysql.exec db sql);; The ocamlopt resulting message File "mysqleventon.ml", line 1, characters 9-28: Error: Unbound value Mysql.quick_connect I hope someone could tell me, where did i'm doing wrong.

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• How do you read a file line by line in your language of choice?

  - by Jon Ericson

  I got inspired to try out Haskell again based on a recent answer. My big block is that reading a file line by line (a task made simple in languages such as Perl) seems complicated in a functional language. How do you read a file line by line in your favorite language? So that we are comparing apples to other types of apples, please write a program that numbers the lines of the input file. So if your input is: Line the first. Next line. End of communication. The output would look like: 1 Line the first. 2 Next line. 3 End of communication. I will post my Haskell program as an example. Ken commented that this question does not specify how errors should be handled. I'm not overly concerned about it because: Most answers did the obvious thing and read from stdin and wrote to stdout. The nice thing is that it puts the onus on the user to redirect those streams the way they want. So if stdin is redirected from a non-existent file, the shell will take care of reporting the error, for instance. The question is more aimed at how a language does IO than how it handles exceptions. But if necessary error handling is missing in an answer, feel free to either edit the code to fix it or make a note in the comments.

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• Is F# a good language for card game AI?

  - by Anthony Brien

  I'm writing a Mahjong Game in C# (the Chinese traditional game, not the solitaire kind). While writing the code for the bot player's AI, I'm wondering if a functional language like F# would be a more suitable language than what I currently use which is C# with a lot of Linq. I don't know much about F# which is why I ask here. To illustrate what I try to solve, here's a quick summary of Mahjong: Mahjong plays a bit like Gin Rummy. You have 13 tiles in your hand, and each turn, you draw a tile and discard another one, trying to improve your hand towards a winning Mahjong hand, which consists or 4 sets and a pair. Sets can be a 3 of a kind (pungs), 4 of a kind (kongs) or a sequence of 3 consecutive tiles (chows). You can also steal another player's discard, if it can complete one of your sets. The code I had to write to detect if the bot can declare 3 consecutive tiles set (chow) is pretty tedious. I have to find all the unique tiles in the hand, and then start checking if there's a sequence of 3 tiles that contain that one in the hand. Detecting if the bot can go Mahjong is even more complicated since it's a combination of detecting if there's 4 sets and a pair in his hand. And that's just a standard Mahjong hand. There's also numerous "special" hands that break those rules but are still a Mahjong hand. For example, "13 unique wonders" consists of 13 specific tiles, "Jade Empire" consists of only tiles colored green, etc. In a perfect world, I'd love to be able to just state the 'rules' of Mahjong, and have the language be able to match a set of 13 tiles against those rules to retrieve which rules it fulfills, for example, checking if it's a Mahjong hand or if it includes a 4 of a kind. Is this something F#'s pattern matching feature can help solve?

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• Patterns for non-layered applications

  - by Paul Stovell

  In Patterns of Enterprise Application Architecture, Martin Fowler writes: This book is thus about how you decompose an enterprise application into layers and how those layers work together. Most nontrivial enterprise applications use a layered architecture of some form, but in some situations other approaches, such as pipes and filters, are valuable. I don't go into those situations, focussing instead on the context of a layered architecture because it's the most widely useful. What patterns exist for building non-layered applications/parts of an application? Take a statistical modelling engine for a financial institution. There might be a layer for data access, but I expect that most of the code would be in a single layer. Would you still expect to see Gang of Four patterns in such a layer? How about a domain model? Would you use OO at all, or would it be purely functional? The quote mentions pipes and filters as alternate models to layers. I can easily imagine a such an engine using pipes as a way to break down the data processing. What other patterns exist? Are there common patterns for areas like task scheduling, results aggregation, or work distribution? What are some alternatives to MapReduce?

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• Reconstructing trees from a "fingerprint"

  - by awshepard

  I've done my SO and Google research, and haven't found anyone who has tackled this before, or at least, anyone who has written about it. My question is, given a "universal" tree of arbitrary height, with each node able to have an arbitrary number of branches, is there a way to uniquely (and efficiently) "fingerprint" arbitrary sub-trees starting from the "universal" tree's root, such that given the universal tree and a tree's fingerprint, I can reconstruct the original tree? For instance, I have a "universal" tree (forgive my poor illustrations), representing my universe of possibilities: Root / / / | \ \ ... \ O O O O O O O (Level 1) /|\/|\...................\ (Level 2) etc. I also have tree A, a rooted subtree of my universe Root / /|\ \ O O O O O / Etc. Is there a way to "fingerprint" the tree, so that given that fingerprint, and the universal tree, I could reconstruct A? I'm thinking something along the lines of a hash, a compression, or perhaps a functional/declarative construction? Big-O analysis (in time or space) is a plus. As a for-instance, a nested expression like: {{(Root)},{(1),(2),(3)},{(2,3),(1),(4,5)}...} representing the actual nodes present at each level in the tree is probably valid, but can it be done more efficiently?

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• standard way to perform a clean shutdown with Boost.Asio

  - by Timothy003

  I'm writing a cross-platform server program in C++ using Boost.Asio. Following the HTTP Server example on this page, I'd like to handle a user termination request without using implementation-specific APIs. I've initially attempted to use the standard C signal library, but have been unable to find a design pattern suitable for Asio. The Windows example's design seems to resemble the signal library closest, but there's a race condition where the console ctrl handler could be called after the server object has been destroyed. I'm trying to avoid race conditions that cause undefined behavior as specified by the C++ standard. Is there a standard (correct) way to stop the server? So far: #include <csignal> #include <functional> #include <boost/asio.hpp> using std::signal; using boost::asio::io_service; extern "C" { static void handle_signal(int); } namespace { std::function<void ()> sighandler; } void handle_signal(int) { sighandler(); } int main() { io_service s; sighandler = std::bind(&io_service::stop, &s); auto res = signal(SIGINT, &handle_signal); // race condition? SIGINT raised before I could set ignore back if (res == SIG_IGN) signal(SIGINT, SIG_IGN); res = signal(SIGTERM, &handle_signal); // race condition? SIGTERM raised before I could set ignore back if (res == SIG_IGN) signal(SIGTERM, SIG_IGN); s.run(); // reset signals signal(SIGTERM, SIG_DFL); signal(SIGINT, SIG_DFL); // is it defined whether handle_signal can still be in execution at this // point? sighandler = nullptr; }

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• Curve fitting: Find a CDF (or any function) that satisfies a list of constraints.

  - by dreeves

  I have some constraints on a CDF in the form of a list of x-values and for each x-value, a pair of y-values that the CDF must lie between. We can represent that as a list of {x,y1,y2} triples such as constraints = {{0, 0, 0}, {1, 0.00311936, 0.00416369}, {2, 0.0847077, 0.109064}, {3, 0.272142, 0.354692}, {4, 0.53198, 0.646113}, {5, 0.623413, 0.743102}, {6, 0.744714, 0.905966}} Graphically that looks like this: And since this is a CDF there's an additional implicit constraint of {Infinity, 1, 1} Ie, the function must never exceed 1. Also, it must be monotone. Now, without making any assumptions about its functional form, we want to find a curve that respects those constraints. For example: (I cheated to get that one: I actually started with a nice log-normal distribution and then generated fake constraints based on it.) One possibility is a straight interpolation through the midpoints of the constraints: mids = ({#1, Mean[{#2,#3}]}&) @@@ constraints f = Interpolation[mids, InterpolationOrder->0] Plotted, f looks like this: That sort of technically satisfies the constraints but it needs smoothing. We can increase the interpolation order but now it violates the implicit constraints (always less than one, and monotone): How can I get a curve that looks as much like the first one above as possible? Note that NonLinearModelFit with a LogNormalDistribution will do the trick in this example but is insufficiently general as sometimes there will sometimes not exist a log-normal distribution satisfying the constraints.

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• Groovy / Scala / Java under the hood

  - by Jack

  I used Java for like 6-7 years, then some months ago I discovered Groovy and started to save a lot of typing.. then I wondered how certain things worked under the hood (because groovy performance is really poor) and understood that to give you dynamic typing every Groovy object is a MetaClass object that handles all the things that the JVM couldn't handle by itself. Of course this introduces a layer in the middle between what you write and what you execute that slows down everything. Then somedays ago I started getting some infos about Scala. How these two languages compare in their byte code translations? How much things they add to the normal structure that it would be obtained by plain Java code? I mean, Scala is static typed so wrapper of Java classes should be lighter, since many things are checked during compile time but I'm not sure about the real differences of what's going inside. (I'm not talking about the functional aspect of Scala compared to the other ones, that's a different thing) Can someone enlighten me? From WizardOfOdds it seems like that the only way to get less typing and same performance would be to write an intermediate translator that translates something in Java code (letting javac compile it) without alterating how things are executed, just adding synctatic sugar withour caring about other fallbacks of the language itself.

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• Custom types as key for a map - C++

  - by Appu

  I am trying to assign a custom type as a key for std::map. Here is the type which I am using as key. struct Foo { Foo(std::string s) : foo_value(s){} bool operator<(const Foo& foo1) { return foo_value < foo1.foo_value; } bool operator>(const Foo& foo1) { return foo_value > foo1.foo_value; } std::string foo_value; }; When used with std::map, I am getting the following error. error C2678: binary '<' : no operator found which takes a left-hand operand of type 'const Foo' (or there is no acceptable conversion) c:\program files\microsoft visual studio 8\vc\include\functional 143 If I change the struct like the below, everything worked. struct Foo { Foo(std::string s) : foo_value(s) {} friend bool operator<(const Foo& foo,const Foo& foo1) { return foo.foo_value < foo1.foo_value; } friend bool operator>(const Foo& foo,const Foo& foo1) { return foo.foo_value > foo1.foo_value; } std::string foo_value; }; Nothing changed except making the operator overloads as friend. I am wondering why my first code is not working? Any thoughts?

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• When is it better to use a method versus a property for a class definition?

  - by ccomet

  Partially related to an earlier question of mine, I have a system in which I have to store complex data as a string. Instead of parsing these strings as all kinds of separate objects, I just created one class that contains all of those objects, and it has some parser logic that will encode all properties into strings, or decode a string to get those objects. That's all fine and good. This question is not about the parser itself, but about where I should house the logic for the parser. Is it a better choice to put it as a property, or as a method? In the case of a property, say public string DataAsString, the get accessor would house the logic to encode all of the data into a string, while the set accessor would decode the input value and set all of the data in the class instance. It seems convenient because the input/output is indeed a string. In the case of a method, one method would be Encode(), which returns the encoded string. Then, either the constructor itself would house the logic for the decoding a string and require the string argument, or I write a Decode(string str) method which is called separately. In either case, it would be using a method instead of a property. So, is there a functional difference between these paths, in terms of the actual running of the code? Or are they basically equivalent and it then boils down to a choice of personal preference or which one looks better? And in that kind of question... which would look cleaner anyway?

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• Exiting from the Middle of an Expression Without Using Exceptions

  - by Jon Purdy

  Is there a way to emulate the use of flow-control constructs in the middle of an expression? Is it possible, in a comma-delimited expression x, y, for y to cause a return? Edit: I'm working on a compiler for something rather similar to a functional language, and the target language is C++. Everything is an expression in the source language, and the sanest, simplest translation to the destination language leaves as many things expressions as possible. Basically, semicolons in the target language become C++ commas. In-language flow-control constructs have presented no problems thus far; it's only return. I just need a way to prematurely exit a comma-delimited expression, and I'd prefer not to use exceptions unless someone can show me that they don't have excessive overhead in this situation. The problem of course is that most flow-control constructs are not legal expressions in C++. The only solution I've found so far is something like this: try { return x(), // x(); (1 ? throw Return(0) : 0); // return 0; } catch (Return& ret) { return ref.value; } The return statement is always there (in the event that a Return construct is not reached), and as such the throw has to be wrapped in ?: to get the compiler to shut up about its void result being used in an expression. I would really like to avoid using exceptions for flow control, unless in this case it can be shown that no particular overhead is incurred; does throwing an exception cause unwinding or anything here? This code needs to run with reasonable efficiency. I just need a function-level equivalent of exit().

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• R: outlier cleaning for each column in a dataframe by using quantiles 0.05 and 0.95

  - by Rainer

  hi, I am a R-novice. I want to do some outlier cleaning and over-all-scaling from 0 to 1 before putting the sample into a random forest. g<-c(1000,60,50,60,50,40,50,60,70,60,40,70,50,60,50,70,10) If i do a simple scaling from 0 - 1 the result would be: > round((g - min(g))/abs(max(g) - min(g)),1) [1] 1.0 0.1 0.0 0.1 0.0 0.0 0.0 0.1 0.1 0.1 0.0 0.1 0.0 0.1 0.0 0.1 0.0 So my idea is to replace the values of each column that are greater than the 0.95-quantile with the next value smaller than the 0.95-quantile - and the same for the 0.05-quantile. So the pre-scaled result would be: g<-c(**70**,60,50,60,50,40,50,60,70,60,40,70,50,60,50,70,**40**) and scaled: > round((g - min(g))/abs(max(g) - min(g)),1) [1] 1.0 0.7 0.3 0.7 0.3 0.0 0.3 0.7 1.0 0.7 0.0 1.0 0.3 0.7 0.3 1.0 0.0 I need this formula for a whole dataframe, so the functional implementation within R should be something like: > apply(c, 2, function(x) x[x`<quantile(x, 0.95)]`<-max(x[x, ... max without the quantile(x, 0.95)) Can anyone help? Spoken beside: if there exists a function that does this job directly, please let me know. I already checked out cut and cut2. cut fails because of not-unique breaks; cut2 would work, but only gives back string values or the mean value, and I need a numeric vector from 0 - 1. for trial: a<-c(100,6,5,6,5,4,5,6,7,6,4,7,5,6,5,7,1) b<-c(1000,60,50,60,50,40,50,60,70,60,40,70,50,60,50,70,10) c<-cbind(a,b) c<-as.data.frame(c) Regards and thanks for help, Rainer

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• How to take advantage of an auto-property when refactoring this .Net 1.1 sample?

  - by Hamish Grubijan

  I see a lot of legacy .Net 1.1-style code at work like in example below, which I would like to shrink with the help of an auto-property. This will help many classes shrink by 30-40%, which I think would be good. public int MyIntThingy { get { return _myIntThingy; } set { _myIntThingy = value; } } private int _myIntThingy = -1; This would become: public int MyIntThingy { get; set; } And the only question is - where do I set MyIntThingy = -1;? If I wrote the class from the start, then I would have a better idea, but I did not. An obvious answer would be: put it in the constructor. Trouble is: there are many constructors in this class. Watching the initialization to -1 in the debugger, I see it happen (I believe) before the constructor gets called. It is almost as if I need to use a static constructor as described here: http://www.c-sharpcorner.com/uploadfile/cupadhyay/staticconstructors11092005061428am/staticconstructors.aspx except that my variables are not static. Java's static initializer comes to mind, but again - my variables are not static. http://www.glenmccl.com/tip_003.htm I want to make stylistic but not functional changes to this class. As crappy as it is, it has been tested and working for a few years now. breaking the functionality would be bad. So ... I am looking for shorter, sweeter, cuter, and yet EQUIVALENT code. Let me know if you have questions.

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• FIFO dequeueing in python?

  - by Aaron Ramsey

  hello again everybody— I'm looking to make a functional (not necessarily optimally efficient, as I'm very new to programming) FIFO queue, and am having trouble with my dequeueing. My code looks like this: class QueueNode: def __init__(self, data): self.data = data self.next = None def __str__(self): return str(self.data) class Queue: def__init__(self): self.front = None self.rear = None self.size = 0 def enqueue(self, item) newnode = QueueNode(item) newnode.next = None if self.size == 0: self.front = self.rear = newnode else: self.rear = newnode self.rear.next = newnode.next self.size = self.size+1 def dequeue(self) dequeued = self.front.data del self.front self.size = self.size-1 if self.size == 0: self.rear = None print self.front #for testing if I do this, and dequeue an item, I get the error "AttributeError: Queue instance has no attribute 'front'." I guess my function doesn't properly assign the new front of the queue? I'm not sure how to fix it though. I don't really want to start from scratch, so if there's a tweak to my code that would work, I'd prefer that—I'm not trying to minimize runtime so much as just get a feel for classes and things of that nature. Thanks in advance for any help, you guys are the best.

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• How can I SETF an element in a tree by an accessor?

  - by Willi Ballenthin

  We've been using Lisp in my AI course. The assignments I've received have involved searching and generating tree-like structures. For each assignment, I've ended up writing something like: (defun initial-state () (list 0 ; score nil ; children 0 ; value 0)) ; something else and building my functions around these "states", which are really just nested lists with some loosely defined structure. To make the structure more rigid, I've tried to write accessors, such as: (defun state-score ( state ) (nth 2 state)) This works for reading the value (which should be all I need to do in a nicely functional world. However, as time crunches, and I start to madly hack, sometimes I want a mutable structure). I don't seem to be able to SETF the returned ...thing (place? value? pointer?). I get an error with something like: (setf (state-score *state*) 10) Sometimes I seem to have a little more luck writing the accessor/mutator as a macro: (defmacro state-score ( state ) `(nth 2 ,state)) However I don't know why this should be a macro, so I certainly shouldn't write it as a macro (except that sometimes it works. Programming by coincidence is bad). What is an appropriate strategy to build up such structures? More importantly, where can I learn about whats going on here (what operations affect the memory in what way)?

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