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  • Finding the intersection of two vector equations.

    - by Matthew Mitchell
    I've been trying to solve this and I found an equation that gives the possibility of zero division errors. Not the best thing: v1 = (a,b) v2 = (c,d) d1 = (e,f) d2 = (h,i) l1: v1 + ?d1 l2: v2 + µd2 Equation to find vector intersection of l1 and l2 programatically by re-arranging for lambda. (a,b) + ?(e,f) = (c,d) + µ(h,i) a + ?e = c + µh b +?f = d + µi µh = a + ?e - c µi = b +?f - d µ = (a + ?e - c)/h µ = (b +?f - d)/i (a + ?e - c)/h = (b +?f - d)/i a/h + ?e/h - c/h = b/i +?f/i - d/i ?e/h - ?f/i = (b/i - d/i) - (a/h - c/h) ?(e/h - f/i) = (b - d)/i - (a - c)/h ? = ((b - d)/i - (a - c)/h)/(e/h - f/i) Intersection vector = (a + ?e,b + ?f) Not sure if it would even work in some cases. I haven't tested it. I need to know how to do this for values as in that example a-i. Thank you.

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  • Python/PyParsing: Difficulty with setResultsName

    - by Rosarch
    I think I'm making a mistake in how I call setResultsName(): from pyparsing import * DEPT_CODE = Regex(r'[A-Z]{2,}').setResultsName("Dept Code") COURSE_NUMBER = Regex(r'[0-9]{4}').setResultsName("Course Number") COURSE_NUMBER.setParseAction(lambda s, l, toks : int(toks[0])) course = DEPT_CODE + COURSE_NUMBER course.setResultsName("course") statement = course From IDLE: >>> myparser import * >>> statement.parseString("CS 2110") (['CS', 2110], {'Dept Code': [('CS', 0)], 'Course Number': [(2110, 1)]}) The output I hope for: >>> myparser import * >>> statement.parseString("CS 2110") (['CS', 2110], {'Course': ['CS', 2110], 'Dept Code': [('CS', 0)], 'Course Number': [(2110, 1)]}) Does setResultsName() only work for terminals?

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  • Linking Boost to my C++ project in Eclipse

    - by MahlerFive
    I'm trying to get the Boost library working in my C++ projects in Eclipse. I can successfully build when using header-only libraries in Boost such as the example simple program in the "Getting Started" guide using the lambda header. I cannot get my project to successfully link to the regex Boost library as shown later in the guide. Under my project properties - c/c++ build - settings - tool settings tab - libraries, I have added "libboost_regex" to the Libraries box, and "C:\Program Files\boost\boost_1_42_0\bin.v2\libs" to the Library search path box since this is where all the .lib files are. I've even tried adding "libboost_regex-mgw34-mt-d-1_42.lib" to the libraries box instead of "libboost_regex" since that is the exact file name, but this did not work either. I keep getting an error that says "cannot find -llibboost_regex" when I try to build my project. Any ideas as to how I can fix this?

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  • Do I need to include the 'this' when using a property name in a closure?

    - by Scott Whitlock
    I'm using a list of Actions to store an undo history for an object. Let's say I have a property of my object called myChildObject and it's being changed, so I want to store the undo action where I would set it back to it's current value: public class Class1 { public Class1() { } private readonly List<Action> m_undoActions = new List<Action>(); private SomeObject myChildObject { get; set; } public void ChangeState(SomeObject newChildObject) { // copies the reference SomeObject existingObject = myChildObject; m_undoActions.Add(() => myChildObject = existingObject); myChildObject = newChildObject; } } Looking at the lambda expression, existingObject is a local variable, so it's using a closure to pass a reference to that variable, but what about the property myChildObject? Do I need to use 'this' to preface it? Do I need to make a copy of the 'this' reference to a local variable first? Thanks for helping me understand this closure stuff.

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  • declarative authorization and has_and_belongs_to_many

    - by Michael Balsiger
    Hi, I have a little problem with declarative-authorization. I have a User and Role Model with a has_and_belongs_to_many association. I've created a Role named :moderator in my authorization_rules.rb Is it possible that a User with the Role Moderator only gets the Users that have the Moderator Role assigned to it?? -- User.with_permissions_to(:index) I thought it would be possible like that: role :moderator do has_permission_on :users, :to => :index do if_attribute :roles => contains { ????? } end end I also created a named_scope in my User Model because I thought it would help... class User has_and_belongs_to_many :roles named_scope :by_role, lambda { |role| { :include => :roles, :conditions => {"roles.name" => role} } } end Does anyone knows if it's possible to do this with declarative_authorization? Thanks for your help!

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  • named_scope and substings

    - by Philb28
    I have a named_scope in rails that finds episodes by there directors given name named_scope :director_given, lambda { |dr| {:joins => :director, :conditions => ['given = ?', dr]} } It works great but I would like it to also work on substrings one the name. e.g. instead of having to search for 'Lucy' you could just search 'Lu'. P.S. I also have another named scope which does exactly the same thing but on the directors last name. It there a way to combine the two? Thanks,

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  • Python How to make a cross-module function?

    - by Evan
    I want to be able to call a global function from an imported class, for example In file PetStore.py class AnimalSound(object): def __init__(self): if 'makenoise' in globals(): self.makenoise = globals()['makenoise'] else: self.makenoise = lambda: 'meow' def __str__(self): return self.makenoise() Then when I test in the Python Interpreter >>> def makenoise(): ... return 'bark' ... >>> from PetStore import AnimalSound >>> sound = AnimalSound() >>> sound.makenoise() 'meow' I get a 'meow' instead of 'bark'. I have tried using the solutions provided in python-how-to-make-a-cross-module-variable with no luck.

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  • Why wont numpy matrix let me print its rows?

    - by uberjumper
    Okay this is probably a really dumb question, however its really starting to hurt. I have a numpy matrix, and basically i print it out row by row. However i want to make each row be formatted and separated properly. >>> arr = numpy.matrix([[x for x in range(5)] for y in range(5)]) >>> arr matrix([[0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4], [0, 1, 2, 3, 4]]) Lets say i want to print the first row, and add a '|' between each element: >>> '|'.join(map(str, arr[0,])) '[[0 1 2 3 4]]' Err... >>> '|'.join(map(lambda x: str(x[0]), arr[0])) '[[0 1 2 3 4]]' I am really confused by this behavior why does it do this?

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  • Java style FOR loop in a clojure interpeter ?

    - by Kevin
    I have a basic interpreter in clojure. Now i need to implement for (initialisation; finish-test; loop-update) { statements } inside my interpreter. I will attach my interpreter code I got so far. Any help is appreciated. Interpreter (declare interpret make-env) ;; (def do-trace false) ;; ;; simple utilities (def third ; return third item in a list (fn [a-list] (second (rest a-list)))) (def fourth ; return fourth item in a list (fn [a-list] (third (rest a-list)))) (def run ; make it easy to test the interpreter (fn [e] (println "Processing: " e) (println "=> " (interpret e (make-env))))) ;; for the environment (def make-env (fn [] '())) (def add-var (fn [env var val] (cons (list var val) env))) (def lookup-var (fn [env var] (cond (empty? env) 'error (= (first (first env)) var) (second (first env)) :else (lookup-var (rest env) var)))) ;; -- define numbers (def is-number? (fn [expn] (number? expn))) (def interpret-number (fn [expn env] expn)) ;; -- define symbols (def is-symbol? (fn [expn] (symbol? expn))) (def interpret-symbol (fn [expn env] (lookup-var env expn))) ;; -- define boolean (def is-boolean? (fn [expn] (or (= expn 'true) (= expn 'false)))) (def interpret-boolean (fn [expn env] expn)) ;; -- define functions (def is-function? (fn [expn] (and (list? expn) (= 3 (count expn)) (= 'lambda (first expn))))) (def interpret-function (fn [expn env] expn)) ;; -- define addition (def is-plus? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '+ (first expn))))) (def interpret-plus (fn [expn env] (+ (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define subtraction (def is-minus? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '- (first expn))))) (def interpret-minus (fn [expn env] (- (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define multiplication (def is-times? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '* (first expn))))) (def interpret-times (fn [expn env] (* (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define division (def is-divides? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '/ (first expn))))) (def interpret-divides (fn [expn env] (/ (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define equals test (def is-equals? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '= (first expn))))) (def interpret-equals (fn [expn env] (= (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define greater-than test (def is-greater-than? (fn [expn] (and (list? expn) (= 3 (count expn)) (= '> (first expn))))) (def interpret-greater-than (fn [expn env] (> (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define not (def is-not? (fn [expn] (and (list? expn) (= 2 (count expn)) (= 'not (first expn))))) (def interpret-not (fn [expn env] (not (interpret (second expn) env)))) ;; -- define or (def is-or? (fn [expn] (and (list? expn) (= 3 (count expn)) (= 'or (first expn))))) (def interpret-or (fn [expn env] (or (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define and (def is-and? (fn [expn] (and (list? expn) (= 3 (count expn)) (= 'and (first expn))))) (def interpret-and (fn [expn env] (and (interpret (second expn) env) (interpret (third expn) env)))) ;; -- define with (def is-with? (fn [expn] (and (list? expn) (= 3 (count expn)) (= 'with (first expn))))) (def interpret-with (fn [expn env] (interpret (third expn) (add-var env (first (second expn)) (interpret (second (second expn)) env))))) ;; -- define if (def is-if? (fn [expn] (and (list? expn) (= 4 (count expn)) (= 'if (first expn))))) (def interpret-if (fn [expn env] (cond (interpret (second expn) env) (interpret (third expn) env) :else (interpret (fourth expn) env)))) ;; -- define function-application (def is-function-application? (fn [expn env] (and (list? expn) (= 2 (count expn)) (is-function? (interpret (first expn) env))))) (def interpret-function-application (fn [expn env] (let [function (interpret (first expn) env)] (interpret (third function) (add-var env (first (second function)) (interpret (second expn) env)))))) ;; the interpreter itself (def interpret (fn [expn env] (cond do-trace (println "Interpret is processing: " expn)) (cond ; basic values (is-number? expn) (interpret-number expn env) (is-symbol? expn) (interpret-symbol expn env) (is-boolean? expn) (interpret-boolean expn env) (is-function? expn) (interpret-function expn env) ; built-in functions (is-plus? expn) (interpret-plus expn env) (is-minus? expn) (interpret-minus expn env) (is-times? expn) (interpret-times expn env) (is-divides? expn) (interpret-divides expn env) (is-equals? expn) (interpret-equals expn env) (is-greater-than? expn) (interpret-greater-than expn env) (is-not? expn) (interpret-not expn env) (is-or? expn) (interpret-or expn env) (is-and? expn) (interpret-and expn env) ; special syntax (is-with? expn) (interpret-with expn env) (is-if? expn) (interpret-if expn env) ; functions (is-function-application? expn env) (interpret-function-application expn env) :else 'error)))

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  • Installing a condition handler in Common Lisp

    - by Paul Nathan
    The HTTP library Drakma on CLISP generates an error USOCKET:UNSUPPORTED due to a bug in Drakma+CLISP. However, it turns out that the CONTINUE restart seems to work fine. Therefore, I spent some time with CLtL and other references trying to determine how to write a restart handler. (defun http-request (url param) (handler-bind ((USOCKET:UNSUPPORTED #'(lambda (x) (invoke-restart 'continue))))) (drakma:http-request url :method :post :parameters param)) According to my best understanding, the above code should trap the error USOCKET:UNSUPPORTED. It doesn't; it seems to ignore the error binder. How do I fix this?

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  • Entity Framework query builder methods: why "it" and not lambdas?

    - by nlawalker
    I'm just getting started with EF and a query like the following strikes me as odd: var departmentQuery = schoolContext.Departments.Include("Courses"). OrderBy("it.Name"); Specifically, what sticks out to me is "it.Name." When I was tooling around with LINQ to SQL, pretty much every filter in a query-builder query could be specified with a lambda, like, in this case, d = d.Name. I see that there are overrides of OrderBy that take lambdas that return an IOrderedQueryable or an IOrderedEnumable, but those obviously don't have the Execute method needed to get the ObjectResult that can then be databound. It seems strange to me after all I've read about how lambdas make so much sense for this kind of stuff, and how they are translated into expression trees and then to a target language - why do I need to use "it.Name"?

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  • Find all paths from root to leaves of tree in Scheme

    - by grifaton
    Given a tree, I want to find the paths from the root to each leaf. So, for this tree: D / B / \ A E \ C-F-G has the following paths from root (A) to leaves (D, E, G): (A B D), (A B E), (A C F G) If I represent the tree above as (A (B D E) (C (F G))) then the function g does the trick: (define (g tree) (cond ((empty? tree) '()) ((pair? tree) (map (lambda (path) (if (pair? path) (cons (car tree) path) (cons (car tree) (list path)))) (map2 g (cdr tree)))) (else (list tree)))) (define (map2 fn lst) (if (empty? lst) '() (append (fn (car lst)) (map2 fn (cdr lst))))) But this looks all wrong. I've not had to do this kind of thinking for a while, but I feel there should be a neater way of doing it. Any ideas for a better solution (in any language) would be appreciated.

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  • ruby block and returning something from block

    - by dorelal
    I am using ruby 1.8.7. p = lambda { return 10;} def lab(block) puts 'before' puts block.call puts 'after' end lab p Above code output is before 10 after I refactored same code into this def lab(&block) puts 'before' puts block.call puts 'after' end lab { return 10; } Now I am getting LocalJumpError: unexpected return. To me both the code are doing same thing. Yes in the first case I am passing a proc and in the second case I am passing a block. But &block converts that block into proc. So proc.call should behave same. And yes I have seen this post http://stackoverflow.com/questions/2325471/using-return-in-a-ruby-block

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  • Why is one Func valid and the other (almost identical) not.

    - by runrunraygun
    private static Dictionary<Type, Func<string, object>> _parseActions = new Dictionary<Type, Func<string, object>> { { typeof(bool), value => {Convert.ToBoolean(value) ;}} }; The above gives an error Error 14 Not all code paths return a value in lambda expression of type 'System.Func<string,object>' However this below is ok. private static Dictionary<Type, Func<string, object>> _parseActions = new Dictionary<Type, Func<string, object>> { { typeof(bool), value => Convert.ToBoolean(value) } }; I don't understand the difference between the two. I thought the extra braces in example1 are to allow us to use multiple lines in the anon function so why have they affected the meaning of the code?

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  • (C++) Linking with namespaces causes duplicate symbol error

    - by user577072
    Hello. For the past few days, I have been trying to figure out how to link the files for a CLI gaming project I have been working on. There are two halves of the project, the Client and the Server code. The client needs two libraries I've made. The first is a general purpose game board. This is split between GameEngine.h and GameEngine.cpp. The header file looks something like this namespace gfdGaming { // struct sqr_size { // Index x; // Index y; // }; typedef struct { Index x, y; } sqr_size; const sqr_size sPos = {1, 1}; sqr_size sqr(Index x, Index y); sqr_size ePos; class board { // Prototypes / declarations for the class } } And the CPP file is just giving everything content #include "GameEngine.h" type gfdGaming::board::functions The client also has game-specific code (in this case, TicTacToe) split into declarations and definitions (TTT.h, Client.cpp). TTT.h is basically #include "GameEngine.h" #define TTTtar "localhost" #define TTTport 2886 using namespace gfdGaming; void* turnHandler(void*); namespace nsTicTacToe { GFDCON gfd; const char X = 'X'; const char O = 'O'; string MPhostname, mySID; board TTTboard; bool PlayerIsX = true, isMyTurn; char Player = X, Player2 = O; int recon(string* datHolder = NULL, bool force = false); void initMP(bool create = false, string hn = TTTtar); void init(); bool isTie(); int turnPlayer(Index loc, char lSym = Player); bool checkWin(char sym = Player); int mainloop(); int mainloopMP(); }; // NS I made the decision to put this in a namespace to group it instead of a class because there are some parts that would not work well in OOP, and it's much easier to implement later on. I have had trouble linking the client in the past, but this setup seems to work. My server is also split into two files, Server.h and Server.cpp. Server.h contains exactly: #include "../TicTacToe/TTT.h" // Server needs a full copy of TicTacToe code class TTTserv; struct TTTachievement_requirement { Index id; Index loc; bool inUse; }; struct TTTachievement_t { Index id; bool achieved; bool AND, inSameGame; bool inUse; bool (*lHandler)(TTTserv*); char mustBeSym; int mustBePlayer; string name, description; TTTachievement_requirement steps[safearray(8*8)]; }; class achievement_core_t : public GfdOogleTech { public: // May be shifted to private TTTachievement_t list[safearray(8*8)]; public: achievement_core_t(); int insert(string name, string d, bool samegame, bool lAnd, int lSteps[8*8], int mbP=0, char mbS=0); }; struct TTTplayer_t { Index id; bool inUse; string ip, sessionID; char sym; int desc; TTTachievement_t Ding[8*8]; }; struct TTTgame_t { TTTplayer_t Player[safearray(2)]; TTTplayer_t Spectator; achievement_core_t achievement_core; Index cTurn, players; port_t roomLoc; bool inGame, Xused, Oused, newEvent; }; class TTTserv : public gSserver { TTTgame_t Game; TTTplayer_t *cPlayer; port_t conPort; public: achievement_core_t *achiev; thread threads[8]; int parseit(string tDat, string tsIP); Index conCount; int parseit(string tDat, int tlUser, TTTplayer_t** retval); private: int parseProto(string dat, string sIP); int parseProto(string dat, int lUser); int cycleTurn(); void setup(port_t lPort = 0, bool complete = false); public: int newEvent; TTTserv(port_t tlPort = TTTport, bool tcomplete = true); TTTplayer_t* userDC(Index id, Index force = false); int sendToPlayers(string dat, bool asMSG = false); int mainLoop(volatile bool *play); }; // Other void* userHandler(void*); void* handleUser(void*); And in the CPP file I include Server.h and provide main() and the contents of all functions previously declared. Now to the problem at hand I am having issues when linking my server. More specifically, I get a duplicate symbol error for every variable in nsTicTacToe (and possibly in gfdGaming as well). Since I need the TicTacToe functions, I link Client.cpp ( without main() ) when building the server ld: duplicate symbol nsTicTacToe::PlayerIsX in Client.o and Server.o collect2: ld returned 1 exit status Command /Developer/usr/bin/g++-4.2 failed with exit code 1 It stops once a problem is encountered, but if PlayerIsX is removed / changed temporarily than another variable causes an error Essentially, I am looking for any advice on how to better organize my code to hopefully fix these errors. Disclaimers: -I apologize in advance if I provided too much or too little information, as it is my first time posting -I have tried using static and extern to fix these problems, but apparently those are not what I need Thank you to anyone who takes the time to read all of this and respond =)

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  • Change text_factory in Django/sqlite

    - by Krumelur
    I have a django project that uses a sqlite database that can be written to by an external tool. The text is supposed to be UTF-8, but in some cases there will be errors in the encoding. The text is from an external source, so I cannot control the encoding. Yes, I know that I could write a "wrapping layer" between the external source and the database, but I prefer not having to do this, especially since the database already contains a lot of "bad" data. The solution in sqlite is to change the text_factory to something like: lambda x: unicode(x, "utf-8", "ignore") However, I don't know how to tell the Django model driver this.

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  • Rails / JBuilder - Entity array with has_many attributes

    - by seufagner
    I have two models, Person and Image and I want return an json array of Persons with your Images. But I dont want return all Image attributes, but produces a different result. Code below: class Person < ActiveRecord::Base has_many :images, as: :imageable validates :name, presence: true accepts_nested_attributes_for :images, :reject_if => lambda { |img| img['asset'].blank? } end class Image < ActiveRecord::Base belongs_to :imageable, polymorphic: true mount_uploader :asset, ImageUploader validates :asset, presence: true end zzz.jbuilder.json template json.persons(@rodas, :id, :name, :images) json produced: { "rodas": [{ "id": 4, "name": "John", "images": [ { "asset": { "url": "/uploads/image/xxxx.png" } }, { "asset": { "url": "/uploads/image/yyyyy.jpeg" } } ]}, { "id": 19, "name": "Mary", "images": [ { "asset": { "url": "/uploads/image/kkkkkkk.png" } } ] }] } I want something like: { "rodas": [ { "id": 4, "name": "John", "images": [ "/uploads/image/xxxx.png" , "/uploads/image/yyyy.jpeg" ] }, { "id": 10, "name": "Mary", "images": [ "/uploads/image/dddd.png" , "/uploads/image/xxxx.jpeg" ] } ]}

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  • Should I use C++0x Features Now?

    - by svu2g
    With the official release of VS 2010, is it safe for me to start using the partially-implemented C++0x feature set in my new code? The features that are of interest to me right now are both implemented by VC++ 2010 and recent versions of GCC. These are the only two that I have to support. In terms of the "safety" mentioned in the first sentence: can I start using these features (e.g., lambda functions) and still be guaranteed that my code will compile in 10 years on a compiler that properly conforms to C++0x when it is officially released? I guess I'm asking if there is any chance that VC++ 2010 or GCC will end up like VC++ 6; it was released before the language was officially standardized and consequently allowed grossly ill-formed code to compile. After all, Microsoft does say that "10 is the new 6". ;)

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  • What, if any, is wrong with this definition of letrec in Scheme?

    - by Lajla
    R5RS gives proposed macro definitions for library forms of syntax: http://schemers.org/Documents/Standards/R5RS/HTML/r5rs-Z-H-10.html#%_sec_7.3 Which also defines letrec, in a very complicated way, certainly not how I would define it, I would simply use: (define-syntax letrec2 (syntax-rules () ((letrec2 ((name val) ...) body bodies ...) ((lambda () (define name val) ... body bodies ...))))) As far as I understand the semantics of letrec, which I use very often as a named let. It works in this way, however as I've had my fair share of debates with philosophers who think they can just disprove special relativity or established phonological theories, I know that when you think you have a simple solution to a complex problem, it's probably WRONG. There has got to be some point where this macro does not satify the semantics of letrec else they'd probably have used it. In this definition, the definitions are local to the body of the letrec, they can refer to each other for mutual recursion, I'm not quite sure what (if any) is wrong.

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  • How to binding to bit datatype in SQL to Booleans/CheckBoxes in ASP.NET MVC 2 with DataAnnotations

    - by nvtthang
    I've got problem with binding data type boolean to checkbox in MVC 2 data annotations Here's my code sample: label> Is Hot </label> <%=Html.CheckBoxFor(model => model.isHot, new {@class="input" })%> It always raise this error message below at (model=model.isHot). Cannot convert lambda expression to delegate type 'System.Func<Framework.Models.customer,bool>' because some of the return types in the block are not implicitly convertible to the delegate return type Cannot implicitly convert type 'bool?' to 'bool'. An explicit conversion exists (are you missing a cast?) Please suggest me how can I solve this problem? Thanks in advance.

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  • How to always return a set number of records when using find_related_tags with acts-as-taggable-on

    - by hadees
    I'm using the acts-as-taggable-on gem and I need to use find_related_tags on my survey model to get back 3 surveys every time. In the event there aren't always 3 related I need to pick how ever many are related plus some random ones to get to 3. Additionally I have a method I wrote called completed_survey_ids which return an array of survey_ids that shouldn't be used because the user has already completed them. Also there is a rare case that there won't be enough surveys because the user has completed them all so in that event it is okay to return less surveys then requested. I did write a named_scope to handle getting rid of the completed_survey_ids that I think works named_scope :not, lambda { |survey_ids| {:conditions => "id NOT IN (#{survey_ids.join(',')})" } }

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  • How to save and load an array of complex numbers using numpy.savetxt?

    - by ptomato
    I want to use numpy.savetxt() to save an array of complex numbers to a text file. Problems: If you save the complex array with the default format string, the imaginary part is discarded. If you use fmt='%s', then numpy.loadtxt() can't load it unless you specify dtype=complex, converters={0: lambda s: complex(s)}. Even then, if there are NaN's in the array, loading still fails. It looks like someone has inquired about this multiple times on the Numpy mailing list and even filed a bug, but has not gotten a response. Before I put something together myself, is there a canonical way to do this?

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  • C++ boost or STL `y += f(x)` type algorithm

    - by aaa
    hello. I know I can do this y[i] += f(x[i]) using transform with two input iterators. however it seems somewhat counterintuitive and more complicated than for loop. Is there a more natural way to do so using existing algorithm in boost or Stl. I could not find clean equivalent. here is transform (y = y + a*x): using boost::lambda; transform(y.begin(), y.end(), x.begin(), y.begin(), (_1 + scale*_2); // I thought something may exist: transform2(x.begin(), x.end(), y.begin(), (_2 + scale*_1); // it does not, so no biggie. I will write wrapper Thanks

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  • Error using MVCContrib TestHelper

    - by Brian McCord
    While trying to implement the second answer to a previous question, I am receiving an error. I have implemented the methods just as the post shows, and the first three work properly. The fourth one (HomeController_Delete_Action_Handler_Should_Redirect_If_Model_Successfully_Delete) gives this error: Could not find a parameter named 'controller' in the result's Values collection. If I change the code to: actual .AssertActionRedirect() .ToAction("Index"); it works properly, but I don't like the "magic string" in there and prefer to use the lambda method that the other poster used. My controller method looks like this: [HttpPost] public ActionResult Delete(State model) { try { if( model == null ) { return View( model ); } _stateService.Delete( model ); return RedirectToAction("Index"); } catch { return View( model ); } } What am I doing wrong?

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  • How to handle failure to release a resource which is contained in a smart pointer?

    - by cj
    How should an error during resource deallocation be handled, when the object representing the resource is contained in a shared pointer? Smart pointers are a useful tool to manage resources safely. Examples of such resources are memory, disk files, database connections, or network connections. // open a connection to the local HTTP port boost::shared_ptr<Socket> socket = Socket::connect("localhost:80"); In a typical scenario, the class encapsulating the resource should be noncopyable and polymorphic. A good way to support this is to provide a factory method returning a shared pointer, and declare all constructors non-public. The shared pointers can now be copied from and assigned to freely. The object is automatically destroyed when no reference to it remains, and the destructor then releases the resource. /** A TCP/IP connection. */ class Socket { public: static boost::shared_ptr<Socket> connect(const std::string& address); virtual ~Socket(); protected: Socket(const std::string& address); private: // not implemented Socket(const Socket&); Socket& operator=(const Socket&); }; But there is a problem with this approach. The destructor must not throw, so a failure to release the resource will remain undetected. A common way out of this problem is to add a public method to release the resource. class Socket { public: virtual void close(); // may throw // ... }; Unfortunately, this approach introduces another problem: Our objects may now contain resources which have already been released. This complicates the implementation of the resource class. Even worse, it makes it possible for clients of the class to use it incorrectly. The following example may seem far-fetched, but it is a common pitfall in multi-threaded code. socket->close(); // ... size_t nread = socket->read(&buffer[0], buffer.size()); // wrong use! Either we ensure that the resource is not released before the object is destroyed, thereby losing any way to deal with a failed resource deallocation. Or we provide a way to release the resource explicitly during the object's lifetime, thereby making it possible to use the resource class incorrectly. There is a way out of this dilemma. But the solution involves using a modified shared pointer class. These modifications are likely to be controversial. Typical shared pointer implementations, such as boost::shared_ptr, require that no exception be thrown when their object's destructor is called. Generally, no destructor should ever throw, so this is a reasonable requirement. These implementations also allow a custom deleter function to be specified, which is called in lieu of the destructor when no reference to the object remains. The no-throw requirement is extended to this custom deleter function. The rationale for this requirement is clear: The shared pointer's destructor must not throw. If the deleter function does not throw, nor will the shared pointer's destructor. However, the same holds for other member functions of the shared pointer which lead to resource deallocation, e.g. reset(): If resource deallocation fails, no exception can be thrown. The solution proposed here is to allow custom deleter functions to throw. This means that the modified shared pointer's destructor must catch exceptions thrown by the deleter function. On the other hand, member functions other than the destructor, e.g. reset(), shall not catch exceptions of the deleter function (and their implementation becomes somewhat more complicated). Here is the original example, using a throwing deleter function: /** A TCP/IP connection. */ class Socket { public: static SharedPtr<Socket> connect(const std::string& address); protected: Socket(const std::string& address); virtual Socket() { } private: struct Deleter; // not implemented Socket(const Socket&); Socket& operator=(const Socket&); }; struct Socket::Deleter { void operator()(Socket* socket) { // Close the connection. If an error occurs, delete the socket // and throw an exception. delete socket; } }; SharedPtr<Socket> Socket::connect(const std::string& address) { return SharedPtr<Socket>(new Socket(address), Deleter()); } We can now use reset() to free the resource explicitly. If there is still a reference to the resource in another thread or another part of the program, calling reset() will only decrement the reference count. If this is the last reference to the resource, the resource is released. If resource deallocation fails, an exception is thrown. SharedPtr<Socket> socket = Socket::connect("localhost:80"); // ... socket.reset();

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