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• Haskell Cons Operator (:)

  - by Carson Myers

  I am really new to Haskell (Actually I saw "Real World Haskell" from O'Reilly and thought "hmm, I think I'll learn functional programming" yesterday) and I am wondering: I can use the construct operator to add an item to the beginning of a list: 1 : [2,3] [1,2,3] I tried making an example data type I found in the book and then playing with it: --in a file data BillingInfo = CreditCard Int String String | CashOnDelivery | Invoice Int deriving (Show) --in ghci $ let order_list = [Invoice 2345] $ order_list [Invoice 2345] $ let order_list = CashOnDelivery : order_list $ order_list [CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, CashOnDelivery, ...- etc... it just repeats forever, is this because it uses lazy evaluation? -- EDIT -- okay, so it is being pounded into my head that let order_list = CashOnDelivery:order_list doesn't add CashOnDelivery to the original order_list and then set the result to order_list, but instead is recursive and creates an infinite list, forever adding CashOnDelivery to the beginning of itself. Of course now I remember that Haskell is a functional language and I can't change the value of the original order_list, so what should I do for a simple "tack this on to the end (or beginning, whatever) of this list?" Make a function which takes a list and BillingInfo as arguments, and then return a list? -- EDIT 2 -- well, based on all the answers I'm getting and the lack of being able to pass an object by reference and mutate variables (such as I'm used to)... I think that I have just asked this question prematurely and that I really need to delve further into the functional paradigm before I can expect to really understand the answers to my questions... I guess what i was looking for was how to write a function or something, taking a list and an item, and returning a list under the same name so the function could be called more than once, without changing the name every time (as if it was actually a program which would add actual orders to an order list, and the user wouldn't have to think of a new name for the list each time, but rather append an item to the same list).

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• Programming texts and reference material for my Kindle DX, creating the ultimate reference device?

  - by mwilliams

  (Revisiting this topic with the release of the Kindle DX) Having owned both generation Kindle readers and now getting a Kindle DX; I'm very excited for true PDF handling on an e-ink device! An image of _Why's book on my Kindle (from my iPhone). This gives me a device capable of storing hundreds of thousands of pages that are full text search capable in the form factor of a magazine. What references (preferably PDF to preserve things such as code samples) would you recommend? Ultimately I would like reference material for every modern and applicable programming language (C, C++, Objective-C, Python, Ruby, Java, .NET (C#, Visual Basic, ASP.NET), Erlang, SQL references) as well as general programming texts and frameworks (algorithms, design patterns, theory, Rails, Django, Cocoa, ORMs, etc) and anything else that could be thought of. With so many developers here using such a wide array of languages, as a professional in your particular field, what books or references would you recommend to me for my Kindle? Creative Commons material a plus (translate that to free) as well as the material being in the PDF file format. File size is not an issue. If this turns out to be a success, I will update with a follow-up with a compiled list generated from all of the answers. Thanks for the assistance and contributing! UPDATE I have been using the Kindle DX a lot now for technical books. Check out this blog post I did for high resolution photos of different material: http://www.matthewdavidwilliams.com/2009/06/12/technical-document-pdfs-on-the-kindle-dx/

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• Pascals Triangle by recursion

  - by Olpers

  Note : My Class Teacher gave me this question as an assignment... I am not asked to do it but please tell me how to do it with recursion Binomial coefficients can be calculated using Pascal's triangle: 1 n = 0 1 1 1 2 1 1 3 3 1 1 4 6 4 1 n = 4 Each new level of the triangle has 1's on the ends; the interior numbers are the sums of the two numbers above them. Task: Write a program that includes a recursive function to produce a list of binomial coefficients for the power n using the Pascal's triangle technique. For example, Input = 2 Output = 1 2 1 Input = 4 Output = 1 4 6 4 1 done this So Far but tell me how to do this with recursion... #include<stdio.h> int main() { int length,i,j,k; //Accepting length from user printf("Enter the length of pascal's triangle : "); scanf("%d",&length); //Printing the pascal's triangle for(i=1;i<=length;i++) { for(j=1;j<=length-i;j++) printf(" "); for(k=1;k<i;k++) printf("%d",k); for(k=i;k>=1;k--) printf("%d",k); printf("\n"); } return 0; }

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• Does it ever make sense to make a fundamental (non-pointer) parameter const?

  - by Scott Smith

  I recently had an exchange with another C++ developer about the following use of const: void Foo(const int bar); He felt that using const in this way was good practice. I argued that it does nothing for the caller of the function (since a copy of the argument was going to be passed, there is no additional guarantee of safety with regard to overwrite). In addition, doing this prevents the implementer of Foo from modifying their private copy of the argument. So, it both mandates and advertises an implementation detail. Not the end of the world, but certainly not something to be recommended as good practice. I'm curious as to what others think on this issue. Edit: OK, I didn't realize that const-ness of the arguments didn't factor into the signature of the function. So, it is possible to mark the arguments as const in the implementation (.cpp), and not in the header (.h) - and the compiler is fine with that. That being the case, I guess the policy should be the same for making local variables const. One could make the argument that having different looking signatures in the header and source file would confuse others (as it would have confused me). While I try to follow the Principle of Least Astonishment with whatever I write, I guess it's reasonable to expect developers to recognize this as legal and useful.

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• Best Functional Approach

  - by dbyrne

  I have some mutable scala code that I am trying to rewrite in a more functional style. It is a fairly intricate piece of code, so I am trying to refactor it in pieces. My first thought was this: def iterate(count:Int,d:MyComplexType) = { //Generate next value n //Process n causing some side effects return iterate(count - 1, n) } This didn't seem functional at all to me, since I still have side effects mixed throughout my code. My second thought was this: def generateStream(d:MyComplexType):Stream[MyComplexType] = { //Generate next value n return Stream.cons(n, generateStream(n)) } for (n <- generateStream(initialValue).take(2000000)) { //process n causing some side effects } This seemed like a better solution to me, because at least I've isolated my functional value-generation code from the mutable value-processing code. However, this is much less memory efficient because I am generating a large list that I don't really need to store. This leaves me with 3 choices: Write a tail-recursive function, bite the bullet and refactor the value-processing code Use a lazy list. This is not a memory sensitive app (although it is performance sensitive) Come up with a new approach. I guess what I really want is a lazily evaluated sequence where I can discard the values after I've processed them. Any suggestions?

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• Are certain open-source licenses more suitable than others for career growth?

  - by Francisco Garcia

  As a software engineer/programmer myself, I love the possibility to download the code and learn from it. However building software is what brings food to my table. I have doubts regarding the type of license I should use for my own personal projects or when picking up one project to learn from. There are already many questions about licenses on Stackoverflow, but I would like to make this one much more specific. If your main profession and way of living is building software: which type of license do you find more useful for you? And I mean, the license that can benefit you most as a professional because it gives you more freedom to reuse the experience you gain. GPL is a great license to build communities because it forces you to give back your work. However I like BSD licenses because of their extra freedom. I know that if the code I am exploring is BSD licensed, I might be able to expand not only my skills, but also my programmer toolbox. Whenever I am working for a company, I might recall that something similar was done in another project and I will be able to copy or imitate certain part of the code. I know that there are religious wars regarding GPL vs BSD and it is not my intention to start one. Probably many companies already take snipsets from GPL projects anyway. I just want to insist in the factor of professional enrichment. I do not intend to discriminate any license. I said I prefer BSD licenses but I also use Linux because the user base is bigger and also the market demand.

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• Scheme: Mysterious void in pattern match.

  - by Schemer

  Hi. I am writing a function called annotate that uses match-lambda -- often with recursive calls to annotate. Here is one of the pattern matches: (`(lambda (,<param1> . ,<params>) ,<stmts>) `(CLOSURE ENV (,<param1> . ,<params>) (lambda (ENV) ,(map annotate (map (lambda (x) (append `(,<param1> . ,<params>) (list x))) `(,<stmts>)))))) However, when this pattern is matched this is what returns: '(CLOSURE ENV (x) (lambda (ENV) ((CLOSURE ENV (x y) (lambda (ENV) ((+ x y)))))) #<void>) Specifically I can't figure out where "void" is coming from. In fact, if I include the line: ,(displayln (map annotate (map (lambda (x) (append `(,<param1> . ,<params>) (list x))) `(,<stmts>)))) it prints: ((CLOSURE ENV (x y) (lambda (ENV) ((+ x y))))) notably without "void". If someone could tell me what the problem is it would be greatly appreciated. Thanks.

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• Matlab fft function

  - by CTZStef

  The code below is from the Matlab 2011a help about fft function. I think there is a problem here : why do they multiply t(1:50) by Fs, and then say it's time in millisecond ? Certainly, it happens to be true in this very particular case, but change the value of Fs to, say, 2000, and it won't work anymore, obviously because of this factor of 2. Right ? Quite misleading, isn't it ? What do I miss ? Fs = 1000; % Sampling frequency T = 1/Fs; % Sample time L = 1000; % Length of signal t = (0:L-1)*T; % Time vector % Sum of a 50 Hz sinusoid and a 120 Hz sinusoid x = 0.7*sin(2*pi*50*t) + sin(2*pi*120*t); y = x + 2*randn(size(t)); % Sinusoids plus noise plot(Fs*t(1:50),y(1:50)) title('Signal Corrupted with Zero-Mean Random Noise') xlabel('time (milliseconds)') Clearer with this : fs = 2000; % Sampling frequency T = 1 / fs; % Sample time L = 1000; % Length of signal t2 = (0:L-1)*T; % Time vector f = 50; % signal frequency s2 = sin(2*pi*f*t2); figure, plot(fs*t2(1:50),s2(1:50)); % NOT good figure, plot(t2(1:50),s2(1:50)); % good

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• Multiple Solution Layout for ASP.NET Web Portal?

  - by Jared S

  At work, we've developed a custom ASP.NET Web Portal (That's very similar to iGoogle). We have "Apps" (self-contained, large web forms) and "Modules" (similar to Google Gadgets). Currently, we use a single-solution model. Right now, we have: 3 core projects 60 application projects 80 module projects To reduce copy and pasting between projects, we're going to factor out common functionality (Data Access, Business Logic) into separate projects. I'd also like to introduce Unit Tests, which is going to increase the number of projects even more. We've already reached the point where Visual Studio is choking on the number of projects. We generally only load the 3 core projects and then whatever app's/module's project we're working on. Would a different solution structure help us out? Our number of projects is only going to increase. In general, an app or module only references the 3 core projects. Soon, apps/modules may start referencing the Data Access/Business Logic projects. But in general, apps and modules do not make references between themselves. So to recap, what is the best practice for solution structure when there are MANY projects that use a small number of core projects?

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• How to write a simple Lexer/Parser with antlr 2.7?

  - by Burkhard

  Hello, I have a complex grammar (in antlr 2.7) which I need to extend. Having never used antlr before, I wanted to write a very simple Lexer and Parser first. I found a very good explanation for antlr3 and tried to adapt it: header{ #include <iostream> using namespace std; } options { language="Cpp"; } class P2 extends Parser; /* This will be the entry point of our parser. */ eval : additionExp ; /* Addition and subtraction have the lowest precedence. */ additionExp : multiplyExp ( "+" multiplyExp | "-" multiplyExp )* ; /* Multiplication and addition have a higher precedence. */ multiplyExp : atomExp ( "*" atomExp | "/" atomExp )* ; /* An expression atom is the smallest part of an expression: a number. Or when we encounter parenthesis, we're making a recursive call back to the rule 'additionExp'. As you can see, an 'atomExp' has the highest precedence. */ atomExp : Number | "(" additionExp ")" ; /* A number: can be an integer value, or a decimal value */ number : ("0".."9")+ ("." ("0".."9")+)? ; /* We're going to ignore all white space characters */ protected ws : (" " | "\t" | "\r" | "\n") { newline(); } ; It does generate four files without errors: P2.cpp, P2.hpp, P2TokenTypes.hpp and P2TokenTypes.txt. But now what? How do I create a working programm with that? I tried to add these files to a VS2005-WinConsole-Project but it does not compile: p2.cpp(277) : fatal error C1010: unexpected end of file while looking for precompiled header. Did you forget to add '#include "stdafx.h"' to your source?

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• Entity Framework LINQ Query using Custom C# Class Method - Once yes, once no - because executing on the client or in SQL?

  - by BrooklynDev

  I have two Entity Framework 4 Linq queries I wrote that make use of a custom class method, one works and one does not: The custom method is: public static DateTime GetLastReadToDate(string fbaUsername, Discussion discussion) { return (discussion.DiscussionUserReads.Where(dur => dur.User.aspnet_User.UserName == fbaUsername).FirstOrDefault() ?? new DiscussionUserRead { ReadToDate = DateTime.Now.AddYears(-99) }).ReadToDate; } The linq query that works calls a from after a from, the equivalent of SelectMany(): from g in oc.Users.Where(u => u.aspnet_User.UserName == fbaUsername).First().Groups from d in g.Discussions select new { UnReadPostCount = d.Posts.Where(p => p.CreatedDate > DiscussionRepository.GetLastReadToDate(fbaUsername, p.Discussion)).Count() }; The query that does not work is more like a regular select: from d in oc.Discussions where d.Group.Name == "Student" select new { UnReadPostCount = d.Posts.Where(p => p.CreatedDate > DiscussionRepository.GetLastReadToDate(fbaUsername, p.Discussion)).Count(), }; The error I get is: LINQ to Entities does not recognize the method 'System.DateTime GetLastReadToDate(System.String, Discussion)' method, and this method cannot be translated into a store expression. My question is, why am I able to use my custom GetLastReadToDate() method in the first query and not the second? I suppose this has something to do with what gets executed on the db server and what gets executed on the client? These queries seem to use the GetLastReadToDate() method so similarly though, I'm wondering why would work for the first and not the second, and most importantly if there's a way to factor common query syntax like what's in the GetLastReadToDate() method into a separate location to be reused in several different places LINQ queries. Please note all these queries are sharing the same object context.

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• Out-of-memory algorithms for addressing large arrays

  - by reve_etrange

  I am trying to deal with a very large dataset. I have k = ~4200 matrices (varying sizes) which must be compared combinatorially, skipping non-unique and self comparisons. Each of k(k-1)/2 comparisons produces a matrix, which must be indexed against its parents (i.e. can find out where it came from). The convenient way to do this is to (triangularly) fill a k-by-k cell array with the result of each comparison. These are ~100 X ~100 matrices, on average. Using single precision floats, it works out to 400 GB overall. I need to 1) generate the cell array or pieces of it without trying to place the whole thing in memory and 2) access its elements (and their elements) in like fashion. My attempts have been inefficient due to reliance on MATLAB's eval() as well as save and clear occurring in loops. for i=1:k [~,m] = size(data{i}); cur_var = ['H' int2str(i)]; %# if i == 1; save('FileName'); end; %# If using a single MAT file and need to create it. eval([cur_var ' = cell(1,k-i);']); for j=i+1:k [~,n] = size(data{j}); eval([cur_var '{i,j} = zeros(m,n,''single'');']); eval([cur_var '{i,j} = compare(data{i},data{j});']); end save(cur_var,cur_var); %# Add '-append' when using a single MAT file. clear(cur_var); end The other thing I have done is to perform the split when mod((i+j-1)/2,max(factor(k(k-1)/2))) == 0. This divides the result into the largest number of same-size pieces, which seems logical. The indexing is a little more complicated, but not too bad because a linear index could be used. Does anyone know/see a better way?

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• How to transform a production to LL(1) grammar for a list separated by a semicolon?

  - by Subb

  Hi, I'm reading this introductory book on parsing (which is pretty good btw) and one of the exercice is to "build a parser for your favorite language." Since I don't want to die today, I thought I could do a parser for something relatively simple, ie a simplified CSS. Note: This book teach you how to right a LL(1) parser using the recursive-descent algorithm. So, as a sub-exercice, I am building the grammar from what I know of CSS. But I'm stuck on a production that I can't transform in LL(1) : //EBNF block = "{", declaration, {";", declaration}, [";"], "}" //BNF <block> =:: "{" <declaration> "}" <declaration> =:: <single-declaration> <opt-end> | <single-declaration> ";" <declaration> <opt-end> =:: "" | ";" This describe a CSS block. Valid block can have the form : { property : value } { property : value; } { property : value; property : value } { property : value; property : value; } ... The problem is with the optional ";" at the end, because it overlap with the starting character of {";", declaration}, so when my parser meet a semicolon in this context, it doesn't know what to do. The book talk about this problem, but in its example, the semicolon is obligatory, so the rule can be modified like this : block = "{", declaration, ";", {declaration, ";"}, "}" So, Is it possible to achieve what I'm trying to do using a LL(1) parser?

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• Difference between two PHP times as years, months and days for PHP version 5.2

  - by Dominor Novus

  Forward: I've scanned through the existing questions/answers on this matter. This is not a duplicitous question; I cannot find a working solution from the accepted answers. The main questions/answers I've reviewed can be found here: How to calculate the difference between two dates using PHP? What I need: A calucalation of the difference between two dates expressed as years, months and days that works with PHP version: 5.2. <?php $current_date = date('d-M-Y'); $future_date = '2012-11-01'; ?> What I've tried: Most answers I find online don't seem to exact in that they don't factor in leap years. This highly rated answer won't work because DateTime-diff() is php 5.3+. This accepted answer results in (i.e. the second block of code aimed at PHP 5.2) results in the following being parsed: Array ( [y] = 25 [m] = 11 [d] = 7 [h] = 3 [i] = 15 [s] = 19 [invert] = 0 [days] = 9473 ) Array ( [y] = 25 [m] = 11 [d] = 7 [h] = 3 [i] = 15 [s] = 19 [invert] = 1 [days] = 9473 ) I can't tell if I've incorrectly applied the code or it's simply a case of me not knowing how to manipulate the array.

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• A scripting engine for Ruby?

  - by Earlz

  Hello, I am creating a Ruby On Rails website, and for one part it needs to be dynamic so that (sorta) trusted users can make parts of the website work differently. For this, I need a scripting language. In a sort of similar project in ASP.Net, I wrote my own scripting language/DSL. I can not use that source code(written at work) though, and I don't want to make another scripting language if I don't have to. So, what choices do I have? The scripting must be locked down and not be able to crash my server or anything. I'd really like if I could use Ruby as the scripting language, but it's not strictly necessary. Also, this scripting part will be called on almost every request for the website, sometimes more than once. So, speed is a factor. I looked at the RubyLuaBridge but it is Alpha status and seems dead. What choices for a scripting language do I have in a Ruby project? Also, I will have full control over where this project is deployed(root access), so there are no real limits..

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• How to handle build rule with unknown targets in OMake when target list generator is built

  - by Michael E

  I have a project which uses OMake for its build system, and I am trying to handle a rather tough corner case. I have some definition files and a tool which can take these definition files and create GraphViz files. There are two problems, though: Each definition file can produce multiple graphs, and the list of graphs it can produce is encoded in the file. My dump tool does have a -list option which lists all the graphs a definition file will produce. This dump tool is built in the source tree. I want this list available in the OMakefile so I can use other rules to convert the DOT files to SVG, and have a phony target depend on all the SVGs (goal: a single build command which builds SVG descriptions of all my graphs). If I only had the first problem, it would be easy - I would run the tool to build a list, and then use that list to build a target which invokes the dumper to output the GraphViz files. However, I am rather stuck on forcing the dump tool to be built before it is needed. If this were make, I would just run make recursively to build the dump tool. OMake does not allow recursive invocation, however, and the build function is only usable from osh. Any suggestions for a good solution to this problem?

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• Why is my quick sort so slow?

  - by user513075

  Hello, I am practicing writing sorting algorithms as part of some interview preparation, and I am wondering if anybody can help me spot why this quick sort is not very fast? It appears to have the correct runtime complexity, but it is slower than my merge sort by a constant factor of about 2. I would also appreciate any comments that would improve my code that don't necessarily answer the question. Thanks a lot for your help! Please don't hesitate to let me know if I have made any etiquette mistakes. This is my first question here. private class QuickSort implements Sort { @Override public int[] sortItems(int[] ts) { List<Integer> toSort = new ArrayList<Integer>(); for (int i : ts) { toSort.add(i); } toSort = partition(toSort); int[] ret = new int[ts.length]; for (int i = 0; i < toSort.size(); i++) { ret[i] = toSort.get(i); } return ret; } private List<Integer> partition(List<Integer> toSort) { if (toSort.size() <= 1) return toSort; int pivotIndex = myRandom.nextInt(toSort.size()); Integer pivot = toSort.get(pivotIndex); toSort.remove(pivotIndex); List<Integer> left = new ArrayList<Integer>(); List<Integer> right = new ArrayList<Integer>(); for (int i : toSort) { if (i > pivot) right.add(i); else left.add(i); } left = partition(left); right = partition(right); left.add(pivot); left.addAll(right); return left; } }

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• TortoiseSVN Error: "OPTIONS of 'https://...' could not connect to server (...)"

  - by Zack Peterson

  I'm trying to setup a new computer to synchronize with my SVN repository that's hosted with cvsdude.com. I get this error: Here's what I did (these have worked in the past): Downloaded and installed TortoiseSVN Created a new folder C:\aspwebsite Right-clicked, chose SVN Checkout... Entered the following information, clicked OK: URL of repository: https://<reponame>-svn.cvsdude.com/aspwebsite Checkout directory: C:\aspwebsite Checkout depth: Fully recursive Omit externals: Unchecked Revision: HEAD revision Got TortoiseSVN error: OPTIONS of 'https://<reponame>-svn.cvsdude.com/aspwebsite': could not connect to server (https://<reponame>-svn.cvsdude.com) Rather than getting the error, TortoiseSVN should have asked for my username and password and then downloaded about 90MB. Why can't I checkout from my Subversion repository? Kent Fredric wrote: Either their security certificate has expired, or their hosting is broken/down. Contact CVSDude and ask them whats up. It could also be a timeout, because for me their site is exhaustively slow.. It errors after only a couple seconds. I don't think it's a timeout. Matt wrote: Try visiting https://[redacted]-svn.cvsdude.com/aspwebsite and see what happens. If you can visit it in your browser, you ought to be able to get the files in your SVN client and we can work from there. If it fails, then there's your answer. I can access the site in a web browser.

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• Why might one app connect to SQL backend OK and a second app fail if they share the same connectionstring?

  - by hawbsl

  Trying to figure out a SQL connection error 26 in our app. We've got two closely related apps Foo and FooAddIn. Foo is a Winforms app built in VS2010 and runs fine and connects fine to our SQLExpress back end. FooAddIn is an Outlook AddIn which references Foo.exe and connects to the same SQL Express instance. Or rather, it doesn't connect, instead reporting: A network-related or instance-specific error occurred while establishing a connection to SQL Server. The server was not found or was not accessible. Verify that the instance name is correct and that SQL Server is configured to allow remote connections. (provider: SQL Network Interfaces, error: 26 - Error Locating Server/Instance Specified) Now, both apps share the same connectionstring and we've verified they really do share the same connectionstring. At this stage we're just testing from within the same developer machine, so the apps are on the same machine, going via the same VS2010 IDE. So a lot of the advice online for this error doesn't apply because the fact that Foo connects through to SQL Express tells us the database is there and available and can be reached. What else is there to check? One thing is that Foo and FooAddIn are running different runtime versions of System.Data (v2.0.50727 and v4.0.30319). Could that be a factor?

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• Avoiding resource (localizable string) duplication with String.Format

  - by Hrvoje Prgeša

  I'm working on a application (.NET, but not relevant) where there is large potential for resource/string duplication - most of these strings are simple like: Volume: 33 Volume: 33 (dB) Volume 33 dB Volume (dB) Command - Volume: 33 (dB) where X, Y and unit are the same. Should I define a new resource for each of the string or is it preferable to use String.Format to simplify some of these, eg.: String.Format("{0}: {1}", Resource.Volume, 33) String.Format("{0}: {1} {2}", Resource.Volume, 33, Resource.dB) Resource.Volume String.Format("{0} ({1})", 33, Resource.dB) String.Format("{0} ({1})", Resource.Volume, Resource.dB) String.Format("Command - {0}: {1} {2}", Resource.Volume, 33, Resource.dB) I would also define string formats like "{0}: {1}" in the resources so there would be a possibility of reordering words... I would not use this approach selectivly and not throughout the whole application.. And how about: String.Format("{0}: {1}", Volume, Resource.Muted_Volume) // = Volume: Muted Resource.Muted_Volume String.Format("{0}: {1} (by user {2})", Volume, Resource.Muted_Volume, "xy") // = Volume: Muted (by user xy) The advantage is cutting the number of resource by the factor of 4-5. Are there any hidden dangers of using this approach? Could someone give me an example (language) where this would not work correctly?

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• Has anyone ever had OpenCV work with Python 2.7 on MacOS 10.6?

  - by ?????

  I've been trying on and off for the past 6 months to get OpenCV to work with Python on MacOS. Every time there's a new release, I try again and fail. I've tried both 64-bit and 32-bit, and both the xcode gcc and gcc installed via macports. I just spend the past two days on it, hopeful that the latest OpenCV release, that appears to include Python support directly would work. It doesn't. I've also tried and failed to use this: http://code.google.com/p/pyopencv/ I've been using OpenCV with C++ or Microsoft C++/CLI for the past few years, but I'd love to use it with Python on a Mac because that is my primary development environment. I'd love to hear from anyone who's actually been able to get the opencv python examples to run under Mac OS 10.6, either 32 or 64-bit. My last attempt was to follow the instructions on this page http://recursive-design.com/blog/2010/12/14/face-detection-with-osx-and-python/ with a clean, fresh install of 10.6 on a 64-bit capable Mac. My PYTHONPATH is set, and I can see the cv library in it. But an "import cv" from python fails. Previously, the closest I've ever gotten (again, staring on a clean, fresh 10.6 install) was this: Python 2.7.1 (r271:86882M, Nov 30 2010, 10:35:34) [GCC 4.2.1 (Apple Inc. build 5664)] on darwin Type "help", "copyright", "credits" or "license" for more information. >>> import cv Fatal Python error: Interpreter not initialized (version mismatch?) Abort trap thrilllap-2:~ swirsky$ I've seen a lot of folks answering similar questions here, but have never seen an definitive answer for it.

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• Why can't you return a List from a Compiled Query?

  - by Andrew

  I was speeding up my app by using compiled queries for queries which were getting hit over and over. I tried to implement it like this: Function Select(ByVal fk_id As Integer) As List(SomeEntity) Using db As New DataContext() db.ObjectTrackingEnabled = False Return CompiledSelect(db, fk_id) End Using End Function Shared CompiledSelect As Func(Of DataContext, Integer, List(Of SomeEntity)) = _ CompiledQuery.Compile(Function(db As DataContext, fk_id As Integer) _ (From u In db.SomeEntities _ Where u.SomeLinkedEntity.ID = fk_id _ Select u).ToList()) This did not work and I got this error message: Type : System.ArgumentNullException, mscorlib, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 Message : Value cannot be null. Parameter name: value However, when I changed my compiled query to return IQueryable instead of List like so: Function Select(ByVal fk_id As Integer) As List(SomeEntity) Using db As New DataContext() db.ObjectTrackingEnabled = False Return CompiledSelect(db, fk_id).ToList() End Using End Function Shared CompiledSelect As Func(Of DataContext, Integer, IQueryable(Of SomeEntity)) = _ CompiledQuery.Compile(Function(db As DataContext, fk_id As Integer) _ From u In db.SomeEntities _ Where u.SomeLinkedEntity.ID = fk_id _ Select u) It worked fine. Can anyone shed any light as to why this is? BTW, compiled queries rock! They sped up my app by a factor of 2.

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• Emulating a computer running MS-DOS

  - by Richard

  Writing emulators has always fascinated me. Now I want to write an emulator for an IBM PC and run MS-DOS on it (I've got the floppy image files). I have good experience in C++ and C and basic knowledge of assembler and the architecture of a CPU. I also know that there are thousands of emulators out there doing exactly what I want to do, but I'd be doing this for pure joy only. How much work do I have to expect? (If my goal is to boot DOS and create a text file with it, all emulated) What CPU should I emulate ? Where can I find documentation on how the machine code is organized and which opcodes mean what, so I can unpack and execute them correctly with my emulator? Does MS-DOS still run on the newest generations of processors? Would it theoretically be able to natively run on a 64-bit AMD Phenom 2 processor w/ a modern mainboard, HDD, RAM, etc.? What else, besides emulating the CPU, could be an important factor (in terms of difficulty)? I would only aim for outputting / inputting text to the system via the host system's console, no sound or other more advanced IO etc. Have you written an emulator yet? What was your first one for? How hard was it? Do you have any special tips for me? Thanks in advance

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• Return multiple IDs from a function and use the result in a query

  - by NewK

  I have this function that returns me all children of a tree node: CREATE OR REPLACE FUNCTION fn_category_get_childs_v2(id_pai integer) RETURNS integer[] AS $BODY$ DECLARE ids_filhos integer array; BEGIN SELECT array ( SELECT category_id FROM category WHERE category_id IN ( (WITH RECURSIVE parent AS ( SELECT category_id , parent_id from category WHERE category_id = id_pai UNION ALL SELECT t.category_id , t.parent_id FROM parent INNER JOIN category t ON parent.category_id = t.parent_id ) SELECT category_id FROM parent WHERE category_id <> id_pai ) ) ) into ids_filhos; return ids_filhos; END; and I would like to use it in a select statement like this: select * from teste1_elements where category_id in (select * from fn_category_get_childs_v2(12)) I've also tried this way with the same result: select * from teste1_elements where category_id=any(select * from fn_category_get_childs_v2(12))) But I get the following error: ERROR: operator does not exist: integer = integer[] LINE 1: select * from teste1_elements where category_id in (select *... ^ HINT: No operator matches the given name and argument type(s). You might need to add explicit type casts. The function returns an integer array, is that the problem? SELECT * from fn_category_get_childs_v2(12) retrieves the following array (integer[]): '{30,32,34,20,19,18,17,16,15,14}'

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• How lucene indexing ?

  - by user312140

  Hello I read some document about lucene ; also i read the document in this link ( http://lucene.sourceforge.net/talks/pisa ) . I don't really understand how lucene index documents and don't understand lucene work with which algorithm for indexing ? On above link , said lucene use this algorithm for indexing : * incremental algorithm: o maintain a stack of segment indices o create index for each incoming document o push new indexes onto the stack o let b=10 be the merge factor; M=8 for (size = 1; size < M; size *= b) { if (there are b indexes with size docs on top of the stack) { pop them off the stack; merge them into a single index; push the merged index onto the stack; } else { break; } } How this algorithm help us to have an optimize indexing ? Does lucene use B-tree algorithm or any other algorithm like that for indexing or have a paticular algorithm ? Thank you for reading my post .

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