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• Trouble on setting SSL certificates for Virtual Hosts using Apache\Phusion Passenger in localhost

  - by user502052

  I am using Ruby on Rails 3 and I would like to make to work HTTPS connections on localhost. I am using: Apache v2 + Phusion Passenger Mac OS + Snow Leopard v10.6.6 My Ruby on Rails installation use the Typhoeus gem (it is possible to use the Ruby net\http library but the result doesn't change) to make HTTP requests over HTTPS. I created self-signed ca.key, pjtname.crt and pjtname.key as detailed on the Apple website. Notice: Following instruction from the Apple website, on running the openssl req -new -key server.key -out server.csr command (see the link) at this point Common Name (eg, YOUR name) []: (this is the important one) I entered *pjtname.com so that is valid for all sub_domain of that site. In my Apache httpd.conf I have two virtual hosts configured in this way: # Secure (SSL/TLS) connections #Include /private/etc/apache2/extra/httpd-ssl.conf # # Note: The following must must be present to support # starting without SSL on platforms with no /dev/random equivalent # but a statically compiled-in mod_ssl. # <IfModule ssl_module> SSLRandomSeed startup builtin SSLRandomSeed connect builtin </IfModule> Include /private/etc/apache2/other/*.conf # Passenger configuration LoadModule passenger_module /Users/<my_user_name>/.rvm/gems/ruby-1.9.2-p136/gems/passenger-3.0.2/ext/apache2/mod_passenger.so PassengerRoot /Users/<my_user_name>/.rvm/gems/ruby-1.9.2-p136/gems/passenger-3.0.2 PassengerRuby /Users/<my_user_name>/.rvm/wrappers/ruby-1.9.2-p136/ruby # Go ahead and accept connections for these vhosts # from non-SNI clients SSLStrictSNIVHostCheck off # Ensure that Apache listens on port 443 Listen 443 # Listen for virtual host requests on all IP addresses NameVirtualHost *:80 NameVirtualHost *:443 # # PJTNAME.COM and subdomains SETTING # <VirtualHost *:443> # Because this virtual host is defined first, it will # be used as the default if the hostname is not received # in the SSL handshake, e.g. if the browser doesn't support # SNI. ServerName pjtname.com:443 DocumentRoot "/Users/<my_user_name>/Sites/pjtname.com/pjtname.com/public" ServerAdmin [email protected] ErrorLog "/private/var/log/apache2/error_log" TransferLog "/private/var/log/apache2/access_log" RackEnv development <Directory "/Users/<my_user_name>/Sites/pjtname.com/pjtname.com/public"> Order allow,deny Allow from all </Directory> # SSL Configuration SSLEngine on # Self Signed certificates # Server Certificate SSLCertificateFile /private/etc/apache2/ssl/wildcard.certificate/pjtname.crt # Server Private Key SSLCertificateKeyFile /private/etc/apache2/ssl/wildcard.certificate/pjtname.key # Server Intermediate Bundle SSLCertificateChainFile /private/etc/apache2/ssl/wildcard.certificate/ca.crt </VirtualHost> # HTTP Setting <VirtualHost *:80> ServerName pjtname.com DocumentRoot "/Users/<my_user_name>/Sites/pjtname.com/pjtname.com/public" RackEnv development <Directory "/Users/<my_user_name>/Sites/pjtname.com/pjtname.com/public"> Order allow,deny Allow from all </Directory> </VirtualHost> <VirtualHost *:443> ServerName users.pjtname.com:443 DocumentRoot "/Users/<my_user_name>/Sites/pjtname.com/users.pjtname.com/public" ServerAdmin [email protected] ErrorLog "/private/var/log/apache2/error_log" TransferLog "/private/var/log/apache2/access_log" RackEnv development <Directory "/Users/<my_user_name>/Sites/pjtname.com/users.pjtname.com/public"> Order allow,deny Allow from all </Directory> # SSL Configuration SSLEngine on # Self Signed certificates # Server Certificate SSLCertificateFile /private/etc/apache2/ssl/wildcard.certificate/pjtname.crt # Server Private Key SSLCertificateKeyFile /private/etc/apache2/ssl/wildcard.certificate/pjtname.key # Server Intermediate Bundle SSLCertificateChainFile /private/etc/apache2/ssl/wildcard.certificate/ca.crt </VirtualHost> # HTTP Setting <VirtualHost *:80> ServerName users.pjtname.com DocumentRoot "/Users/<my_user_name>/Sites/pjtname.com/users.pjtname.com/public" RackEnv development <Directory "/Users/<my_user_name>/Sites/pjtname.com/users.pjtname.com/public"> Order allow,deny Allow from all </Directory> </VirtualHost> In the host file I have: ## # Host Database # # localhost is used to configure the loopback interface # when the system is booting. Do not change this entry. ## 127.0.0.1 localhost 255.255.255.255 broadcasthost ::1 localhost fe80::1%lo0 localhost # PJTNAME.COM SETTING 127.0.0.1 pjtname.com 127.0.0.1 users.pjtname.com All seems to work properly because I have already set everything (I think correctly): I generated a wildcard certificate for my domains and sub-domains (in this example: *.pjtname.com) I have set base-named virtualhosts in the http.conf file listening on port :433 and :80 My browser accept certificates also if it alerts me that those aren't safe (notice: I must accept certificates for each domain\sub-domain; that is, [only] at the first time I access a domain or sub-domain over HTTPS I must do the same procedure for acceptance) and I can have access to pages using HTTPS After all this work, when I make a request using Typhoeus (I can use also the Ruby Net::Http library and the result doesn't change) from the pjtname.com RoR application: # Typhoeus request Typhoeus::Request.get("https://users.pjtname.com/") I get something like a warning about the certificate: --- &id001 !ruby/object:Typhoeus::Response app_connect_time: 0.0 body: "" code: 0 connect_time: 0.000625 # Here is the warning curl_error_message: Peer certificate cannot be authenticated with known CA certificates curl_return_code: 60 effective_url: https://users.pjtname.com/ headers: "" http_version: mock: false name_lookup_time: 0.000513 pretransfer_time: 0.0 request: !ruby/object:Typhoeus::Request after_complete: auth_method: body: ... All this means that something is wrong. So, what I have to do to avoid the "Peer certificate cannot be authenticated with known CA certificates" warning and make the HTTPS request to work? Where is\are the error\errors (I think in the Apache configuration, but where?!)? P.S.: if you need some more info, let me know.

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• How to Load a VRML model on PaperVision3d?

  - by DomingoSL

  Hello, im very new at papervision3d, i know in order to display a .DAE model i use this import flash.events.Event; import org.papervision3d.objects.parsers.DAE; [SWF(width=640, height=480, backgroundColor=0x808080, frameRate=30)] public class Earth extends PV3DARApp { private var _earth:DAE; public function Earth() { addEventListener(Event.INIT, _onInit); init('Data/camera_para.dat', 'Data/flarlogo.pat'); } private function _onInit(e:Event):void { _earth = new DAE(); _earth.load('model/amorfo.dae'); But how can i do in order to use VRML models (.WRL). Thanks.

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• What library is used for the main menu in the facebook iOS app?

  - by Seth

  I'm making an app that has more options than will easily fit into a UITabBarController. I wanted to use something like what the facebook app has for its main menu. My guess is that it isn't proprietary to facebook, because the SCVNGR app uses something similar. This library lets you re-order the icons the way you can re-order the apps from the main menu (i.e. press and hold - icons jiggle - you can drag them around). Does anyone know what library provides this UIView? I'd like to use it if possible. Thanks!

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• Calculate year for end date: PostgreSQL

  - by Dave Jarvis

  Background Users can pick dates as shown in the following screen shot: Any starting month/day and ending month/day combinations are valid, such as: Mar 22 to Jun 22 Dec 1 to Feb 28 The second combination is difficult (I call it the "tricky date scenario") because the year for the ending month/day is before the year for the starting month/day. That is to say, for the year 1900 (also shown selected in the screen shot above), the full dates would be: Dec 22, 1900 to Feb 28, 1901 Dec 22, 1901 to Feb 28, 1902 ... Dec 22, 2007 to Feb 28, 2008 Dec 22, 2008 to Feb 28, 2009 Problem Writing a SQL statement that selects values from a table with dates that fall between the start month/day and end month/day, regardless of how the start and end days are selected. In other words, this is a year wrapping problem. Inputs The query receives as parameters: Year1, Year2: The full range of years, independent of month/day combination. Month1, Day1: The starting day within the year to gather data. Month2, Day2: The ending day within the year (or the next year) to gather data. Previous Attempt Consider the following MySQL code (that worked): end_year = start_year + greatest( -1 * sign( datediff( date( concat_ws('-', year, end_month, end_day ) ), date( concat_ws('-', year, start_month, start_day ) ) ) ), 0 ) How it works, with respect to the tricky date scenario: Create two dates in the current year. The first date is Dec 22, 1900 and the second date is Feb 28, 1900. Count the difference, in days, between the two dates. If the result is negative, it means the year for the second date must be incremented by 1. In this case: Add 1 to the current year. Create a new end date: Feb 28, 1901. Check to see if the date range for the data falls between the start and calculated end date. If the result is positive, the dates have been provided in chronological order and nothing special needs to be done. This worked in MySQL because the difference in dates would be positive or negative. In PostgreSQL, the equivalent functionality always returns a positive number, regardless of their relative chronological order. Question How should the following (broken) code be rewritten for PostgreSQL to take into consideration the relative chronological order of the starting and ending month/day pairs (with respect to an annual temporal displacement)? SELECT m.amount FROM measurement m WHERE (extract(MONTH FROM m.taken) >= month1 AND extract(DAY FROM m.taken) >= day1) AND (extract(MONTH FROM m.taken) <= month2 AND extract(DAY FROM m.taken) <= day2) Any thoughts, comments, or questions? (The dates are pre-parsed into MM/DD format in PHP. My preference is for a pure PostgreSQL solution, but I am open to suggestions on what might make the problem simpler using PHP.) Versions PostgreSQL 8.4.4 and PHP 5.2.10

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• Best Educational Operating System For Study?

  - by mudge

  What's the best operating system to study in order to write your own x86 operating system from scratch?

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• MS Web Browser on Pocket PC

  - by PoweRoy

  Hi all, I'm trying to create a custom web browser for on a pocket pc in C++ MFC. When I add the Microsoft Web Browser activeX control and run the app on the pocket pc (emulator) then this error pops up: "Debug assertion failed. occcont.cpp line: 916" When I look in the debug window of VS2005: "CoCreateInstance of OLE control {8856F961-340A-11D0-A96B-00C04FD705A2} failed. Result code: 0x80040154 Is the control is properly registered? Warning: Resource items and Win32 Z-order lists are out of sync. Tab order may be not defined well." How can I use this control on a pocket pc? Or is there a similar control that works?

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• Lucene numDocs and doqFreq on custom similarity class

  - by David A

  Hi All, im doing an aplication with Lucene (im a noob with it) and im facing some problems. My aplication uses the Lucene 2.4.0 library with a custom similaraty implementation (the jar is imported) In my app im calculating doqFreq and numDocs manually (im adding the values of all indexes and then i calculate a global value in order to use it on every query) and i want to use that values on a custom similarity implementation in order to calculate a new IDF. The problem is that I dont know how to use (or send) the new doqFreq and numDocs values from my app on that new similarty implementation as I dont want to change lucene´s code apart from this extra class. Any suggestions or examples? I read the docs but i dont now how to aproach this :s Thanks

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• WCF - calling back to client (duplex ?)

  - by MüllerDK

  Hi, I have a problem with what solution to choose.. I have a server running having a Service running that can receive orders from a website. To this server several client (remote computers) are connected somehow. I would really like to use WCF for all comunication, but not sure it's possible. I dont wanna configure all client firewall settings in their routers, so the clients would have to connect to the server. But when an order is recieved on the server, it should be transferred to a specific client. One solution could be to have the Client connect using a duplex binding, but it will have to somehow keep the connection alive in order to be able to received data from server... Is this a good way to do this ?? Normally the connection times out and probably for a good reason... Anyone have insight to this problem. Thx alot for any advise :-) Best Regards Søren Müller

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• Query for props list with or without values

  - by vitto

  Hi, I'm trying to make a SELECT on three relational tables like these ones: table_materials -> material_id - material_name table_props -> prop_id - prop_name table_materials_props - row_id -> material_id -> prop_id - prop_value On my page, I'd like to get a result like this one but i have some problem with the query: material prop A prop B prop C prop D prop E wood 350 NULL NULL 84 16 iron NULL 17 NULL NULL 201 copper 548 285 99 NULL NULL so the query should return something like: material prop_name prop_value wood prop A 350 wood prop B NULL wood prop C NULL wood prop D 84 wood prop E 16 // and go on with others rows i thought to use something like: SELECT * FROM table_materials AS m INNER JOIN table_materials_props AS mp ON m.material_id = mp.material_id INNER JOIN table_materials_props AS p ON mp.prop_id = p.prop_id ORDER BY p.prop_name the problem is the query doesn't return the NULL values, and I need the same prop order for all the materials regardless of prop values are NULL or not I hope this example is clear!

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• mysql first record retrieval

  - by Sammy

  While very easy to do in Perl or PHP, I cannot figure how to use mysql only to extract the first unique occurence of a record. For example, given the following table: Name Date Time Sale John 2010-09-12 10:22:22 500 Bill 2010-08-12 09:22:37 2000 John 2010-09-13 10:22:22 500 Sue 2010-09-01 09:07:21 1000 Bill 2010-07-25 11:23:23 2000 Sue 2010-06-24 13:23:45 1000 I would like to extract the first record for each individual in asc time order. After sorting the table is ascending time order, I need to extract the first unique record by name. So the output would be : Name Date Time Sale John 2010-09-12 10:22:22 500 Bill 2010-07-25 11:23:23 2000 Sue 2010-06-24 13:23:45 1000 Is this doable in an easy fashion with mySQL? Thanks, Sammy

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• Java EE 6 - ordering Servlet Request Listeners

  - by Walter White

  Hi all, I finally updated to Java EE 6 (web profile) and would like to control the ordering of my servlet request listeners. I did that before through the XML ordering by listing the listeners in a particular order. Now, I have placed the @WebListener annotation on the classes which are listeners and am trying to figure out how to order the listeners such that they work properly. One must run before another one, otherwise, it won't have the information it needs and won't work. Also, it doesn't seem my listeners are actually being invoked even though they're marked with @WebListener. I am running embedded glassfish 3.0. Another question that is somewhat related - ServletRequestListeners in Java EE 6 by default are still synchronous meaning they're hit first, then servlet filters, right? ServletRequestListeners are not asynchronous where they merely get notified of an event without interrupting the execution? Walter

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• C++: Dependency injection, circular dependency and callbacks

  - by Jonathan

  Consider the (highly simplified) following case: class Dispatcher { public: receive() {/*implementation*/}; // callback } class CommInterface { public: send() = 0; // call } class CommA : public CommInterface { public: send() {/*implementation*/}; } Various classes in the system send messages via the dispatcher. The dispatcher uses a comm to send. Once an answer is returned, the comm relays it back to the dispatcher which dispatches it back to the appropriate original sender. Comm is polymorphic and which implementation to choose can be read from a settings file. Dispatcher has a dependency on the comm in order to send. Comm has a dependency on dispatcher in order to callback. Therefor there's a circular dependency here and I can't seem to implement the dependency injection principle (even after encountering this nice blog post).

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• Can't transfer list<T> to web service?

  - by iTayb

  I have the same classes on my server and on my web service. I have the following WebMethod: [WebMethod] public int CreateOrder(List<Purchase> p, string username) { o.Add(new Order(p,username)); return o.Count; } However the following code, run at server: protected void CartRepeater_ItemCommand(object source, RepeaterCommandEventArgs e) { List<Purchase> l = ((List<Purchase>)Session["Cart"]); if (e.CommandName == "Order") { localhost.ValidateService WS = new localhost.ValidateService(); WS.CreateOrder(l, Session["username"].ToString()); } } gives the following error: Argument '1': cannot convert from 'System.Collections.Generic.List<Purchase>' to 'localhost.Purchase[]'. How can I transfer the list<Purchase> object to the web service? Thank you very much.

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• Nested queries in Arel

  - by Schrockwell

  I am attempting to nest SELECT queries in Arel and/or Active Record in Rails 3 to generate the following SQL statement. SELECT sorted.* FROM (SELECT * FROM points ORDER BY points.timestamp DESC) AS sorted GROUP BY sorted.client_id An alias for the subquery can be created by doing points = Table(:points) sorted = points.order('timestamp DESC').alias but then I'm stuck as how to pass it into the parent query (short of calling #to_sql, which sounds pretty ugly). How do you use a SELECT statement as a sub-query in Arel (or Active Record) to accomplish the above? Maybe there's an altogether different way to accomplish this query that doesn't use nested queries?

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• App engine datastore - query on Enum fields.

  - by Gopi

  I am using GAE(Java) with JDO for persistence. I have an entity with a Enum field which is marked as @Persistent and gets saved correctly into the datastore (As observed from the Datastore viewer in Development Console). But when I query these entities putting a filter based on the Enum value, it is always returning me all the entities whatever value I specify for the enum field. I know GAE java supports enums being persisted just like basic datatypes. But does it also allow retrieving/querying based on them? Google search could not point me to any such example code. Details: I have printed the Query just before being executed. So in two cases the query looks like - SELECT FROM com.xxx.yyy.User WHERE role == super ORDER BY key desc RANGE 0,50 SELECT FROM com.xxx.yyy.User WHERE role == admin ORDER BY key desc RANGE 0,50 Both above queries return me all the User entities from datastore in spite of datastore viewer showing some Users are of type 'admin' and some are of type 'super'.

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• Need to upload files to google docs from my application and store reference to the uploaded file...

  - by Ali

  Hi guys - I'm working on a basic google applications based system. Like I earlier defined I'm building a simple ordering system and to each order placed I attach a file or document. I would like to be able to set it such that whatever file I upload is uploaded into google docs and I somehow am able to maintain a reference to that file from my own application i.e these files are concerned with such and such order. My application is google application based hopefully and I'm building it to be such for later on deployment into the cloud. How do I start on this and what do I need to do? Are there already working widgets I can use and apply or would I need to create my own customised solution for this? I'm working in Php MySQL.

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• SQL orderby / case issue: orderdirection fail

  - by Joris

  I got a stored procedure that delivers a table of students, and it needs to order by surname, name etc... it also needs to sort ascending, descending, depending on the parameter @orderby... code: ORDER BY CASE WHEN @orderby = 'studentkey' THEN Studentkey END ASC, CASE WHEN @orderby = 'studentkey' and @desc = 1 THEN Studentkey END DESC, CASE WHEN @orderby = 'initials' THEN Initials END ASC, CASE WHEN @orderby = 'initials' and @desc = 1 THEN Initials END DESC, CASE WHEN @orderby = 'firstname' THEN Firstname END ASC, CASE WHEN @orderby = 'firstname' and @desc = 1 THEN Firstname END DESC, CASE WHEN @orderby = 'nickname' THEN Nickname END ASC, CASE WHEN @orderby = 'nickname' and @desc = 1 THEN Nickname END DESC, CASE WHEN @orderby = 'insertion' THEN Insertion END ASC, CASE WHEN @orderby = 'insertion' and @desc = 1 THEN Insertion END DESC, CASE WHEN @orderby = 'surname' THEN Surname END ASC, CASE WHEN @orderby = 'surname' and @desc = 1 THEN Surname END DESC NED There is a difference in output between @desc = 1 and @desc = 0, but not what i desire... Does anyone have a solution?

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• Rows dropping when I try to join data from two tables

  - by blcArmadillo

  I have a fairly simple query I'm try to write. If I run the following query: SELECT parts.id, parts.type_id FROM parts WHERE parts.type_id=1 OR parts.type_id=2 OR parts.type_id=4 ORDER BY parts.type_id; I get all the rows I expect to be returned. Now when I try to grab the parent_unit from another table with the following query six rows suddenly drop out of the result: SELECT parts.id, parts.type_id, sp.parent_unit FROM parts, serialized_parts sp WHERE (parts.type_id=1 OR parts.type_id=2 OR parts.type_id=4) AND sp.parts_id = parts.id ORDER BY parts.type_id In the past I've never really dealt with ORs in my queries so maybe I'm just doing it wrong. That said I'm guessing it's just a simple mistake. Let me know if you need sample data and I'll post some. Thanks.

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• iphone: view index

  - by Mike

  I have a view between other views, the order may vary. For example: viewA on top of viewB, on top of viewC, or in another order. Suppose I have viewA, viewB, viewC, viewD and viewE A is on the top and E on the bottom. I need to replace viewC with viewZ, but I need to insert viewZ in the same index of viewC. How do I know, before removing viewC, what index it has, so I can insert viewZ using [self.view insertSubview:viewZ atIndex:?????) and it will be at the same level? thanks for any help.

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• ruby on rails, searchlogic and refactoring

  - by JohnMerlino

  Hey all, I'mt not too familiar with searchlogic plugin for rails (I did view the railscasts but wasn't helpful in relation to the specific code below). Can anyone briefly describe how it is being used in the three methods below? Thanks for any response. def extract_order @order_by = if params[:order].present? field = params[:order].gsub(".", "_") field = field.starts_with?('-') ? 'descend_by_'+field[1..-1] : 'ascend_by_'+field field.to_sym else # Workaround 'searchlogic'.to_sym end end def find_resources @search_conditions = params[:search_conditions] || {} # See http://www.binarylogic.com/2008/11/30/searchlogic-1-5-7-complex-searching-no-longer-a-problem/ @resources = @resource_model.send(@order_by).searchlogic(:conditions => @search_conditions) end def apply_filters f = filter_by f.each do |filter_field| filter_constraints = params[filter_field.to_sym] if filter_constraints.present? # Apply searchlogic's scope @resources.send(filter_field,filter_constraints) end end end

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• Mysql query problem....

  - by Avinash

  I have below values in my database. been Lorem Ipsum and scrambled ever scrambledtexttextofandtooktooktypetexthastheunknownspecimenstandardsincetypesett Here is my query: SELECT nBusinessAdID, MATCH (`sHeadline`) AGAINST ("text" IN BOOLEAN MODE) AS score FROM wiki_businessads WHERE MATCH (`sHeadline`) AGAINST ("text" IN BOOLEAN MODE) AND bDeleted ="0" AND nAdStatus ="1" ORDER BY score DESC, bPrimeListing DESC, dDateCreated DESC It's not fetching first result, why? It should fetch first result because its contain text word in it. I have disabled the stopword filtering. This one is also not working SELECT nBusinessAdID, MATCH (`sHeadline`) AGAINST ('"text"' IN BOOLEAN MODE) AS score FROM wiki_businessads WHERE MATCH (`sHeadline`) AGAINST ('"text"' IN BOOLEAN MODE) AND bDeleted ="0" AND nAdStatus ="1" ORDER BY score DESC, bPrimeListing DESC, dDateCreated DESC Thanks Avinash

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• Multi page ordering form php or ajax

  - by Adam

  I'm looking to create a multi page ordering form the first page would contain some dropdown and text fields, the second page would contain more text fields, the third page would be an order summery with paypal payment option. I'm just wondering what the best way to create this order form is. I've used sessions in the past but never with users entering in text and picking items from drop downs, does anyone have any resources for doing this? Or does anyone know of a jquery or other ajax example or plugin I might be able to use and modify. any insight would be a big help. thanks

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• return to merchent problme in paypal.

  - by Avinash

  Hi, I am using paypal standard as my payment gateway. My problem is that, in Paypal Standard payment method, I have done my code as below: When user click on return to merchant button from paypal then user return to the site with order data , and on that page my order entry will be inserted in my DB. So my problem occur when someone pays but don't click on return to merchant link. So in this case customer gets paid, but due to no entry my DB its not working proper. Hope I am clear to all. Thanks Avinash

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• How do I sort a hash table in javascript?

  - by Colen

  I have a javascript hash table, like so: var things = [ ]; things["hello"] = {"name" : "zzz I fell asleep", "number" : 7}; things["one"] = {"name" : "something", "number" : 18}; things["two"] = {"name" : "another thing", "number" : -2}; I want to sort these into order by name, so if I iterate through the hash table it will go in order another thing something zzz I fell asleep I tried doing this: function compareThings(thing1, thing2) { var name1 = thing1["name"].toLowerCase(); var name2 = thing2["name"].toLowerCase(); if (name1 < name2) { return -1; } if (name1 > name2) { return 1; } return 0; } things.sort(compareThings); But it doesn't seem to work. Edit: it occurs to me that perhaps a sorted hash table is an oxymoron. If so, what's the best way to get access to a sorted list of the things here?

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• More Fun with C# Iterators and Generators

  - by James Michael Hare

  In my last post, I talked quite a bit about iterators and how they can be really powerful tools for filtering a list of items down to a subset of items.  This had both pros and cons over returning a full collection, which, in summary, were:   Pros: If traversal is only partial, does not have to visit rest of collection. If evaluation method is costly, only incurs that cost on elements visited. Adds little to no garbage collection pressure.    Cons: Very slight performance impact if you know caller will always consume all items in collection. And as we saw in the last post, that con for the cost was very, very small and only really became evident on very tight loops consuming very large lists completely.    One of the key items to note, though, is the garbage!  In the traditional (return a new collection) method, if you have a 1,000,000 element collection, and wish to transform or filter it in some way, you have to allocate space for that copy of the collection.  That is, say you have a collection of 1,000,000 items and you want to double every item in the collection.  Well, that means you have to allocate a collection to hold those 1,000,000 items to return, which is a lot especially if you are just going to use it once and toss it.   Iterators, though, don't have this problem.  Each time you visit the node, it would return the doubled value of the node (in this example) and not allocate a second collection of 1,000,000 doubled items.  Do you see the distinction?  In both cases, we're consuming 1,000,000 items.  But in one case we pass back each doubled item which is just an int (for example's sake) on the stack and in the other case, we allocate a list containing 1,000,000 items which then must be garbage collected.   So iterators in C# are pretty cool, eh?  Well, here's one more thing a C# iterator can do that a traditional "return a new collection" transformation can't!   It can return **unbounded** collections!   I know, I know, that smells a lot like an infinite loop, eh?  Yes and no.  Basically, you're relying on the caller to put the bounds on the list, and as long as the caller doesn't you keep going.  Consider this example:   public static class Fibonacci {     // returns the infinite fibonacci sequence     public static IEnumerable<int> Sequence()     {         int iteration = 0;         int first = 1;         int second = 1;         int current = 0;         while (true)         {             if (iteration++ < 2)             {                 current = 1;             }             else             {                 current = first + second;                 second = first;                 first = current;             }             yield return current;         }     } }   Whoa, you say!  Yes, that's an infinite loop!  What the heck is going on there?  Yes, that was intentional.  Would it be better to have a fibonacci sequence that returns only a specific number of items?  Perhaps, but that wouldn't give you the power to defer the execution to the caller.   The beauty of this function is it is as infinite as the sequence itself!  The fibonacci sequence is unbounded, and so is this method.  It will continue to return fibonacci numbers for as long as you ask for them.  Now that's not something you can do with a traditional method that would return a collection of ints representing each number.  In that case you would eventually run out of memory as you got to higher and higher numbers.  This method, though, never runs out of memory.   Now, that said, you do have to know when you use it that it is an infinite collection and bound it appropriately.  Fortunately, Linq provides a lot of these extension methods for you!   Let's say you only want the first 10 fibonacci numbers:       foreach(var fib in Fibonacci.Sequence().Take(10))     {         Console.WriteLine(fib);     }   Or let's say you only want the fibonacci numbers that are less than 100:       foreach(var fib in Fibonacci.Sequence().TakeWhile(f => f < 100))     {         Console.WriteLine(fib);     }   So, you see, one of the nice things about iterators is their power to work with virtually any size (even infinite) collections without adding the garbage collection overhead of making new collections.    You can also do fun things like this to make a more "fluent" interface for for loops:   // A set of integer generator extension methods public static class IntExtensions {     // Begins counting to inifity, use To() to range this.     public static IEnumerable<int> Every(this int start)     {         // deliberately avoiding condition because keeps going         // to infinity for as long as values are pulled.         for (var i = start; ; ++i)         {             yield return i;         }     }     // Begins counting to infinity by the given step value, use To() to     public static IEnumerable<int> Every(this int start, int byEvery)     {         // deliberately avoiding condition because keeps going         // to infinity for as long as values are pulled.         for (var i = start; ; i += byEvery)         {             yield return i;         }     }     // Begins counting to inifity, use To() to range this.     public static IEnumerable<int> To(this int start, int end)     {         for (var i = start; i <= end; ++i)         {             yield return i;         }     }     // Ranges the count by specifying the upper range of the count.     public static IEnumerable<int> To(this IEnumerable<int> collection, int end)     {         return collection.TakeWhile(item => item <= end);     } }   Note that there are two versions of each method.  One that starts with an int and one that starts with an IEnumerable<int>.  This is to allow more power in chaining from either an existing collection or from an int.  This lets you do things like:   // count from 1 to 30 foreach(var i in 1.To(30)) {     Console.WriteLine(i); }     // count from 1 to 10 by 2s foreach(var i in 0.Every(2).To(10)) {     Console.WriteLine(i); }     // or, if you want an infinite sequence counting by 5s until something inside breaks you out... foreach(var i in 0.Every(5)) {     if (someCondition)     {         break;     }     ... }     Yes, those are kinda play functions and not particularly useful, but they show some of the power of generators and extension methods to form a fluid interface.   So what do you think?  What are some of your favorite generators and iterators?

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