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• How are lists implemented in Haskell (GHC)?

  - by eman

  I was just curious about some exact implementation details of lists in Haskell (GHC-specific answers are fine)--are they naive linked lists, or do they have any special optimizations? More specifically: Do length and (!!) (for instance) have to iterate through the list? If so, are their values cached in any way (i.e., if I call length twice, will it have to iterate both times)? Does access to the back of the list involve iterating through the whole list? Are infinite lists and list comprehensions memoized? (i.e., for fib = 1:1:zipWith (+) fib (tail fib), will each value be computed recursively, or will it rely on the previous computed value?) Any other interesting implementation details would be much appreciated. Thanks in advance!

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• Base class for windows service

  - by user187020

  My new project has a design in which there are number windows services for performing different tasks. I have been given a task to create base class from which all of the windows service will inherit. This base class will perform common functions like creating instances of other windows services by iterating through the config file (may be like Activator.CreateInstance), do event logging on onStart, onStop etc. and may contain some more functionality. Before I start developing stuff, wondering if there is any pattern already in place or someone has good understanding of how to implement this kind of functionality. Any help appreciated.

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• as3 loops and event listeners

  - by Ross

  I have an array that returns multidimensional data from an mysql database, when this is collected the createNews function creates the user interface. The problem I am having is the loop is iterating quicker than the ui is being created, is there a way to use event listeners with loops so it only continues after my function has completed its work. Any solutions or ideas are much appreciated; Thanks, R. var t:Array = responds.serverInfo.initialData; for (var i:uint = 0; i < t.length; i++) { var date = t[i][1]; var newstitle = t[i][2]; var story= t[i][3]; var image = t[i][4]; createNews(date, newstitle, story, image); }

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• How to "interleave" two DataTables.

  - by Brent

  Take these two lists: List 1 Red Green Blue List 2 Brown Red Blue Purple Orange I'm looking for a way to combine these lists together to produce: List 3 Brown Red Green Blue Purple Orange I think the basic rules are these: 1) Insert on top the list any row falling before the first common row (e.g., Brown comes before the first common row, Red); 2) Insert items between rows if both lists have two items (e.g., List 1 inserts Green between Red and Blue); and 3) Insert rows on the bottom if the there's no "between-ness" found in 2 (e.g., List 2 inserts Orange at the bottom). The lists are stored in a DataTable. I'm guessing I'll have to switch between them while iterating, but I'm having a hard time figuring out a method of combining the rows. Thanks for any help. --Brent

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• Passing sql results to views hard-codes views to database column names

  - by Galen

  I just realized that i may not be following best practices in regards to the MVC pattern. My issue is that my views "know" information about my database Here's my situation in psuedo code... My controller invokes a method from my model and passes it directly to the view view.records = tableGateway.getRecords() view.display() in my view each records as record print record.name print record.address ... In my view i have record.name and record.address, info that's hard-coded to my database. Is this bad? What other ways around it are there other than iterating over everything in the controller and basically rewriting the records collection. And that just seems silly. Thanks

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• How to define an Integer bean in Struts 1.x

  - by ian_scho_es

  Hi. How do you instantiate an Integer bean, assigning a value, in the Struts 1.x framework? <bean:define id="index" type="java.lang.Integer" value="0"/> or <bean:define id="index" type="java.lang.Integer" value="${0}"/> Results in a: java.lang.ClassCastException: java.lang.String <bean:define id="index" type="java.lang.Integer" value="<%=0%>"/> Results in: The method setValue(String) in the type DefineTag is not applicable for the arguments (int) <% java.lang.Integer index = new java.lang.Integer(0); %> Works, but makes my eyes bleed. Note that I had to refactor iterating over a list but am now applying a filter within the iteration. This was the cleanest solution of all! <logic:equal name="aplicacion" property="generico" value="false" indexId="index"> Maybe I need to go about this completely differently. Many thanks.

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• Ruby on Rails: Model.all.each vs find_by_sql("SELECT * FROM model").each ?

  - by B_

  I'm fairly new to RoR. In my controller, I'm iterating over every tuple in the database. For every table, for every column I used to call SomeOtherModel.find_by_sql("SELECT column FROM model").each {|x| #etc } which worked fine enough. When I later changed this to Model.all(:select => "column").each {|x| #etc } the loop starts out at roughly the same speed but quickly slows down to something like 100 times slower than the the find_by_sql command. These calls should be identical so I really don't know what's happening. I know these calls are not the most efficient but this is just an intermediate step and I will optimize it more once this works correctly. Thanks!

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• Recursion and Iteration

  - by Doug

  What is the difference? Are these the same? If not, can someone please give me an example? MW: Iteration - 1 : the action or a process of iterating or repeating: as a : a procedure in which repetition of a sequence of operations yields results successively closer to a desired result b : the repetition of a sequence of computer instructions a specified number of times or until a condition is met Recusion - 3 : a computer programming technique involving the use of a procedure, subroutine, function, or algorithm that calls itself one or more times until a specified condition is met at which time the rest of each repetition is processed from the last one called to the first

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• Changing a Container while using Visitor

  - by Florian

  Hi everyone, I implemented the Visitor pattern in C++ using a STL-like iterator for storing the Visitor's current position in the container. Now I would like to change the container while I iterate over it, and I'm especially interested in deleting items from the container, even the one I'm currently visiting. Now obviously this will invalidate the Visitors internal iterator, because it was pointing to exactly this item. Currently, I store a list of all iterators in the container and update them, as soon as anything is added to or removed from the list. So in a way this is similar to the Observer pattern applied to the iterator (as Observer) and the list (as Observable). Alternatively I considered having the visitor() methods return some hint to the Visitor about what happend to the current item and how to proceed iterating, but that doesn't sound like such a good idea either, because the visit() implementation shouldn't really care about finding the next item. So, my question is: What's the best way to keep a visitor working, even when items are added to the container or removed from it. Regards, Florian

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• How to populate List<string> with Datarow values from single columns...

  - by James

  Hi, I'm still learning (baby steps). Messing about with a function and hoping to find a tidier way to deal with my datatables. For the more commonly used tables throughout the life of the program, I'll dump them to datatables and query those instead. What I'm hoping to do is query the datatables for say column x = "this", and convert the values of column "y" directly to a List to return to the caller: private List<string> LookupColumnY(string hex) { List<string> stringlist = new List<string>(); DataRow[] rows = tblDataTable.Select("Columnx = '" + hex + "'", "Columny ASC"); foreach (DataRow row in rows) { stringlist.Add(row["Columny"].ToString()); } return stringlist; } Anyone know a slightly simpler method? I guess this is easy enough, but I'm wondering if I do enough of these if iterating via foreach loop won't be a performance hit. TIA!

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• Number of lines in csv.DictReader

  - by Alan Harris-Reid

  Hi there, I have a csv DictReader object (using Python 3.1), but I would like to know the number of lines/rows contained in the reader before I iterate through it. Something like as follows... myreader = csv.DictReader(open('myFile.csv', newline='')) totalrows = ? rowcount = 0 for row in myreader: rowcount +=1 print("Row %d/%d" % (rowcount,totalrows)) I know I could get the total by iterating through the reader, but then I couldn't run the 'for' loop. I could iterate through a copy of the reader, but I cannot find how to copy an iterator. I could also use totalrows = len(open('myFile.csv').readlines()) but that seems an unnecessary re-opening of the file. I would rather get the count from the DictReader if possible. Any help would be appreciated. Alan

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• Are Multiple Iterators possible in php?

  - by artvolk

  Good day! I know that C# allows multiple iterators using yield, like described here: http://stackoverflow.com/questions/1754041/is-multiple-iterators-is-possible-in-c In PHP there is and Iterator interface. Is it possible to implement more than one iteration scenario for a class? More details (EDIT): For example I have class TreeNode implementing single tree node. The whole tree can be expressed using only one this class. I want to provide iterators for iterating all direct and indirect children of current node, for example using BreadthFirst or DepthFirst order. I can implement this Iterators as separate classes but doing so I need that tree node should expose it's children collection as public. C# pseudocode: public class TreeNode<T> { ... public IEnumerable<T> DepthFirstEnumerator { get { // Some tree traversal using 'yield return' } } public IEnumerable<T> BreadthFirstEnumerator { get { // Some tree traversal using 'yield return' } } }

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• How can I switch timezones in Perl's Template::Plugin::Date?

  - by aidan

  I have a calendar on my website, generated in Perl using Template::Toolkit and Template::Plugin::Date. It highlights the current day. I achieve this by iterating through all the dates (as I print the calendar) and comparing against the current date. Something like this: [% IF cur_date == date.format(format = '%Y-%m-%d') %] ... [% END %] It all works well until someone in Australia looks at it. (They are in a different timezone to me and my server in the UK). What's the best way to get Template::Plugin::Date to use a different time zone? It accepts a 'locale' parameter, but AFAIK this is only used for formatting.

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• Lucene search taking TOOO long.

  - by Josh Handel

  I;m using Lucene.net (2.9.2.2) on a (currently) 70Gig index.. I can do a fairly complicated search and get all the document IDs back in 1 ~ 2 seconds.. But to actually load up all the hits (about 700 thousand in my test queries) takes 5+ minutes. We aren't using lucene for UI, this is a datastore between processes where we have hundreds of millions of pre-cached data elements, and the part I am working on exports a few specific fields from each found document. (ergo, pagination doesn't make since as this is an export between processes). My question is what is the best way to get all of the documents in a search result? currently I am using a custom collector that does a get on the document (with a MapFieldSelector) as its collecting.. I've also tried iterating through the list after the collector has finished.. but that was even worse. I'm open to ideas :-). Thanks in advance.

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• Should one use < or <= in a for loop

  - by Eugene Katz

  If you had to iterate through a loop 7 times, would you use: for (int i = 0; i < 7; i++) or: for (int i = 0; i <= 6; i++) There are two considerations: performance readability For performance I'm assuming Java or C#. Does it matter if "less than" or "less than or equal to" is used? If you have insight for a different language, please indicate which. For readability I'm assuming 0-based arrays. UPD: My mention of 0-based arrays may have confused things. I'm not talking about iterating through array elements. Just a general loop. There is a good point below about using a constant to which would explain what this magic number is. So if I had "int NUMBER_OF_THINGS = 7" then "i <= NUMBER_OF_THINGS - 1" would look weird, wouldn't it.

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• Groovy MetaClass - Add category methods to appropriate metaClasses

  - by noah

  I have several categories that I use in my Grails plugin. e.g., class Foo { static foo(ClassA a,Object someArg) { ... } static bar(ClassB b,Object... someArgs) { ... } } I'm looking for the best way to add these methods to the meta-classes so that I don't have to use the category classes and can just invoke them as instance methods. e.g., aInstance.foo(someArg) bInstance.bar(someArgs) Is there a Groovy/Grails class or method that will help me do this or am I stuck iterating through the methods and adding them all myself?

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• How to Obtain Data to Pre-Populate Forms.

  - by Stan

  The objective is to have a form reflect user's defined constraints on a search. At first, I relied entirely upon server-side scripting to achieve this; recently I tried to shift the functionality to JavaScript. On the server side, the search parameters are stored in a ColdFusion struct which makes it particularly convenient to have the data JSON'ed and sent to the client. Then it's just a matter of separately iterating over 'checkable' and text fields to reflect the user's search parameters; jQuery proved to be exceptionally effective in simplifying the workload. One observable difference lies in performance. The second method appeared to be somewhat slower and didn't work in IE8. Evidently, the returned JSON'ed struct was seen as an empty object. I'm sure it can be fixed, though before spending any more time with it, I'm curious to hear how others would approach the task. I'd gladly appreciate any suggestions. --Stan

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• Find element with attribute with minidom

  - by Xster

  Given <field name="frame.time_delta_displayed" showname="Time delta from previous displayed frame: 0.000008000 seconds" size="0" pos="0" show="0.000008000"/> <field name="frame.time_relative" showname="Time since reference or first frame: 0.000008000 seconds" size="0" pos="0" show="0.000008000"/> <field name="frame.number" showname="Frame Number: 2" size="0" pos="0" show="2"/> <field name="frame.pkt_len" showname="Packet Length: 1506 bytes" hide="yes" size="0" pos="0" show="1506"/> <field name="frame.len" showname="Frame Length: 1506 bytes" size="0" pos="0" show="1506"/> <field name="frame.cap_len" showname="Capture Length: 1506 bytes" size="0" pos="0" show="1506"/> <field name="frame.marked" showname="Frame is marked: False" size="0" pos="0" show="0"/> <field name="frame.protocols" showname="Protocols in frame: eth:ip:tcp:http:data" size="0" pos="0" show="eth:ip:tcp:http:data"/> How do I get the field with name="frame.len" right away without iterating through every tag and checking the attributes?

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• how to get group total in self refrenced data in data table ?

  - by Nikhil Vaghela

  I have three columns in my data table. 1) ProductID 2) ProductParentID 3) ProductTotal ProductID and ProductParentID are self refrencing columns where i can set parent child relationship and get child rows based on my relationship. Let us say i have following data Product1     Product11     Product12     Product13         Product131         Product132         Product133 Product2     Product21     Product22     Product23 Next to above hierarchy in Product total column what i want is total of each child rows and sum of those child rows product total should be rolled up to it parent product. E.g if Product 131 total is 10,Product 13 total is 15 and Product 133 total is 5 then the product 13 total should be 30. The logic should work for n number of self hierarchy. Is there any functionality in data table itself where i can achieve this without iterating through each row and do it manually ? Thanks.

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• How to show that the double-checked-lock pattern with Dictionary's TryGetValue is not threadsafe in

  - by Amir

  Recently I've seen some C# projects that use a double-checked-lock pattern on a Dictionary. Something like this: private static readonly object _lock = new object(); private static volatile IDictionary<string, object> _cache = new Dictionary<string, object>(); public static object Create(string key) { object val; if (!_cache.TryGetValue(key, out val)) { lock (_lock) { if (!_cache.TryGetValue(key, out val)) { val = new object(); // factory construction based on key here. _cache.Add(key, val); } } } return val; } This code is incorrect, since the Dictionary can be "growing" the collection in _cache.Add() while _cache.TryGetValue (outside the lock) is iterating over the collection. It might be extremely unlikely in many situations, but is still wrong. Is there a simple program to demonstrate that this code fails? Does it make sense to incorporate this into a unit test? And if so, how?

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• XML databinding to TreeView (or Tab control), bind attribute based on different attribute

  - by Number8

  Hello, I have some xml: <Test> <thing location="home" status="good"/> <thing location=work" status="bad"/> <thing location="mountains" status="good"/> </Test> The leaves on the TreeView are the values of the status attribute; the nodes will be the value of the location attribute. +--bad ¦.....+--work +--good .......+--home .......+--mountains Currently, I populate the TreeView (or Tab control) manually, iterating through the xml, adding the nodes to the appropriate leaf. Can this be done via databinding? I'm guessing a Converter will be involved... Thanks for any advice.

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• Is System.AddIn mostly about making it easier to use Remoting or does it make it harder to do so?

  - by MatthewMartin

  It takes at least 7 assemblies and restricting my AddIn's data model to data types that remoting can deal with before the appdomain isolation features begin to work. It is so complex! The System.AddIn teams blog implies to me they were trying to re-create a mental model of COM, a model I never understood very well in the first place and am not sold on the benefits. (If COM is so good why's it dead?-rhetorical question.) If I don't need to mirror or interop with legacy COM (like VSTO does using System.AddIn), is it possible to just create some classes that load load in a new AppDomain? I can write the discovery code my self, I've done it before and a naive implementation is pretty fast because I'm not like iterating over the assemblies in the GAC! So my specific question is, can I get the AppDomain isolation that AddIns provide with a few code Remoting snippets, and what would those be?

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• Can ActionScript tell when a SWF was published?

  - by spiralganglion

  I'd like to write a little class that adds a Day/Month box showing the date a SWF was published from Flash. The company I work for regularly produces many, many SWFs and many versions of each, iterating over the course of months. A version-tracking system we've been using to communicate with our clients is a Day/Month date-box that gives the date the SWF was published. Up until now, we've been filling in the publish date by hand. If there's any way I can do this programatically with ActionScript that'd be fantastic. Any insight? Basically, all I need is the call that gives me the publish date, or even.. anything about the circumstances under which a SWF was published that I could use to roll into some form of.. automated version identification, unique to this SWF. So, can ActionScript tell when a SWF was published?

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• MarkupBuilder using list

  - by tathamr

  I am currently using sql.row("statement") and storing to a list. I then am trying to setup my xml file using MarkupBuilder. Is there a better way than iterating over the list poping off an item and then parsing it to add my different column names and values? What is stored by list entry is ID='X' Period='Yearly' Lengh='test' So the XML would be something similar to: <table name='test'> <row> <column name=ID>X</column> <column name=Period>Yearly</column> <column name=Length>test</column> </row> </table>

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• .NET DataGridView rows' default alignment all set to left

  - by Xster

  DataGridView gets bound to DataTable. I then try to set some columns' DefaultCellStyle.Alignment to right. It doesn't work. After a bunch of tinkering, it turns out that every single row's DefaultCellStyle.Alignment is set to left and overrides it. DataGridView.DefaultCellStyle or RowsDefaultCellStyle doesn't do anything. Makes sense since a particular row's DefaultCellStyle is more specific but what gives. Why are they all set to something and not NotSet? How do I get rid of it instead of iterating through all rows?

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