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• What techniques can be used to detect so called "black holes" (a spider trap) when creating a web crawler?

  - by Tom

  When creating a web crawler, you have to design somekind of system that gathers links and add them to a queue. Some, if not most, of these links will be dynamic, which appear to be different, but do not add any value as they are specifically created to fool crawlers. An example: We tell our crawler to crawl the domain evil.com by entering an initial lookup URL. Lets assume we let it crawl the front page initially, evil.com/index The returned HTML will contain several "unique" links: evil.com/somePageOne evil.com/somePageTwo evil.com/somePageThree The crawler will add these to the buffer of uncrawled URLs. When somePageOne is being crawled, the crawler receives more URLs: evil.com/someSubPageOne evil.com/someSubPageTwo These appear to be unique, and so they are. They are unique in the sense that the returned content is different from previous pages and that the URL is new to the crawler, however it appears that this is only because the developer has made a "loop trap" or "black hole". The crawler will add this new sub page, and the sub page will have another sub page, which will also be added. This process can go on infinitely. The content of each page is unique, but totally useless (it is randomly generated text, or text pulled from a random source). Our crawler will keep finding new pages, which we actually are not interested in. These loop traps are very difficult to find, and if your crawler does not have anything to prevent them in place, it will get stuck on a certain domain for infinity. My question is, what techniques can be used to detect so called black holes? One of the most common answers I have heard is the introduction of a limit on the amount of pages to be crawled. However, I cannot see how this can be a reliable technique when you do not know what kind of site is to be crawled. A legit site, like Wikipedia, can have hundreds of thousands of pages. Such limit could return a false positive for these kind of sites. Any feedback is appreciated. Thanks.

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• What are the worst examples of moral failure in the history of software engineering?

  - by Amanda S

  Many computer science curricula include a class or at least a lecture on disasters caused by software bugs, such as the Therac-25 incidents or Ariane 5 Flight 501. Indeed, Wikipedia has a list of software bugs with serious consequences, and a question on StackOverflow addresses some of them too. We study the failures of the past so that we don't repeat them, and I believe that rather than ignoring them or excusing them, it's important to look at these failures squarely and remind ourselves exactly how the mistakes made by people in our profession cost real money and real lives. By studying failures caused by uncaught bugs and bad process, we learn certain lessons about rigorous testing and accountability, and we make sure that our innocent mistakes are caught before they cause major problems. There are kinds of less innocent failure in software engineering, however, and I think it's just as important to study the serious consequences caused by programmers motivated by malice, greed, or just plain amorality. Thus we can learn about the ethical questions that arise in our profession, and how to respond when we are faced with them ourselves. Unfortunately, it's much harder to find lists of these failures--the only one I can come up with is that apocryphal "DOS ain't done 'til Lotus won't run" story. What are the worst examples of moral failure in the history of software engineering?

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• Partially constructed object / Multi threading

  - by reto

  Heya! I'm using joda due to it's good reputation regarding multi threading. It goes great distances to make multi threaded date handling efficient, for example by making all Date/Time/DateTime objects immutable. But here's a situation where I'm not sure if Joda is really doing the right thing. It probably is correct, but I'd be very interested to see the explanation for it. When a toString() of a DateTime is being called Joda does the following: /* org.joda.time.base.AbstractInstant */ public String toString() { return ISODateTimeFormat.dateTime().print(this); } All formatters are thread safe, as they are as well ready-only. But what's about the formatter-factory: private static DateTimeFormatter dt; /* org.joda.time.format.ISODateTimeFormat */ public static DateTimeFormatter dateTime() { if (dt == null) { dt = new DateTimeFormatterBuilder() .append(date()) .append(tTime()) .toFormatter(); } return dt; } This is a common pattern in single threaded applications. I see the following dangers: Race condition during null check -- worst case: two objects get created. No Problem, as this is solely a helper object (unlike a normal singleton pattern situation), one gets saved in dt, the other is lost and will be garbage collected sooner or later. the static variable might point to a partially constructed object before the objec has been finished initialization (before calling me crazy, read about a similar situation in this Wikipedia article. So how does Joda ensure that not partially created formatter gets published in this static variable? Thanks for your explanations! Reto

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• remove values from an array php

  - by LiveEn

  I have a array of links and i have another array which contains certain values i would like to filter in the list eg: http://www.liquidshredder.co.uk/shop%3Fkw%3Dsurfboards%26fl%3D330343%26ci%3D3889610385%26network%3Ds http://www.bournemouth-surfing.co.uk/index.php%3FcPath%3D32 http://www.stcstores.co.uk/wetsuit-range-sizing--pricing-info-1-w.asp http://www.kingofwatersports.com/wetsuit-sale-c227.html http://www.uk.best-price.com/search/landing/query/bodyboards/s/google/altk/Surf%2Band/koid/1944273223/ http://www.surfinghardware.co.uk/Results.cfm%3Fcategory%3D20%26kw%3Dbodyboards%26fl%3D11407%26ci%3D3326979552%26network%3Ds http://www.teste.co.uk/adtrack/baod.html http://www.teste.co.uk/bodyboards/ www.sandskater.co.uk/ www.sandskater.co.uk/bodyboards/+Bodyboards&sa=X&ei=GwSWS-KaGM24rAeF-vCKDA&ved=0CBMQHzAKOAo http://www.extremesportstrader.co.uk/buy/water/bodyboarding/ www.extremesportstrader.co.uk/buy/water/bodyboarding/+Bodyboards&sa=X&ei=GwSWS-KaGM24rAeF-vCKDA&ved=0CBYQHzALOAo www.circle-one.co.uk/+Bodyboards&sa=X&ei=GwSWS-KaGM24rAeF-vCKDA&ved=0CBkQHzAMOAo http://www.teste.co.uk/bodyboards/p1 http://www.teste.co.uk/bodyboards/p2 http://www.amazon.co.uk/s/%3Fie%3DUTF8%26keywords%3Dbodyboards%26tag%3Dgooghydr-21%26index%3Daps%26hvadid%3D4764625891%26ref%3Dpd_sl_2lyzsfw1ar_e http://www.teste.co.uk/bodyboards/p3 www.extremesportstrader.co.uk/buy/water/ and i would like to remove all the instances of "http://www.teste.co.uk"? i tried the below code but it doesn't work :( $remove=array("teste.co.uk","127.0.0.1","localhost","wikipedia.org","gmail.com","answers.yahoo.com"); foreach ($list[0] as $key=>$clean) { if (in_array($clean,$remove)) { unset($list[0][$key]); } echo $clean; echo '<br>'; }

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• Question about decorator pattern and the abstract decorator class?

  - by es11

  This question was asked already here, but rather than answering the specific question, descriptions of how the decorator pattern works were given instead. I'd like to ask it again because the answer is not immediately evident to me just by reading how the decorator pattern works (I've read the wikipedia article and the section in the book Head First Design Patterns). Basically, I want to know why an abstract decorator class must be created which implements (or extends) some interface (or abstract class). Why can't all the new "decorated classes" simply implement (or extend) the base abstract object themselves (instead of extending the abstract decorator class)? To make this more concrete I'll use the example from the design patterns book dealing with coffee beverages: There is an abstract component class called Beverage Simple beverage types such as HouseBlend simply extend Beverage To decorate beverage, an abstract CondimentDecorator class is created which extends Beverage and has an instance of Beverage Say we want to add a "milk" condiment, a class Milk is created which extends CondimentDecorator I'd like to understand why we needed the CondimentDecorator class and why the class Milk couldn't have simply extended the Beverage class itself and been passed an instance of Beverage in its constructor. Hopefully this is clear...if not I'd simply like to know why is the abstract decorator class necessary for this pattern? Thanks. Edit: I tried to implement this, omitting the abstract decorator class, and it seems to still work. Is this abstract class present in all descriptions of this pattern simply because it provides a standard interface for all of the new decorated classes?

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• Image processing on bifurcation diagram to get small eps size

  - by yCalleecharan

  Hello, I'm producing bifurcation diagrams (which are normally used in nonlinear dynamics). These diagrams identify abrupt changes in topologies due to stability changes. These abrupt changes occur as one or more parameters pass through some critical value(s). An example is here: http://en.wikipedia.org/wiki/File:LogisticMap_BifurcationDiagram.png On the above figure, some image processing has been done so as to make the plot more visually pleasant. A bifurcation diagram usually contains hundreds of thousands of points and the resulting eps file can become very big. Journal submission in the LaTeX format require that figures are to be submitted in the eps format. In my case one of such figures can result in about 6 MB in Matlab and even much more in Gnuplot. For the example in the above figure, 100,000 x values are calculated for each r and one can imagine that the resulting eps file would be huge. The site however explains some image processing that makes the plot more visually pleasing. Can anyone explain to me stepwise how go about? I can't understand the explanation provided in the "summary" section. Will the resulting image processing also reduce the figure size? Furthermore, any tips on reducing the file size of such a huge eps figure? Thanks a lot...

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• How can I tackle 'profoundly found elsewhere' syndrome (inverse of NIH)?

  - by Alistair Knock

  How can I encourage colleagues to embrace small-scale innovation within our team(s), in order to get things done quicker and to encourage skills development? (the term 'profoundly found elsewhere' comes from Wikipedia, although it is scarcely used anywhere else apart from a reference to Proctor & Gamble) I've worked in both environments where there is a strong opposition to software which hasn't been developed in-house (usually because there's a large community of developers), and more recently (with far fewer central developers) where off-the-shelf products are far more favoured for the usual reasons: maintenance, total cost over product lifecycle, risk management and so on. I think the off the shelf argument works in the majority of cases for the majority of users, even though as a developer the product never quite does what I'd like it to do. However, in some cases there are clear gaps where the market isn't able to provide specifically what we would need, or at least it isn't able to without charging astronomical consultancy rates for a bespoke solution. These can be small web applications which provide a short-term solution to a particular need in one specific department, or could be larger developments that have the potential to serve a wider audience, both across the organisation and into external markets. The problem is that while development of these applications would be incredibly cheap in terms of developer hours, and delivered very quickly without the need for glacial consultation, the proposal usually falls flat because of risk: 'Who'll maintain the project tracker that hasn't had any maintenance for the past 7 years while you're on holiday for 2 weeks?' 'What if one of our systems changes and the connector breaks?' 'How can you guarantee it's secure/better/faster/cheaper/holier than Company X's?' With one developer behind these little projects, the answers are invariably: 'Nobody, but...' 'It will break, just like any other application would...' 'I, uh...' How can I better answer these questions and encourage people to take a little risk in order to stimulate creativity and fast-paced, short-lifecycle development instead of using that 6 months to consult about what tender process we might use?

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• How can the Three-Phase Commit Protocol (3PC) guarantee atomicity?

  - by AndiDog

  I'm currently exploring worst case scenarios of atomic commit protocols like 2PC and 3PC and am stuck at the point that I can't find out why 3PC can guarantee atomicity. That is, how does it guarantee that if cohort A commits, cohort B also commits? Here's the simplified 3PC from the Wikipedia article: Now let's assume the following case: Two cohorts participate in the transaction (A and B) Both do their work, then vote for commit Coordinator now sends precommit messages... A receives the precommit message, acknowledges, and then goes offline for a long time B doesn't receive the precommit message (whatever the reason might be) and is thus still in "uncertain" state The results: Coordinator aborts the transaction because not all precommit messages were sent and acknowledged successfully A, who is in precommit state, is still offline, thus times out and commits B aborts in any case: He either stays offline and times out (causes abort) or comes online and receives the abort command from the coordinator And there you have it: One cohort committed, another aborted. The transaction is screwed. So what am I missing here? In my understanding, if the automatic commit on timeout (in precommit state) was replaced by infinitely waiting for a coordinator command, that case should work fine.

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• Adding select menu default value via JS?

  - by purpler

  Hi, i'm developing a meta search engine website, Soogle and i've used JS to populate select menu.. Now, after the page is loaded none of engines is loaded by default, user needs to select it on his own or [TAB] to it.. Is there a possibility to preselect one value from the menu via JS after the page loads? This is the code: Javascript: // SEARCH FORM INIT function addOptions(){ var sel=document.searchForm.whichEngine;for(var i=0;i<arr.length;i++){ sel.options[i]=new Option(arr[i][0],i)}} function startSearch(){ searchString=document.searchForm.searchText.value;if(searchString!=""){ var searchEngine=document.searchForm.whichEngine.selectedIndex; var finalSearchString=arr[searchEngine][1]+searchString;location.href=finalSearchString}return false} function checkKey(e){ var character=(e.which)?e.which:event.keyCode;if(character=='13'){ return startSearch()}} // SEARCH ENGINES INIT var arr = new Array(); arr[arr.length] = new Array("Web", "http://www.google.com/search?q="); arr[arr.length] = new Array("Images", "http://images.google.com/images?q="); arr[arr.length] = new Array("Knoweledge","http://en.wikipedia.org/wiki/Special:Search?search="); arr[arr.length] = new Array("Videos","http://www.youtube.com/results?search_query="); arr[arr.length] = new Array("Movies", "http://www.imdb.com/find?q="); arr[arr.length] = new Array("Torrents", "http://thepiratebay.org/search/"); HTML: <body onload="addOptions();document.forms.searchForm.searchText.focus()"> <div id="wrapper"> <div id="logo"></div> <form name="searchForm" method="POST" action="javascript:void(0)"> <input name="searchText" type="text" onkeypress="checkKey(event);"/> <span id="color"></span> <select tabindex="1" name="whichEngine" selected="Web"></select> <br /> <input tabindex="2" type="button" onClick="return startSearch()" value="Search"/> </form> </div> </body>

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• What are Code Smells? What is the best way to correct them?

  - by Rob Cooper

  OK, so I know what a code smell is, and the Wikipedia Article is pretty clear in its definition: In computer programming, code smell is any symptom in the source code of a computer program that indicates something may be wrong. It generally indicates that the code should be refactored or the overall design should be reexamined. The term appears to have been coined by Kent Beck on WardsWiki. Usage of the term increased after it was featured in Refactoring. Improving the Design of Existing Code. I know it also provides a list of common code smells. But I thought it would be great if we could get clear list of not only what code smells there are, but also how to correct them. Some Rules Now, this is going to be a little subjective in that there are differences to languages, programming style etc. So lets lay down some ground rules: ** ONE SMELL PER ANSWER PLEASE! & ADVISE ON HOW TO CORRECT! ** See this answer for a good display of what this thread should be! DO NOT downmod if a smell doesn't apply to your language or development methodology We are all different. DO NOT just quickly smash in as many as you can think of Think about the smells you want to list and get a good idea down on how to work around. DO downmod answers that just look rushed For example "dupe code - remove dupe code". Let's makes it useful (e.g. Duplicate Code - Refactor into separate methods or even classes, use these links for help on these common.. etc. etc.). DO upmod answers that you would add yourself If you wish to expand, then answer with your thoughts linking to the original answer (if it's detailed) or comment if its a minor point. DO format your answers! Help others to be able to read it, use code snippets, headings and markup to make key points stand out!

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• What is the current status of LOGO? (The programming language)

  - by Workshop Alex

  In another Q I saw someone mention LOGO and it reminded me of some programming language from the past, mostly used for educational purposes. Basically, you would have to program a turtle with a pen through it's back. By telling it where to move, the pen would draw lines. It could also lift the pen to move without drawing lines. I have fond memories of this language, since it was one of the first I've ever used, about 30 years ago. (Yeah, I'm old.) Well, I only programmed with LOGO for two days or so, but it got me hooked to programming. But I wonder if the LOGO information on it's Wikipedia page is still correct. And more importantly, are there versions that will create .NET binaries? Are there only LOGO Interpreters and no compilers? What is the current status of this educational language? And more interestingly, are there more experts here at SO who have experimented with LOGO in the past? Yeah, I know. Nowadays this language is a bit antique but I got some warm and comfortable memories when I remembered this interesting language from my history. For a teenager back then, it was fun!

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• Fastest Java way to remove the first/top line of a file (like a stack)

  - by christangrant

  I am trying to improve an external sort implementation in java. I have a bunch of BufferedReader objects open for temporary files. I repeatedly remove the top line from each of these files. This pushes the limits of the Java's Heap. I would like a more scalable method of doing this without loosing speed because of a bunch of constructor calls. One solution is to only open files when they are needed, then read the first line and then delete it. But I am afraid that this will be significantly slower. So using Java libraries what is the most efficient method of doing this. --Edit-- For external sort, the usual method is to break a large file up into several chunk files. Sort each of the chunks. And then treat the sorted files like buffers, pop the top item from each file, the smallest of all those is the global minimum. Then continue until for all items. http://en.wikipedia.org/wiki/External_sorting My temporary files (buffers) are basically BufferedReader objects. The operations performed on these files are the same as stack/queue operations (peek and pop, no push needed). I am trying to make these peek and pop operations more efficient. This is because using many BufferedReader objects takes up too much space.

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• Any diff/merge tool that provides a report (metrics) of conflicts?

  - by cad

  CONTEXT: I am preparing a big C# merge using visual studio 2008 and TFS. I need to create a report with the files and the number of collisions (total changes and conflicts) for each file (and in total of course) PROBLEM: I cannot do it for two reasons (first one is solved): 1- Using TFS merge I can have access to the file comparison but I cannot export the list of conflicting files... I can only try to resolve the conflicts. (I have solved problem 1 using beyond compare. It allows me to export the file list) 2- Using TFS merge I can only access manually for each file to get the number of conflicts... but I have more than 800 files (and probably will have to repeat it in the close future so is not an option doing it manually) There are dozens of file comparison tools (http://en.wikipedia.org/wiki/Comparison_of_file_comparison_tools ) but I am not sure which one could (if any) give me these metrics. I have also read several forums and questions here but are more general questions (which diff tool is better) and I am looking for a very specific report. So my questions are: Is Visual Studio 2010 (using still TFS2008) capable of doing such reports/exportation? Is there any tool that provide this kind of metrics (Now I am trying Beyond Compare)

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• Is the size of a struct required to be an exact multiple of the alignment of that struct?

  - by Steve314

  Once again, I'm questioning a longstanding belief. Until today, I believed that the alignment of the following struct would normally be 4 and the size would normally be 5... struct example { int m_Assume_32_Bits; char m_Assume_8_Bit_Bytes; }; Because of this assumption, I have data structure code that uses offsetof to determine the distance in bytes between two adjacent items in an array. Today, I spotted some old code that was using sizeof where it shouldn't, couldn't understand why I hadn't had bugs from it, coded up a unit test - and the test surprised me by passing. A bit of investigation showed that the sizeof the type I used for the test (similar to the struct above) was an exact multiple of the alignment - ie 8 bytes. It had padding after the final member. Here is an example of why I never expected this... struct example2 { example m_Example; char m_Why_Cant_This_Be_At_Offset_6_Bytes; }; A bit of Googling showed examples that make it clear that this padding after the final member is allowed - for example http://en.wikipedia.org/wiki/Data_structure_alignment#Data_structure_padding (the "or at the end of the structure" bit). This is a bit embarrassing, as I recently posted this comment - Use of struct padding (my first comment to that answer). What I can't seem to determine is whether this padding to an exact multiple of the alignment is guaranteed by the C++ standard, or whether it is just something that is permitted and that some (but maybe not all) compilers do. So - is the size of a struct required to be an exact multiple of the alignment of that struct according to the C++ standard? If the C standard makes different guarantees, I'm interested in that too, but the focus is on C++.

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• Naive Bayesian classification (spam filtering) - Doubt in one calculation? Which one is right? Plz c

  - by Microkernel

  Hi guys, I am implementing Naive Bayesian classifier for spam filtering. I have doubt on some calculation. Please clarify me what to do. Here is my question. In this method, you have to calculate P(S|W) - Probability that Message is spam given word W occurs in it. P(W|S) - Probability that word W occurs in a spam message. P(W|H) - Probability that word W occurs in a Ham message. So to calculate P(W|S), should I do (1) (Number of times W occuring in spam)/(total number of times W occurs in all the messages) OR (2) (Number of times word W occurs in Spam)/(Total number of words in the spam message) So, to calculate P(W|S), should I do (1) or (2)? (I thought it to be (2), but I am not sure, so plz clarify me) I am refering http://en.wikipedia.org/wiki/Bayesian_spam_filtering for the info by the way. I got to complete the implementation by this weekend :( Thanks and regards, MicroKernel :) @sth: Hmm... Shouldn't repeated occurrence of word 'W' increase a message's spam score? In the your approach it wouldn't, right?. Lets take a scenario and discuss... Lets say, we have 100 training messages, out of which 50 are spam and 50 are Ham. and say word_count of each message = 100. And lets say, in spam messages word W occurs 5 times in each message and word W occurs 1 time in Ham message. So total number of times W occuring in all the spam message = 5*50 = 250 times. And total number of times W occuring in all Ham messages = 1*50 = 50 times. Total occurance of W in all of the training messages = (250+50) = 300 times. So, in this scenario, how do u calculate P(W|S) and P(W|H) ? Naturally we should expect, P(W|S) P(W|H)??? right. Please share your thought...

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• List of fonts installed by default in versions of Windows?

  - by Ricket

  I've been seeing more and more websites using fancy antialiased fonts. Every time I hit one, I think to myself "hmm, what web-safe font is that?" - but after looking at the CSS I typically find some font name in quotes, like "Palatino Linotype". Obviously not web-safe, but according to the Wikipedia article, "Palatino Linotype is shipped with Windows 2000 or later, and Microsoft Office Professional Edition 2003." So that covers what, 95% of users that might visit your website? And thanks to the power of CSS, the website can fallback to a similar generic font typename such as 'serif' for non-Windows users with a line like this: font: 16px/20px "Palatino Linotype", serif; Awesome! I want to start using fancy fonts! Is there a set of lists out there, of the fonts that are preinstalled by default in Windows 98, 2000, NT, ME, XP, 2003, etc., and maybe for the Mac OSX versions and various Linux distributions as well? It would be a great reference for picking web font faces! (if not, someone should compile it!) I had never before heard of Palatino Linotype and I want to know what other fonts have existed since old Windows versions that I've never known about!

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• How would you implement a hashtable in language x?

  - by mk

  The point of this question is to collect a list of examples of hashtable implementations using arrays in different languages. It would also be nice if someone could throw in a pretty detailed overview of how they work, and what is happening with each example. Edit: Why not just use the built in hash functions in your specific language? Because we should know how hash tables work and be able to implement them. This may not seem like a super important topic, but knowing how one of the most used data structures works seems pretty important to me. If this is to become the wikipedia of programming, then these are some of the types of questions that I will come here for. I'm not looking for a CS book to be written here. I could go pull Intro to Algorithms off the shelf and read up on the chapter on hash tables and get that type of info. More specifically what I am looking for are code examples. Not only for me in particular, but also for others who would maybe one day be searching for similar info and stumble across this page. To be more specific: If you had to implement them, and could not use built-in functions, how would you do it? You don't need to put the code here. Put it in pastebin and just link it.

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• How do I find all paths through a set of given nodes in a DAG?

  - by Hanno Fietz

  I have a list of items (blue nodes below) which are categorized by the users of my application. The categories themselves can be grouped and categorized themselves. The resulting structure can be represented as a Directed Acyclic Graph (DAG) where the items are sinks at the bottom of the graph's topology and the top categories are sources. Note that while some of the categories might be well defined, a lot is going to be user defined and might be very messy. Example: On that structure, I want to perform the following operations: find all items (sinks) below a particular node (all items in Europe) find all paths (if any) that pass through all of a set of n nodes (all items sent via SMTP from example.com) find all nodes that lie below all of a set of nodes (intersection: goyish brown foods) The first seems quite straightforward: start at the node, follow all possible paths to the bottom and collect the items there. However, is there a faster approach? Remembering the nodes I already passed through probably helps avoiding unnecessary repetition, but are there more optimizations? How do I go about the second one? It seems that the first step would be to determine the height of each node in the set, as to determine at which one(s) to start and then find all paths below that which include the rest of the set. But is this the best (or even a good) approach? The graph traversal algorithms listed at Wikipedia all seem to be concerned with either finding a particular node or the shortest or otherwise most effective route between two nodes. I think both is not what I want, or did I just fail to see how this applies to my problem? Where else should I read?

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• Plotting an Arc in Discrete Steps

  - by phobos51594

  Good afternoon, Background My question relates to the plotting of an arbitrary arc in space using discrete steps. It is unique, however, in that I am not drawing to a canvas in the typical sense. The firmware I am designing is for a gcode interpreter for a CNC mill that will translate commands into stepper motor movements. Now, I have already found a similar question on this very site, but the methodology suggested (Bresenham's Algorithm) appears to be incompatable for moving an object in space, as it only relies on the calculation of one octant of a circle which is then mirrored about the remaining axes of symmetry. Furthermore, the prescribed method of calculation an arc between two arbitrary angles relies on trigonometry (I am implementing on a microcontroller and would like to avoid costly trig functions, if possible) and simply not taking the steps that are out of the range. Finally, the algorithm only is designed to work in one rotational direction (e.g. counterclockwise). Question So, on to the actual question: Does anyone know of a general-purpose algorithm that can be used to "draw" an arbitrary arc in discrete steps while still giving respect to angular direction (CW / CCW)? The final implementation will be done in C, but the language for the purpose of the question is irrelevant. Thank you in advance. References S.O post on drawing a simple circle using Bresenham's Algorithm: "Drawing" an arc in discrete x-y steps Wiki page describing Bresenham's Algorithm for a circle http://en.wikipedia.org/wiki/Midpoint_circle_algorithm Gcode instructions to be implemented (see. G2 and G3) http://linuxcnc.org/docs/html/gcode.html

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• Why can't RB-Tree be a list?

  - by Alex

  Hey everyone. I have a problem with the rb-trees. according to wikipedia, rb-tree needs to follow the following: A node is either red or black. The root is black. (This rule is used in some definitions and not others. Since the root can always be changed from red to black but not necessarily vice-versa this rule has little effect on analysis.) All leaves are black. Both children of every red node are black. Every simple path from a given node to any of its descendant leaves contains the same number of black nodes. As we know, an rb-tree needs to be balanced and has the height of O(log(n)). But, if we insert an increasing series of numbers (1,2,3,4,5...) and theoretically we will get a tree that will look like a list and will have the height of O(n) with all its nodes black, which doesn't contradict the rb-tree properties mentioned above. So, where am I wrong?? thanks.

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• urllib2.Request() with data returns empty url

  - by Mr. Polywhirl

  My main concern is the function: getUrlAndHtml() If I manually build and append the query to the end of the uri, I can get the response.url(), but if I pass a dictionary as the request data, the url does not come back. Is there anyway to guarantee the redirected url? In my example below, if thisWorks = True I get back a url, but the returned url is the request url as opposed to a redirect link. On a sidenote, the encoding for .E2.80.93 does not translate to - for some reason? #!/usr/bin/python import pprint import urllib import urllib2 from bs4 import BeautifulSoup from sys import argv URL = 'http://en.wikipedia.org/w/index.php?' def yesOrNo(boolVal): return 'yes' if boolVal else 'no' def getTitleFromRaw(page): return page.strip().replace(' ', '_') def getUrlAndHtml(title, printable=False): thisWorks = False if thisWorks: query = 'title={:s}&printable={:s}'.format(title, yesOrNo(printable)) opener = urllib2.build_opener() opener.addheaders = [('User-agent', 'Mozilla/5.0')] response = opener.open(URL + query) else: params = {'title':title,'printable':yesOrNo(printable)} data = urllib.urlencode(params) headers = {'User-agent':'Mozilla/5.0'}; request = urllib2.Request(URL, data, headers) response = urllib2.urlopen(request) return response.geturl(), response.read() def getSoup(html, name=None, attrs=None): soup = BeautifulSoup(html) if name is None: return None return soup.find(name, attrs) def setTitle(soup, newTitle): title = soup.find('div', {'id':'toctitle'}) h2 = title.find('h2') h2.contents[0].replaceWith('{:s} for {:s}'.format(h2.getText(), newTitle)) def updateLinks(soup, url): fragment = '#' for a in soup.findAll('a', href=True): a['href'] = a['href'].replace(fragment, url + fragment) def writeToFile(soup, filename='out.html', indentLevel=2): with open(filename, 'wt') as out: pp = pprint.PrettyPrinter(indent=indentLevel, stream=out) pp.pprint(soup) print('Wrote {:s} successfully.'.format(filename)) if __name__ == '__main__': def exitPgrm(): print('usage: {:s} "<PAGE>" <FILE>'.format(argv[0])) exit(0) if len(argv) == 2: help = argv[1] if help == '-h' or help == '--help': exitPgrm() if False:''' if not len(argv) == 3: exitPgrm() ''' page = 'Led Zeppelin' # argv[1] filename = 'test.html' # argv[2] title = getTitleFromRaw(page) url, html = getUrlAndHtml(title) soup = getSoup(html, 'div', {'id':'toc'}) setTitle(soup, page) updateLinks(soup, url) writeToFile(soup, filename)

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• IE6 transparency+radio button can't be clicked

  - by Jonas Byström

  IE6: when I place a partially transparent image in a div, the radio buttons in that div that overlap the non-transparent pixels of the image become unclickable. Example: <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> <head> <style media="screen" type="text/css"> div { position: relative; width: 500px; height: 300px; _filter:progid:DXImageTransform.Microsoft.AlphaImageLoader(src=http://upload.wikimedia.org/wikipedia/commons/thumb/f/fc/Olympic_flag_transparent.svg/200px-Olympic_flag_transparent.svg.png, sizingMethod='crop'); } input { position: absolute; top: 40px; left: 60px; } </style> </head> <body> <div> <input type="radio" value="1" name="1"/> </div> </body> </html> If you test the code, you can also try moving the button from (60, 40) to (40, 40) where the image is transparent, and voilà - the clicking is back in business again. This bug might, or might not, be related to the IE6 links transparency bug, but I'm not knowledgable enough to grasp any resemblence. Have I done something wrong? Or how can I circumvent? Is there some other option apart from removing the _filter:progid?

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• How to make write operation idempotent?

  - by Morgan Cheng

  I'm reading article about recently release Gizzard sharding framework by twitter(http://engineering.twitter.com/2010/04/introducing-gizzard-framework-for.html). It mentions that all write operations must be idempotent to make sure high reliability. According to wikipedia, "Idempotent operations are operations that can be applied multiple times without changing the result." But, IMHO, in Gazzard case, idempotent write operation should be operations that sequence doesn't matter. Now, my question is: How to make write operation idempotent? The only thing I can image is to have a version number attached to each write. For example, in blog system. Each blog must have a $blog_id and $content. In application level, we always write a blog content like this write($blog_id, $content, $version). The $version is determined to be unique in application level. So, if application first try to set one blog to "Hello world" and second want it to be "Goodbye", the write is idempotent. We have such two write operations: write($blog_id, "Hello world", 1); write($blog_id, "Goodbye", 2); These two operations are supposed to changed two different records in DB. So, no matter how many times and what sequence these two operations executed, the results are same. This is just my understanding. Please correct me if I'm wrong.

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• What exactly is a reentrant function?

  - by eSKay

  Most of the times, the definition of reentrance is quoted from Wikipedia: A computer program or routine is described as reentrant if it can be safely called again before its previous invocation has been completed (i.e it can be safely executed concurrently). To be reentrant, a computer program or routine: Must hold no static (or global) non-constant data. Must not return the address to static (or global) non-constant data. Must work only on the data provided to it by the caller. Must not rely on locks to singleton resources. Must not modify its own code (unless executing in its own unique thread storage) Must not call non-reentrant computer programs or routines. How is safely defined? If a program can be safely executed concurrently, does it always mean that it is reentrant? What exactly is the common thread between the six points mentioned that I should keep in mind while checking my code for reentrant capabilities? Also, Are all recursive functions reentrant? Are all thread-safe functions reentrant? Are all recursive and thread-safe functions reentrant? While writing this question, one thing comes to mind: Are the terms like reentrance and thread safety absolute at all i.e. do they have fixed concrete definations? For, if they are not, this question is not very meaningful. Thanks!

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• python crc32 woes

  - by lazyr

  I'm writing a python program to extract data from the middle of a 6 GB bz2 file. A bzip2 file is made up of independently decryptable blocks of data, so I only need to find a block (they are delimited by magic bits), then create a temporary one-block bzip2 file from it in memory, and finally pass that to the bz2.decompress function. Easy, no? The bzip2 format has a crc32 checksum for the file at the end. No problem, binascii.crc32 to the rescue. But wait. The data to be checksummed does not necessarily end on a byte boundary, and the crc32 function operates on a whole number of bytes. My plan: use the binascii.crc32 function on all but the last byte, and then a function of my own to update the computed crc with the last 1-7 bits. But hours of coding and testing has left me bewildered, and my puzzlement can be boiled down to this question: how come crc32("\x00") is not 0x00000000? Shouldn't it be, according to the wikipedia article? You start with 0b00000000 and pad with 32 0's, then do polynomial division with 0x04C11DB7 until there are no ones left in the first 8 bits, which is immediately. Your last 32 bits is the checksum, and how can that not be all zeroes? I've searched google for answers and looked at the code of several crc32 implementations without finding any clue to why this is so.

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