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  • Which term to use when referring to functional data structures: persistent or immutable?

    - by Bob
    In the context of functional programming which is the correct term to use: persistent or immutable? When I Google "immutable data structures" I get a Wikipedia link to an article on "Persistent data structure" which even goes on to say: such data structures are effectively immutable Which further confuses things for me. Do functional programs rely on persistent data structures or immutable data structures? Or are they always the same thing?

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  • MVP pattern. Presenter requires new view instance. Best practice

    - by Andrew Florko
    I try to apply MVP pattern for win.forms application. I have 2 forms: main & child. Main has a button and when you click it - child form should appear. There are 2 views interfaces that forms implement IMainView { event OnClick; ... } IChildView { ... } There are two presenters MainPresenter(IMainView) & ChildPresenter(IChildView) MainPresenter listens to OnClick event and then should create IChildView implementation. MainPresenter { ... MainClicked() { // it's required to create IChildView instance here } } How would you implement such creation typically? Shall IMainView has factory method for IChildView or may be it should be separate Views factory. What would you advise? Or maybe there is some misunderstanding of MVP here? Thank you in advance!

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  • Reusability, testability, code complexity reduction and showing-off-ability programming importance

    - by Andrew Florko
    There are lots of programming and architecture patterns. Patterns allow to make code cleaner, reusable, more testable & at last (but not at least) to feel the follower a real cool developer. How do you rank these considerations for you? What does affect you most when you decide to apply pattern? I wonder how many times code reusability (especially for MVP, MVC patterns) was important? For example DAL library often shared between projects (it's reusable) but how often controllers/views (abstracted via interfaces) are reused?

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  • How strict should I be in the "do the simplest thing that could possible work" while doing TDD

    - by Support - multilanguage SO
    For TDD you have to Create a test that fail Do the simplest thing that could possible work to pass the test Add more variants of the test and repeat Refactor when a pattern emerge With this approach you're supposing to cover all the cases ( that comes to my mind at least) but I'm wonder if am I being too strict here and if it is possible to "think ahead" some scenarios instead of simple discover them. For instance, I'm processing a file and if it doesn't conform to a certain format I am to throw an InvalidFormatException So my first test was: @Test void testFormat(){ // empty doesn't do anything... processor.validate("empty.txt"); try { processor.validate("invalid.txt"); assert false: "Should have thrown InvalidFormatException"; } catch( InvalidFormatException ife ) { assert "Invalid format".equals( ife.getMessage() ); } } I run it and it fails because it doesn't throw an exception. So the next thing that comes to my mind is: "Do the simplest thing that could possible work", so I : public void validate( String fileName ) throws InvalidFormatException { if(fileName.equals("invalid.txt") { throw new InvalidFormatException("Invalid format"); } } Doh!! ( although the real code is a bit more complicated, I found my self doing something like this several times ) I know that I have to eventually add another file name and other test that would make this approach impractical and that would force me to refactor to something that makes sense ( which if I understood correctly is the point of TDD, to discover the patterns the usage unveils ) but: Q: am I taking too literal the "Do the simplest thing..." stuff?

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  • How are a session identifiers generated?

    - by Asaf R
    Most web applications depend on some kind of session with the user (for instance, to retain login status). The session id is kept as a cookie in the user's browser and sent with every request. To make it hard to guess the next user's session these session-ids need to be sparse and somewhat random. The also have to be unique. The question is - how to efficiently generate session ids that are sparse and unique? This question has a good answer for unique random numbers, but it seems not scalable for a large range of numbers, simply because the array will end up taking a lot of memory.

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  • How do i know if this is random enough?

    - by David
    I wrote a program in java that rolls a die and records the total number of times each value 1-6 is rolled. I rolled 6 Million times. Here's the distribution: #of 0's: 0 #of 1's: 1000068 #of 2's: 999375 #of 3's: 999525 #of 4's: 1001486 #of 5's: 1000059 #of 6's: 999487 (0 wasn't an option.) Is this distribution consistant with random dice rolls? What objective statistical tests might confirm that the dice rolls are indeed random enough?

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  • How does the Amazon Recommendation feature work?

    - by Rachel
    What technology goes in behind the screens of Amazon recommendation technology? I believe that Amazon recommendation is currently the best in the market, but how do they provide us with such relevant recommendations? Recently, we have been involved with similar recommendation kind of project, but would surely like to know about the in and outs of the Amazon recommendation technology from a technical standpoint. Any inputs would be highly appreciated. Update: This patent explains how personalized recommendations are done but it is not very technical, and so it would be really nice if some insights could be provided. From the comments of Dave, Affinity Analysis forms the basis for such kind of Recommendation Engines. Also here are some good reads on the Topic Demystifying Market Basket Analysis Market Basket Analysis Affinity Analysis Suggested Reading: Data Mining: Concepts and Technique

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  • Shortest distance between two line segments

    - by Frank
    I need a function to find the shortest distance between two line segments. A line segment is defined by two endpoints. So for example one of my line segments (AB) would be defined by the two points A (x1,y1) and B (x2,y2) and the other (CD) would be defined by the two points C (x1,y1) and D (x2,y2). Feel free to write the solution in any language you want and I can translate it into javascript. Please keep in mind my geometry skills are pretty rusty. I have already seen http://stochastix.wordpress.com/2008/12/28/distance-between-two-lines/ and I am not sure how to translate this into a function. Thank you so much for help.

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  • Testing When Correctness is Poorly Defined?

    - by dsimcha
    I generally try to use unit tests for any code that has easily defined correct behavior given some reasonably small, well-defined set of inputs. This works quite well for catching bugs, and I do it all the time in my personal library of generic functions. However, a lot of the code I write is data mining code that basically looks for significant patterns in large datasets. Correct behavior in this case is often not well defined and depends on a lot of different inputs in ways that are not easy for a human to predict (i.e. the math can't reasonably be done by hand, which is why I'm using a computer to solve the problem in the first place). These inputs can be very complex, to the point where coming up with a reasonable test case is near impossible. Identifying the edge cases that are worth testing is extremely difficult. Sometimes the algorithm isn't even deterministic. Usually, I do the best I can by using asserts for sanity checks and creating a small toy test case with a known pattern and informally seeing if the answer at least "looks reasonable", without it necessarily being objectively correct. Is there any better way to test these kinds of cases?

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  • Fast, Vectorizable method of taking floating point number modulus of special primes?

    - by caffiend
    Is there a fast method for taking the modulus of a floating point number? With integers, there are tricks for Mersenne primes, so that its possible to calculate y = x MOD 2^31 without needing division. Can any similar tricks be applied for floating point numbers? Preferably, in a way that can be converted into vector/SIMD operations, or moved into GPGPU code. The primes I'm interested in would be 2^7 and 2^31, although if there are more efficient ones for floating point numbers, those would be welcome.

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  • What should I tell kids about how great it is to be a programmer?

    - by Sara Chipps
    I am putting a presentation together. I thought about illustrating with websites like Facebook, and MySpace. Does anyone have children around that age that could tell me what they are into? How to hold their attention? Ways to illustrate what we do? Get them interested? Your ideas are greatly appreciated, I really want to be able to convey how fun this is :). I don't have access to a digital projector... which really stinks. I do have access to an old transparency overhead, though. http://stackoverflow.com/questions/207278/career-day-how-do-i-make-computer-programmer-sound-cool-to-8-year-olds

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  • Please quickly help with this problem I got 52 minutes left.

    - by Hamish Grubijan
    Write a program that prints the numbers from 1 to 100. But for multiples of three print "Fizz" instead of the number and for the multiples of five print "Buzz". For numbers which are multiples of both three and five print "FizzBuzz". Woman said use any common language. Please make it short and test it. My screen is small. Thanks. P.S. I have test anxiety particularly after talking to people in suits. I also stayed up all night studying Java codes.

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  • Implementing Transparent Persistence

    - by Jules
    Transparent persistence allows you to use regular objects instead of a database. The objects are automatically read from and written to disk. Examples of such systems are Gemstone and Rucksack (for common lisp). Simplified version of what they do: if you access foo.bar and bar is not in memory, it gets loaded from disk. If you do foo.bar = baz then the foo object gets updated on disk. Most systems also have some form of transactions, and they may have support for sharing objects across programs and even across a network. My question is what are the different techniques for implementing these kind of systems and what are the trade offs between these implementation approaches?

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  • How to implement square root and exponentiation on arbitrary length numbers?

    - by tomp
    I'm working on new data type for arbitrary length numbers (only non-negative integers) and I got stuck at implementing square root and exponentiation functions (only for natural exponents). Please help. I store the arbitrary length number as a string, so all operations are made char by char. Please don't include advices to use different (existing) library or other way to store the number than string. It's meant to be a programming exercise, not a real-world application, so optimization and performance are not so necessary. If you include code in your answer, I would prefer it to be in either pseudo-code or in C++. The important thing is the algorithm, not the implementation itself. Thanks for the help.

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  • Do similar passwords have similar hashes?

    - by SLC
    Our computer system at work requires users to change their password every few weeks, and you cannot have the same password as you had previously. It remembers something like 20 of your last passwords. I discovered most people simply increment a digit at the end of their password, so "thisismypassword1" becomes "thisismypassword2" then 3, 4, 5 etc. Since all of these passwords are stored somewhere, I wondered if there was any weakness in the hashes themselves, for standard hashing algorithms used to store passwords like MD5. Could a hacker increase their chances of brute-forcing the password if they have a list of hashes of similar passwords?

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  • What do you enjoy about programming?

    - by Earlz
    Some of us here(or is it just me?) enjoy programming. Even if we're not being paid for it, and in some cases, even though the end result will not do anything for us. For example, many people do the Project Euler problems just for fun, and in the end nothing was really "accomplished" materially. What is it that makes us enjoy programming? How is programming different from another job? You don't see an accountant going home to do some accounting on their own time just for the pure joy of it. How are we different? (also, if anyone has some ideas on how to tag this, then please do correct it for me.. )

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  • Integration tests - "no exceptions are thrown" approach. Does it make sense?

    - by Andrew Florko
    Sometimes integration tests are rather complex to write or developers have no enough time to check output - does it make sense to write tests that make sure "no exceptions are thrown" only? Such tests provide some input parameters set(s) and doesn't check the result, but only make sure code not failed with exception? May be such tests are not very useful but appropriate in situations when you have no time?

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  • What is the preferred way to indent cases in a switch?

    - by neutrino
    Hey there, As I was writing another switch in Eclipse, I once again came across a rather weird (to me, at least) default indentation, which is applied to 'switch' statements: switch (i) { case 1: ... case n: ... } I tend to prefer another way: switch (i) { case 1: ... case n: ... } Which way is more readable and eye-pleasing for you? I'm still not hundred percent determined what's best for me, so I'd like to stick to what's best for other people who would read my code. BTW, you're free to close this question if this is too subjective. :)

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  • Is there a website to lookup already common code functions?

    - by pinnacler
    I'm sitting here writing a function that I'm positive has been written before, somewhere on earth. It's just too common to have not been attempted, and I'm wondering why I can't just go to a website and search for a function that I can then copy and paste into my project in 2 seconds, instead of wasting my day reinventing the wheel. Sure there are certain libraries you can use, but where do you find these libraries and when they are absent, is there a site like I'm describing?

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  • Flow Based Programming

    - by Software Monkey
    I have been doing a little reading on Flow Based Programming over the last few days. There is a wiki which provides further detail. And wikipedia has a good overview on it too. My first thought was, "Great another proponent of lego-land pretend programming" - a concept harking back to the late 80's. But, as I read more, I must admit I have become intrigued. Have you used FBP for a real project? What is your opinion of FBP? Does FBP have a future? In some senses, it seems like the holy grail of reuse that our industry has pursued since the advent of procedural languages.

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  • Is it advisable to have non-ascii characters in the URL?

    - by Ravi Gummadi
    We are currently working on a I18N project. I was just wondering what are the complications of having the non-ascii characters in the URL. If its not, what are the alternatives to deal with this problem? EDIT (in response to Maxym's answer): The site is going to be local to specific country and I need not worry about the world wide public accessing this site. I understand that from usability point of view, It is really annoying. What are the other technical problem associated with this?

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  • Randomly sorting an array

    - by Cam
    Does there exist an algorithm which, given an ordered list of symbols {a1, a2, a3, ..., ak}, produces in O(n) time a new list of the same symbols in a random order without bias? "Without bias" means the probability that any symbol s will end up in some position p in the list is 1/k. Assume it is possible to generate a non-biased integer from 1-k inclusive in O(1) time. Also assume that O(1) element access/mutation is possible, and that it is possible to create a new list of size k in O(k) time. In particular, I would be interested in a 'generative' algorithm. That is, I would be interested in an algorithm that has O(1) initial overhead, and then produces a new element for each slot in the list, taking O(1) time per slot. If no solution exists to the problem as described, I would still like to know about solutions that do not meet my constraints in one or more of the following ways (and/or in other ways if necessary): the time complexity is worse than O(n). the algorithm is biased with regards to the final positions of the symbols. the algorithm is not generative. I should add that this problem appears to be the same as the problem of randomly sorting the integers from 1-k, since we can sort the list of integers from 1-k and then for each integer i in the new list, we can produce the symbol ai.

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  • Consuming services that consume other services.

    - by phthomas
    What is the best way to confirm that these consumed services are actually up and running before I actually try to invoke its operation contracts? I want to do this so that I can gracefully display some message to the customer to give him/her a more pleasant user experience. Thanks.

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