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• Encrypt/Decrypt SQLite-database and use it "on the fly"

  - by Berschi

  Here's the thing: In my Qt4.6-Project, I use a SQLite-Database. This database shouldn't be unencrypted on my harddrive. So I want, that on every start of my program, the user gets asked to enter a password to decrypt the database. Of course the database never should appear "in clear" (not encrypted) on my harddrive. So is there any possibility to decrypt a SQLite-database "on the fly" and read and write data? What algorithm is here the best (maybe AES)? When it's not possible (or very slow), maybe it's better to encrypt every string in the database and decrypt the string when the password was right (so that a user could open the database, but has no clue what all the entrys could mean)?

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• Java RandomAccessFile - dealing with different newline styles?

  - by waitinforatrain

  Hey, I'm trying to seek through a RandomAccessFile, and as part of an algorithm I have to read a line, and then seek backwards from the end of the line E.g String line = raf.readLine(); raf.seek (raf.getFilePointer() - line.length() + m.start() + m.group().length()); //m is a Matcher for regular expressions I've been getting loads of off-by-one errors and couldn't figure out why. I just discovered it's because some files I'm reading from have UNIX-style linefeeds, \r\n, and some have just windows-style \n. Is there an easy to have the RandomAccessFile treat all linefeeds as windows-style linefeeds?

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• Python KMeans clustering words

  - by sadawd

  Dear Everyone I am interested to perform kmeans clustering on a list of words with the distance measure being Leveshtein. 1) I know there are a lot of frameworks out there, including scipy and orange that has a kmeans implementation. However they all require some sort of vector as the data which doesn't really fit me. 2) I need a good clustering implementation. I looked at python-clustering and realize that it doesn't a) return the sum of all the distance to each centroid, and b) it doesn't have any sort of iteration limit or cut off which ensures the quality of the clustering. python-clustering and the clustering algorithm on daniweb doesn't really work for me. Can someone find me a good lib? Google hasn't been my friend

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• Throughput measurements

  - by dotsid

  I wrote simple load testing tool for testing performance of Java modules. One problem I faced is algorithm of throughput measurements. Tests are executed in several thread (client configure how much times test should be repeated), and execution time is logged. So, when tests are finished we have following history: 4 test executions 2 threads 36ms overall time - idle * test execution 5ms 9ms 4ms 13ms T1 |-*****-*********-****-*************-| 3ms 6ms 7ms 11ms T2 |-***-******-*******-***********-----| <-----------------36ms---------------> For the moment I calculate throughput (per second) in a following way: 1000 / overallTime * threadCount. But there is problem. What if one thread will complete it's own tests more quickly (for whatever reason): 3ms 3ms 3ms 3ms T1 |-***-***-***-***----------------| 3ms 6ms 7ms 11ms T2 |-***-******-*******-***********-| <--------------32ms--------------> In this case actual throughput is much better because of measured throughput is bounded by the most slow thread. So, my question is how should I measure throughput of code execution in multithreaded environment.

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• Python ValueError: not allowed to raise maximum limit

  - by Ricky Bobby

  I'm using python 2.7.2 on mac os 10.7.3 I'm doing a recursive algorithm in python with more than 50 000 recursion levels. I tried to increase the maximum recursion level to 1 000 000 but my python shell still exit after 18 000 recursion levels. I tried to increase the resources available : import resource resource.setrlimit(resource.RLIMIT_STACK, (2**29,-1)) sys.setrecursionlimit(10**6) and I get this error : Traceback (most recent call last): File "<pyshell#58>", line 1, in <module> resource.setrlimit(resource.RLIMIT_STACK,(2**29,-1)) ValueError: not allowed to raise maximum limit I don't know why I cannot raise the maximum limit ? thanks for your suggestions .

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• What is your longest-held programming assumption that turned out to be incorrect?

  - by Demi

  I am doing some research into common errors and poor assumptions made by junior (and perhaps senior) software engineers. What was your longest-held poor assumption that was eventually corrected? For example: I at one point failed to understand that the size of an integer was not a standard (depends on the language and target). A bit embarrassing to state, but there it is. Be frank: what hard-held belief did you have, and roughly how long did you maintain the assumption? It can be about an algorithm, a language, a programming concept, testing, anything under the computer science domain.

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• openssl ssl encryption

  - by deddihp

  Hello, I want to discuss about openssl write and read method. Assume I have an data structure like below: /-----------------------------------------------------\ | my_header | PAYLOAD | \-----------------------------------------------------/ | | \ / \ / not encrypted encrypted I think the proper algorithm would be like this : SEND: build my_header with my own header. encrypt PAYLOAD with encryption function attach my_header and PAYLOAD (encrypted) to one buffer send it using common POSIX function just like send or sendto RECV: using common POSIX function just like recv or recvfrom. extract my_header and PAYLOAD(encrypted) decrypt PAYLOAD with decryption function at last i got my_header and PAYLOAD(decrypted). How is your approach if you face a problem like above. Since openssl encrypt all of data that is sent to SSL_write function (CMIIW). Thanks

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• SharePoint Permissions

  - by Greg

  I have a custom workflow. This workflow removes permissions to items when an item is added (example an item is added by a service account and once added those permissions need to be removed from that item). This works as I have the service account 'hard coded' in the custom workflow. Now I would like to remove this hard coding and when a item is added to a list I would like to iterate through all users that have access to the list item. If a user matches some algorithm then remove that user from the item permissions which will be 0 to many. The piece I'm stuggling with is how to iterage all users with permission to a SPListItem. Any thoughts on how to accomplish this? Thanks in advance!

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• Mass Ball-to-Ball Collision Handling (as in, lots of balls)

  - by BlueThen

  Update: Found out that I was using the radius as the diameter, which was why the mtd was overcompensating. Hi, StackOverflow. I've written a Processing program awhile back simulating ball physics. Basically, I have a large number of balls (1000), with gravity turned on. Detection works great, but my issue is that they start acting weird when they're bouncing against other balls in all directions. I'm pretty confident this involves the handling. For the most part, I'm using Jay Conrod's code. One part that's different is if (distance > 1.0) return; which I've changed to if (distance < 1.0) return; because the collision wasn't even being performed with the first bit of code, I'm guessing that's a typo. The balls overlap when I use his code, which isn't what I was looking for. My attempt to fix it was to move the balls to the edge of each other: float angle = atan2(y - collider.y, x - collider.x); float distance = dist(x,y, balls[ID2].x,balls[ID2].y); x = collider.x + radius * cos(angle); y = collider.y + radius * sin(angle); This isn't correct, I'm pretty sure of that. I tried the correction algorithm in the previous ball-to-ball topic: // get the mtd Vector2d delta = (position.subtract(ball.position)); float d = delta.getLength(); // minimum translation distance to push balls apart after intersecting Vector2d mtd = delta.multiply(((getRadius() + ball.getRadius())-d)/d); // resolve intersection -- // inverse mass quantities float im1 = 1 / getMass(); float im2 = 1 / ball.getMass(); // push-pull them apart based off their mass position = position.add(mtd.multiply(im1 / (im1 + im2))); ball.position = ball.position.subtract(mtd.multiply(im2 / (im1 + im2))); except my version doesn't use vectors, and every ball's weight is 1. The resulting code I get is this: PVector delta = new PVector(collider.x - x, collider.y - y); float d = delta.mag(); PVector mtd = new PVector(delta.x * ((radius + collider.radius - d) / d), delta.y * ((radius + collider.radius - d) / d)); // push-pull apart based on mass x -= mtd.x * 0.5; y -= mtd.y * 0.5; collider.x += mtd.x * 0.5; collider.y += mtd.y * 0.5; This code seems to over-correct collisions. Which doesn't make sense to me because in no other way do I modify the x and y values of each ball, other than this. Some other part of my code could be wrong, but I don't know. Here's the snippet of the entire ball-to-ball collision handling I'm using: if (alreadyCollided.contains(new Integer(ID2))) // if the ball has already collided with this, then we don't need to reperform the collision algorithm return; Ball collider = (Ball) objects.get(ID2); PVector collision = new PVector(x - collider.x, y - collider.y); float distance = collision.mag(); if (distance == 0) { collision = new PVector(1,0); distance = 1; } if (distance < 1) return; PVector velocity = new PVector(vx,vy); PVector velocity2 = new PVector(collider.vx, collider.vy); collision.div(distance); // normalize the distance float aci = velocity.dot(collision); float bci = velocity2.dot(collision); float acf = bci; float bcf = aci; vx += (acf - aci) * collision.x; vy += (acf - aci) * collision.y; collider.vx += (bcf - bci) * collision.x; collider.vy += (bcf - bci) * collision.y; alreadyCollided.add(new Integer(ID2)); collider.alreadyCollided.add(new Integer(ID)); PVector delta = new PVector(collider.x - x, collider.y - y); float d = delta.mag(); PVector mtd = new PVector(delta.x * ((radius + collider.radius - d) / d), delta.y * ((radius + collider.radius - d) / d)); // push-pull apart based on mass x -= mtd.x * 0.2; y -= mtd.y * 0.2; collider.x += mtd.x * 0.2; collider.y += mtd.y * 0.2; Thanks. (Apologies for lack of sources, stackoverflow thinks I'm a spammer)

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• how to pass an arbitrary signature to Certifcate

  - by eskoba

  I am trying to sign certificate (X509) using secret sharing. that is shareholders combine their signatures to produce the final signature. which will be in this case the signed certificate. however practically from my understanding only one entity can sign a certificate. therefore I want to know: which entities or data of the x509certificate are actually taken as input to the signing algorithm? ideally I want this data to be signed by the shareholders and then the final combination will be passed to the X509certificate as valid signature. is this possible? how could it done? if not are they other alternatives?

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• pyplot: really slow creating heatmaps

  - by cvondrick

  I have a loop that executes the body about 200 times. In each loop iteration, it does a sophisticated calculation, and then as debugging, I wish to produce a heatmap of a NxM matrix. But, generating this heatmap is unbearably slow and significantly slow downs an already slow algorithm. My code is along the lines: import numpy import matplotlib.pyplot as plt for i in range(200): matrix = complex_calculation() plt.set_cmap("gray") plt.imshow(matrix) plt.savefig("frame{0}.png".format(i)) The matrix, from numpy, is not huge --- 300 x 600 of doubles. Even if I do not save the figure and instead update an on-screen plot, it's even slower. Surely I must be abusing pyplot. (Matlab can do this, no problem.) How do I speed this up?

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• Does the 80/20 rule of time management apply to developers?

  - by Dean

  Jeff's recent article linked to a time management example of the First Fit Decreasing algorithm, which talked about the Pareto principle (or, the 80/20 rule) of time management, that is, that 80% of the work we produce in 20% of our time. Now we've all heard the programmer quote: The first 90% of the code accounts for the first 90% of the development time. The remaining 10% of the code accounts for the other 90% of the development time. But all jokes aside, it is often as if 20% of your code is to do what you want, and the other 80% is to handle exceptions... so does the 80/20 rule really apply to developers? Does anyone have any examples of why it does / does not apply to us?

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• is there a such thing as a randomly accessible pseudo-random number generator? (preferably open-sour

  - by lucid

  first off, is there a such thing as a random access random number generator, where you could not only sequentially generate random numbers as we're all used to, assuming rand100() always generates a value from 0-100: for (int i=0;i<5;i++) print rand100() output: 14 75 36 22 67 but also randomly access any random value like: rand100(0) would output 14 as long as you didn't change the seed rand100(3) would always output 22 rand100(4) would always output 67 and so on... I've actually found an open-source generator algorithm that does this, but you cannot change the seed. I know that pseudorandomness is a complex field; I wouldn't know how to alter it to add that functionality. Is there a seedable random access random number generator, preferably open source? or is there a better term for this I can google for more information? if not, part 2 of my question would be, is there any reliably random open source conventional seedable pseudorandom number generator so I could port it to multiple platforms/languages while retaining a consistent sequence of values for each platform for any given seed?

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• Can this be done with the ORM? - Django

  - by RadiantHex

  Hi folks, I have a few item listed in a database, ordered through Reddit's algorithm. This is it: def reddit_ranking(post): t = time.mktime(post.created_on.timetuple()) - 1134000000 x = post.score if x>0: y=1 elif x==0: y=-0 else: y=-1 if x<0: z=1 else: z=x return (log(z) + y * t/45000) I'm wondering if there is any clever way of using Django's ORM, in order to UPDATE the models in bulk. Without doing this: items = Item.objects.filter(created_on__gte=datetime.now()-timedelta(days=7)) for item in items: item.reddit_rank = reddit_rank(item) item.save() I know about the F() object, but I can't figure out if this function can be performed inside the ORM. Any ideas? Help would be very much appreciated!

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• Query size of block device file in Python

  - by ??O?????

  Hello. I have a Python script that reads a file (typically from optical media) marking the unreadable sectors, to allow a re-attempt to read said unreadable sectors on a different optical reader. I discovered that my script does not work with block devices (e.g. /dev/sr0), in order to create a copy of the contained ISO9660/UDF filesystem, because os.stat().st_size is zero. The algorithm currently needs to know the filesize in advance; I can change that, but the issue (of knowing the block device size) remains, and it's not answered here, so I open this question. I am aware of the following two related SO questions: Determine the size of a block device (/proc/partitions, ioctl through ctypes) how to check file size in python? (about non-special files) Therefore, I'm asking: in Python, how can I get the file size of a block device file?

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• Writing a search engine

  - by wvd

  Hello all, The title might be a bit misleading, but I couldn't figure out a better title. I'm writing a simple search engine which will search on several sites for the specific domain. To be concrete: I'm writing a search engine for hardstyle livesets/aftermovies/tracks. To do I will search on the sites who provide livesets, tracks, and such. The problem here is speed, I need to pass the search query to 5-7 sites, get the results and then use my own algorithm to display the results in a sorted order. I could just "multithread" it, but it's easier said then done so I have a few questions. What would be the best solution to this problem? Should I just multithread/process this application, so I'm going to get a bit of speed-up? Are there any other solutions or I am doing something really wrong? Thanks, William van Doorn

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• Counting viable sublist lengths from an array.

  - by Ben B.

  This is for a genetic algorithm fitness function, so it is important I can do this as efficiently as possible, as it will be repeated over and over. Lets say there is a function foo(int[] array) that returns true if the array is a "good" array and false if the array is a "bad" array. What makes it good or bad does not matter here. Given the full array [1,6,8,9,5,11,45,16,9], lets say that subarray [1,6,8] is a "good" array and [9,5,11,45] is a "good" array. Furthermore [5,11,45,16,9] is a "good" array, and also the longest "good" subarray. Notice that while [9,5,11,45] is a "good" array, and [5,11,45,16,9] is a "good" array, [9,5,11,45,16,9] is a "bad" array. We wants the length counts of all "good" arrays, but not subarrays of "good" arrays. Furthermore, as described above, a "good" array might begin in the middle of another "good" array, but the combination of the two might be a "bad" array.

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• matlab fit exp2

  - by HelloWorld

  I'm unsuccessfully looking for documentation of fit function using exp2 (sum of 2 exponents). How to operate the function is clear: [curve, gof] = fit(x, y,'exp2'); But since there are multiple ways to fit a sum of exponents I'm trying to find out what algorithm is used. Particularly what happens when I'm fitting one exponent (the raw data) with a bit of noise, how the exponents are spread. I've simulated several cases, and it seems that it "drops" all the weight on the second set of coefficients, but row data analysis often shows different behavior. Does anyone have suggestions of documentation?

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• Any tips of how to handle hierarchical trees in relational model?

  - by George

  Hello all. I have a tree structure that can be n-levels deep, without restriction. That means that each node can have another n nodes. What is the best way to retrieve a tree like that without issuing thousands of queries to the database? I looked at a few other models, like flat table model, Preorder Tree Traversal Algorithm, and so. Do you guys have any tips or suggestions of how to implement a efficient tree model? My objective in the real end is to have one or two queries that would spit the whole tree for me. With enough processing i can display the tree in dot net, but that would be in client machine, so, not much of a big deal. Thanks for the attention

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• C++: call original definition of operator equals

  - by Luis Daniel

  I am overloading the operator equals (==) as show bellow: #include <string> #include <algorithm> bool operator == (std::string str1, std::string str2) { std::transform(str1.begin(), str1.end(), str1.begin(), ::tolower); std::transform(str2.begin(), str2.end(), str2.begin(), ::tolower); return (str1 == str2); } but, the problem appear on line return (str1 == str2), because operator == is called recursively. So, how can I call the original definition for operator equals (not the overloaded) ? Best regards

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• Efficiency of while(true) ServerSocket Listen

  - by Submerged

  I am wondering if a typical while(true) ServerSocket listen loop takes an entire core to wait and accept a client connection (Even when implementing runnable and using Thread .start()) I am implementing a type of distributed computing cluster and each computer needs every core it has for computation. A Master node needs to communicate with these computers (invoking static methods that modify the algorithm's functioning). The reason I need to use sockets is due to the cross platform / cross language capabilities. In some cases, PHP will be invoking these java static methods. I used a java profiler (YourKit) and I can see my running ServerSocket listen thread and it never sleeps and it's always running. Is there a better approach to do what I want? Or, will the performance hit be negligible? Please, feel free to offer any suggestion if you can think of a better way (I've tried RMI, but it isn't supported cross-language. Thanks everyone

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• Java text classification problem

  - by yox

  Hello, I have a set of Books objects, classs Book is defined as following : Class Book{ String title; ArrayList<tags> taglist; } Where title is the title of the book, example : Javascript for dummies. and taglist is a list of tags for our example : Javascript, jquery, "web dev", .. As I said a have a set of books talking about different things : IT, BIOLOGY, HISTORY, ... Each book has a title and a set of tags describing it.. I have to classify automaticaly those books into separated sets by topic, example : IT BOOKS : Java for dummies Javascript for dummies Learn flash in 30 days C++ programming HISTORY BOOKS : World wars America in 1960 Martin luther king's life BIOLOGY BOOKS : .... Do you guys know a classification algorithm/method to apply for that kind of problems ? A solution is to use an external API to define the category of the text, but the problem here is that books are in different languages : french, spanish, english ..

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• C# how to create functions that are interpreted at runtime

  - by Lirik

  I'm making a Genetic Program, but I'm hitting a limitation with C# where I want to present new functions to the algorithm but I can't do it without recompiling the program. In essence I want the user of the program to provide the allowed functions and the GP will automatically use them. It would be great if the user is required to know as little about programming as possible. I want to plug in the new functions without compiling them into the program. In Python this is easy, since it's all interpreted, but I have no clue how to do it with C#. Does anybody know how to achieve this in C#? Are there any libraries, techniques, etc?

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• question about frequency of updating access

  - by I__

  i have a table in an access database this access database is used on a regular basis, basically from 9-5 someone else has a copy of this exact table. sometimes records are added, sometimes deleted, and sometimes data within the records is updated. i need to update the access database table with the offsite table every hour or so. what is the best algorithm of updating the data? there are about 5000 records. would it severely lock up the table for a few seconds every hour? i would like to publicly apologize for my rude comment to david fenton

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• Simple string encryption in .NET and Javascript

  - by jonathanconway

  I'm developing an ASP.NET MVC application in which I want to encrypt a short string on the server, using C#, and send it to the client-side. Then on the client-side it will be decrypted through Javascript code. Any thoughts on how to implement this? Do you know of a simple encryption algorithm (doesn't have to be bullet-proof secure) that can be easily translated from C# to Javascript or vice-versa? NOTE: I could do this entirely in C# and do the decryption through Ajax, but I'd prefer not to do it this way, as I want to reduce website traffic as much as possible.

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