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• Why my UTableView with style UITableViewStyleGrouped is consuming memory?

  - by prathumca

  Hello everyone, Currently in my app, I'm using an UITableView with style UITableViewStyleGrouped as shown below. CGRect imgFrame = CGRectMake(0, 0, 320, 650); UITableView *myTable = [[UITableView alloc] initWithFrame:imgFrame style:UITableViewStyleGrouped]; myTable.dataSource = self; myTable.delegate = self; //make the current object the event handler for view [self.view addSubview:myTable]; [myTable release]; And the data has stored in an array "dataArray". dataArray has collection of arrays, where each array represent a section. Currently I have only one section with 100 records. When I installed my app onto my IPhone, I observed that this UITableView is consuming 20 MB of IPhone memory. If I changed the table view style to "UITableViewStylePlain", then it is consuming only 4MB of memory. I'm trying to figure it out, where is the exact problem, but not. What was wrong with "UITableViewStyleGrouped"? Regards, prathumca.

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• Design suggestion for expression tree evaluation with time-series data

  - by Lirik

  I have a (C#) genetic program that uses financial time-series data and it's currently working but I want to re-design the architecture to be more robust. My main goals are: sequentially present the time-series data to the expression trees. allow expression trees to access previous data rows when needed. to optimize performance of the data access while evaluating the expression trees. keep a common interface so various types of data can be used. Here are the possible approaches I've thought about: I can evaluate the expression tree by passing in a data row into the root node and let each child node use the same data row. I can evaluate the expression tree by passing in the data row index and letting each node get the data row from a shared DataSet (currently I'm passing the row index and going to multiple synchronized arrays to get the data). Hybrid: an immutable data set is accessible by all of the expression trees and each expression tree is evaluated by passing in a data row. The benefit of the first approach is that the data row is being passed into the expression tree and there is no further query done on the data set (which should increase performance in a multithreaded environment). The drawback is that the expression tree does not have access to the rest of the data (in case some of the functions need to do calculations using previous data rows). The benefit of the second approach is that the expression trees can access any data up to the latest data row, but unless I specify what that row is, I'll have to iterate through the rows and figure out which one is the last one. The benefit of the hybrid is that it should generally perform better and still provide access to the earlier data. It supports two basic "views" of data: the latest row and the previous rows. Do you guys know of any design patterns or do you have any tips that can help me build this type of system? Should I use a DataSet to hold and present the data, or are there more efficient ways to present rows of data while maintaining a simple interface? FYI: All of my code is written in C#.

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• How to shuffle pairs

  - by Jessy

  How to shuffle the elements in the pairs? The program below, generate all possible pairs and later shuffle the pairs. e.g. possible pairs before shuffle is ab,ac,ae,af..etc shuffled to ac,ae,af,ab...etc How to make it not only shuffled in pairs but within the elements in the pair itself? e.g. instead of ab, ac, how can I make ba, ac ? String[] pictureFile = {"a.jpg","b.jpg","c.jpg","d.jpg","e.jpg","f.jpg","g.jpg"}; List <String> pic1= Arrays.asList(pictureFile); ... ListGenerator pic2= new ListGenerator(pic1); ArrayList<ArrayList<Integer>> pic2= new ArrayList<ArrayList<Integer>>(); public class ListGenerator { public ListGenerator(List<String> pic1) { int size = pic1.size(); // create a list of all possible combinations for(int i = 0 ; i < size ; i++) { for(int j = (i+1) ; j < size ; j++) { ArrayList<Integer> temp = new ArrayList<Integer>(); temp.add(i); temp.add(j); pic2.add(temp); } } Collections.shuffle(pic2); } //This method return the shuffled list public ArrayList<ArrayList<Integer>> getList() { return pic2; } }

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• Game of life in F# with accelerator

  - by jpalmer

  I'm trying to write life in F# using accelerator v2, but for some odd reason my output isn't square despite all my arrays being square - It appears that everything but a rectangular area in the top left of the matrix is being set to false. I've got no idea how this could be happening as all my operations should treat the entire array equally. Any ideas? open Microsoft.ParallelArrays open System.Windows.Forms open System.Drawing type IPA = IntParallelArray type BPA = BoolParallelArray type PAops = ParallelArrays let RNG = new System.Random() let size = 1024 let arrinit i = Array2D.init size size (fun x y -> i) let target = new DX9Target() let threearr = new IPA(arrinit 3) let twoarr = new IPA(arrinit 2) let onearr = new IPA(arrinit 1) let zeroarr = new IPA(arrinit 0) let shifts = [|-1;-1|]::[|-1;0|]::[|-1;1|]::[|0;-1|]::[|0;1|]::[|1;-1|]::[|1;0|]::[|1;1|]::[] let progress (arr:BPA) = let sums = shifts //adds up whether a neighbor is on or not |> List.fold (fun (state:IPA) t ->PAops.Add(PAops.Cond(PAops.Rotate(arr,t),onearr,zeroarr),state)) zeroarr PAops.Or(PAops.CompareEqual(sums,threearr),PAops.And(PAops.CompareEqual(sums,twoarr),arr)) //rule for life let initrandom () = Array2D.init size size (fun x y -> if RNG.NextDouble() > 0.5 then true else false) type meform () as self= inherit Form() let mutable array = new BoolParallelArray(initrandom()) let timer = new System.Timers.Timer(1.0) //redrawing timer do base.DoubleBuffered <- true do base.Size <- Size(size,size) do timer.Elapsed.Add(fun _ -> self.Invalidate()) do timer.Start() let draw (t:Graphics) = array <- array |> progress let bmap = new System.Drawing.Bitmap(size,size) target.ToArray2D array |> Array2D.iteri (fun x y t -> if not t then bmap.SetPixel(x,y,Color.Black)) t.DrawImageUnscaled(bmap,0,0) do self.Paint.Add(fun t -> draw t.Graphics) do Application.Run(new meform())

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• How to decode base64-encoded <data> (CFData/NSData) property in a property list?

  - by bantic

  I am trying to reverse-engineer a preferences file (not for any nefarious purposes, just so that I can script usage of it) that, among other things, has arrays of coordinates stored within it. This is the salient snippet from the property list: <dict> <key>$class</key> <dict> <key>CF$UID</key> <integer>34</integer> </dict> <key>coordArray</key> <data> AAAAAAAAAAAAAAAAAAAAAT70vS8/M7xSPwAAAD8AAAA/AAAA </data> <key>coordCount</key> <integer>1</integer> </dict> I assume that data string is an array of coordinates (based on its key name). My question is, how can I figure out what data is stored there? If I simply base64-decode that string, I get gibberish. Is there a way to decode it and cast it into whatever format it came from (NSArray, I think)?

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• Fastest way to generate delimited string from 1d numpy array

  - by Abiel

  I have a program which needs to turn many large one-dimensional numpy arrays of floats into delimited strings. I am finding this operation quite slow relative to the mathematical operations in my program and am wondering if there is a way to speed it up. For example, consider the following loop, which takes 100,000 random numbers in a numpy array and joins each array into a comma-delimited string. import numpy as np x = np.random.randn(100000) for i in range(100): ",".join(map(str, x)) This loop takes about 20 seconds to complete (total, not each cycle). In contrast, consider that 100 cycles of something like elementwise multiplication (x*x) would take than one 1/10 of a second to complete. Clearly the string join operation creates a large performance bottleneck; in my actual application it will dominate total runtime. This makes me wonder, is there a faster way than ",".join(map(str, x))? Since map() is where almost all the processing time occurs, this comes down to the question of whether there a faster to way convert a very large number of numbers to strings.

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• Flex 3: should I provide prepared data to my component or make it to process data before display?

  - by grapkulec

  I'm starting to learn a little Flex just for fun and maybe to prove that I still can learn something new :) I have some idea for a project and one of its parts is a tree component which could display data in different ways depending on configuration. The idea There is list of objects having properties like id, date, time, name, description. And sometimes list should be displayed like this: first level: date second level: time third level: name and sometimes like this: first level: year second level: month third level: day fourth level: time and name By level I mean level of nesting of course. So, we can have years, that have months, that have days, that have hours and so forth. The problem What could be the best way to do it? I mean, should I prepare data for different ways of nesting outside of component or even outside of flex? I can do it at web service level in C# where I plan to have database access layer and send to flex nice and ready to display XML or array of objects. But I wonder if that won't cause additional and maybe unneccessary network traffic. I tried to hack some code in my component to convert my data objects into XML or ArrayCollection but I don't know enough of Flex and got stuck on elimination of duplicates or getting specific data by some key value. Usually to do such things I have STL with maps, sets and vectors and I find Flex arrays and even Dictionary a little bit confusing (I've read language reference and googled without any significant luck). The question So, to sum things up: should I give my tree component data prepared just for chosen type of display or should I try to do it internally inside component (or some helper class written in ActionScript)?

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• Design Practice Retrieve Multiple Users - PHP

  - by pws5068

  Greetings all, I'm looking for a more efficient way to grab multiple users without too much code redundancy. I have a Users class, (here's a highly condensed representation) Class Users { function __construct() { ... ... } private static function getNew($id) { // This is all only example code $sql = "SELECT * FROM Users WHERE User_ID=?"; $user = new User(); $user->setID($id); .... return $user; } public static function getNewestUsers($count) { $sql = "SELECT * FROM Users ORDER BY Join_Date LIMIT ?"; $users = array(); // Prepared Statement Data Here while($stmt->fetch()) { $users[] = Users::getNew($id); ... } return $users; } // These have more sanity/security checks, demonstration only. function setID($id) { $this->id = $id; } function setName($name) { $this->name = $name; } ... } So clearly calling getNew() in a loop is inefficient, because it makes $count number of calls to the database each time. I would much rather select all users in a single SQL statement. The problem is that I have probably a dozen or so methods for getting arrays of users, so all of these would now need to know the names of the database columns which is repeating a lot of structure that could always change later. What are some other ways that I could approach this problem? My goals include efficiency, flexibility, scalability, and good design practice.

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• Misalignement in the output Bitmap created from a byte array

  - by Daniel

  I am trying to understand why I have troubles creating a Bitmap from a byte array. I post this after a careful scrutiny of the existing posts about Bitmap creation from byte arrays, like the followings: Creating a bitmap from a byte[], Working with Image and Bitmap in c#?, C#: Bitmap Creation using bytes array My code is aimed to execute a filter on a digital image 8bppIndexed writing the pixel value on a byte [] buffer to be converted again (after some processing to manage gray levels) in a 8BppIndexed Bitmap My input image is a trivial image created by means of specific perl code: https://www.box.com/shared/zqt46c4pcvmxhc92i7ct Of course, after executing the filter the output image has lost the first and last rows and the first and last columns, due to the way the filter manage borders, so from the original 256 x 256 image i get a 254 x 254 image. Just to stay focused on the issue I have commented the code responsible for executing the filter so that the operation really performed is an obvious: ComputedPixel = InputImage.GetPixel(myColumn, myRow).R; I know, i should use lock and unlock but I prefer one headache one by one. Anyway this code should be a sort of identity transform, and at last i use: private unsafe void FillOutputImage() { OutputImage = new Bitmap (OutputImageCols, OutputImageRows , PixelFormat .Format8bppIndexed); ColorPalette ncp = OutputImage.Palette; for (int i = 0; i < 256; i++) ncp.Entries[i] = Color .FromArgb(255, i, i, i); OutputImage.Palette = ncp; Rectangle area = new Rectangle(0, 0, OutputImageCols, OutputImageRows); var data = OutputImage.LockBits(area, ImageLockMode.WriteOnly, OutputImage.PixelFormat); Marshal .Copy (byteBuffer, 0, data.Scan0, byteBuffer.Length); OutputImage.UnlockBits(data); } The output image I get is the following: https://www.box.com/shared/p6tubyi6dsf7cyregg9e It is quite clear that I am losing a pixel per row, but i cannot understand why: I have carefully controlled all the parameters: OutputImageCols, OutputImageRows and the byte [] byteBuffer length and content even writing known values as way to test. The code is nearly identical to other code posted in stackOverflow and elsewhere. Someone maybe could help to identify where the problem is? Thanks a lot

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• Scale image to fit text boxes around borders

  - by nispio

  I have the following plot in Matlab: The image size may vary, and so may the length of the text boxes at the top and left. I dynamically determine the strings that go in these text boxes and then create them with: [M,N] = size(img); imagesc((1:N)-0.5,(1:M)-0.5, img > 0.5); axis image; grid on; colormap([1 1 1; 0.5 0.5 0.5]); set(gca,'XColor','k','YColor','k','TickDir','out') set(gca,'XTick',1:N,'XTickLabel',cell(1,N)) set(gca,'YTick',1:N,'YTickLabel',cell(1,N)) for iter = 1:M text(-0.5, iter-0.5, sprintf(strL, br{iter,:}), ... 'FontSize',16, ... 'HorizontalAlignment','right', ... 'VerticalAlignment','middle', ... 'Interpreter','latex' ); end for iter = 1:N text(iter-0.5, -0.5, {bc{:,iter}}, ... 'FontSize',16, ... 'HorizontalAlignment','center', ... 'VerticalAlignment','bottom', ... 'Interpreter','latex' ); end where br and bc are cell arrays containing the appropriate numbers for the labels. The problem is that most of the time, the text gets clipped by the edges of the figure. I am using this as a workaround: set(gca,'Position',[0.25 0.25 0.5 0.5]); As you can see, I am simply adding a larger border around the plot so that there is more room for the text. While this scaling works for one zoom level, if I maximize my plot window I get way too much empty space, and if I shrink my plot window, I get clipping again. Is there a more intelligent way to add these labels to use the minimum amount of space while making sure that the text does not get clipped?

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• about Quick Sort

  - by matin1234

  Hi I have written this code but it will print these stack traces in the console please help me thanks! (Aslo "p" and "q" are the first and last index of our array ,respectively) public class JavaQuickSort { public static void QuickSort(int A[], int p, int q) { int i, last = 0; Random rand = new Random(); if (q < 1) { return; } **swap(A, p, rand.nextInt() % (q+1));** for (i = p + 1; i <= q; i++) { if (A[i] < A[p]) { swap(A, ++last, i); } } swap(A, p, last); QuickSort(A, p, last - 1); QuickSort(A, last + 1, q); } private static void swap(int[] A, int i, int j) { int temp; temp = A[i]; **A[i] = A[j];** A[j] = temp; } public static void main(String[] args){ int[] A = {2,5,7,3,9,0,1,6,8}; **QuickSort(A, 0,8 );** System.out.println(Arrays.toString(A)); } } the Stack traces : run: Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: -3 at JavaQuickSort.swap(JavaQuickSort.java:38) at JavaQuickSort.QuickSort(JavaQuickSort.java:22) at JavaQuickSort.main(JavaQuickSort.java:45) Java Result: 1 BUILD SUCCESSFUL (total time: 2 seconds) I also bold those statements that cause these stack traces. like == ** ...**

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• rpy2: Converting a data.frame to a numpy array

  - by Mike Dewar

  I have a data.frame in R. It contains a lot of data : gene expression levels from many (125) arrays. I'd like the data in Python, due mostly to my incompetence in R and the fact that this was supposed to be a 30 minute job. I would like the following code to work. To understand this code, know that the variable path contains the full path to my data set which, when loaded, gives me a variable called immgen. Know that immgen is an object (a Bioconductor ExpressionSet object) and that exprs(immgen) returns a data frame with 125 columns (experiments) and tens of thousands of rows (named genes). robjects.r("load('%s')"%path) # loads immgen e = robjects.r['data.frame']("exprs(immgen)") expression_data = np.array(e) This code runs, but expression_data is simply array([[1]]). I'm pretty sure that e doesn't represent the data frame generated by exprs() due to things like: In [40]: e._get_ncol() Out[40]: 1 In [41]: e._get_nrow() Out[41]: 1 But then again who knows? Even if e did represent my data.frame, that it doesn't convert straight to an array would be fair enough - a data frame has more in it than an array (rownames and colnames) and so maybe life shouldn't be this easy. However I still can't work out how to perform the conversion. The documentation is a bit too terse for me, though my limited understanding of the headings in the docs implies that this should be possible. Anyone any thoughts?

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• question/problem regarding assigning an array of char *

  - by Fantastic Fourier

  Hi I'm working with C and I have a question about assigning pointers. struct foo { int _bar; char * _car[MAXINT]; // this is meant to be an array of char * so that it can hold pointers to names of cars } int foofunc (void * arg) { int bar; char * car[MAXINT]; struct foo thing = (struct foo *) arg; bar = arg->_bar; // this works fine car = arg->_car; // this gives compiler errors of incompatible types in assignment } car and _car have same declaration so why am I getting an error about incompatible types? My guess is that it has something to do with them being pointers (because they are pointers to arrays of char *, right?) but I don't see why that is a problem. when i declared char * car; instead of char * car[MAXINT]; it compiles fine. but I don't see how that would be useful to me later when I need to access certain info using index, it would be very annoying to access that info later. in fact, I'm not even sure if I am going about the right way, maybe there is a better way to store a bunch of strings instead of using array of char *?

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• How to more effectively debug PHP code in the vein JavaScript with Firebug or XCode?

  - by racl101

  Hi everyone, I'm kind of a newbie with PHP so please bear with me. I would like to know if there was an effective way of debugging PHP code so that I don't have to have debugging messages display on the browser. For example, I find var_dump and print_r functions to be excellent for debugging variables, function calls and arrays respectively. The problem is I have been asked to debug code on a live site (no dev site, I know it is a horrible practice but this is not my project from the start.) So I would like to know what core function or php library or whatever else it is that I could use to log debugging calls in to log that I can check so I don't have to send debugging calls to the browser on a live site? I like the way you can use the console.log function in JavaScript code and check it in Firebug or the Webkit Console and I also like like the console window in Xcode and I was wondering if there was some similar tool for PHP debugging. Any extra info and pearls of wisdom would be greatly appreciated. Thanks, racl101.

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• "multiply frog enemy" timer and array AS3

  - by VideoDnd

  How can I use the counter value to multiply the frogs in the array? My counter goes from 0-100. I want to prove that I can increment the enemies using a counter. EXPLAINED BETTER I have 10 frogs in an array. I want to use a timer to add 10 more frogs on each iteration of the TimerEvent.TIMER firing. //currentCount var timer:Timer = new Timer(1000, 50); timer.addEventListener(TimerEvent.TIMER, countdown); timer.start(); function countdown(event:TimerEvent) { // myText.text = String(0 + timer.currentCount); } //Creates 10 enemies "I want enemies to multiply 0-100" var enemyArray:Array = new Array(); for (var i:int = 0; i < 10; i++) { var noname:FrogClass = new FrogClass(); noname.x = i*10; //this will just assign some different x and y value depending on i. noname.y = i*11; enemyArray.push(noname); //put the enemy into the array addChild(noname); //puts it on the stage } SYMBOL PROPERTIES NAME "noname" CLASS "FrogClass" WHY I need specific examples using strings and arrays, because I'm stuck in a learning curve. Stupid examples are more fun!

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• PHP - "Fat Free Framework" Find Methods and Showing Results in Template

  - by user1672808

  Just started trying the "Fat Free Framework" I'm building a site using a MySQL DB with 265 fields, and 5000+ rows in the DB; I can load() a specific record easily, no problems. When using find(), afind(), and even "select()", template will show blank lines or lines with "filler" text, with the correct number of rows for the query results, but no text/data from the DB itself; Same problem whether using objects or simply arrays from result (afind() and find()). I've copied/pasted the code verbatim from examples and from documentation, with only the DB specific items changed. Still, no luck. CODE IN PHP FILE (function from CLASS): static function home() { $featured=new Axon('boats'); $F3::set('boatlist',$featured->afind('D_CustomerID=173')); F3::set('content',TEMPLATE_DIR .'/home.html'); echo Template::serve(TEMPLATE_DIR .'/layout.html'); } TEMPLATE home.html: <div class="span8"> <h3> Featured Boats </h3> <F3:repeat group="{{@boatlist}}" value="{{@boat}}"> <div style="margin-left: 2em" class="thumbnails"> <p> <a href="boat/{{@boat['D_BoatNum']}}">{{trim(@boat['D_Description'])}}</a> by {{@boat['D_CustomerID']}} </p> <p> {{@boat['D_Price']}} </p> </div> </F3:repeat> </div> The number of rows this produces coincides with the correct number of rows in the DB. However, the actual data from each field does not show. Any ideas?

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• Storing multiple inputs with the same name in a CodeIgniter session

  - by Joshua Cody

  I've posted this in the CodeIgniter forum and exhausted the forum search engine as well, so apologies if cross-posting is frowned upon. Essentially, I've got a single input, set up as <input type="text" name="goal">. At a user's request, they may add another goal, which throws a duplicate to the DOM. What I need to do is grab these values in my CodeIgniter controller and store them in a session variable. My controller is currently constructed thusly: function goalsAdd(){ $meeting_title = $this->input->post('topic'); $meeting_hours = $this->input->post('hours'); $meeting_minutes = $this->input->post('minutes'); $meeting_goals = $this->input->post('goal'); $meeting_time = $meeting_hours . ":" . $meeting_minutes; $sessionData = array( 'totaltime' => $meeting_time, 'title' => $meeting_title, 'goals' => $meeting_goals ); $this->session->set_userdata($sessionData); $this->load->view('test', $sessionData); } Currently, obviously, my controller gets the value of each input, writing over previous values in its wake, leaving only a string of the final value. I need to store these, however, so I can print them on a subsequent page. What I imagine I'd love to do is extend the input class to be able to call $this-input-posts('goal'). Eventually I will need to store other arrays to session values. But I'm totally open to implementation suggestion. Thanks so much for any help you can give.

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• How to mark array value types in PHP (Java)Doc?

  - by Christian Sciberras

  It might be a bit difficult to explain, so I'll give some example code. Note that I'm using NetBeans IDE (latest). class Dummy { public function say(){ } } /** * Builds dummy class and returns it. * @return Dummy The dummy class. */ function say_something(){ return new Dummy(); } $s=say_something(); While developing in netbeans I can invoke auto-complete by hitting ctrl+space after typing "$s-". In the the hint window that follows, there is the item "say()". This is because the javadoc says say_something returns a Dummy and NetBeans parsed Dummy class to know that it has a method called "say()". So far so good. My problem is with arrays. Example code follows: /** * Builds array of 2 dummy classes and returns it. * @return Array The dummy class. (*) */ function say_something2(){ return array(new Dummy(),new Dummy()); } $s=say_something2(); If I try the auto-complete thing again but with "$s[0]-" instead, I don't get the methods fro Dummy class. This is because in the JavaDoc I only said that it is an array, but not the values' type. So the question would be, is there any JavaDoc syntax, cheat, whatever which allows me to tell JavaDoc what type of variables to expect in an array?

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• How to make an equation span the whole page / line in LaTeX?

  - by Reed Richards

  I have this equation and it's quite big (basically a FDM one) but it aligns with the text and then continues out on the right side to the nothingness. I've tried stuff like \begin{center} and \hspace*{-2.5cm} but to no avail. I want it to use the whole line not just from the left-margin and out to the right. How do I do it and do I need to install some special package for it? I use the \[ instead of the displaymath like this \[ Equation arrays here \] The code \[ \left( \begin{array}{cccccc} -(2\kappa+\frac{hV\rho}{2}) & (\frac{hV\rho}{2}-\kappa) & 0 & \cdots & 0 \\ -\kappa & -(2\kappa+\frac{hV\rho}{2}) & (\frac{hV\rho}{2}-\kappa) & 0 & \cdots \\ 0 & -\kappa & -(2\kappa+\frac{hV\rho}{2}) & (\frac{hV\rho}{2}-\kappa) & 0 & \cdots \\ \vdots & 0 & \ddots & \vdots \\ \vdots & \vdots & \vdots & -\kappa & -(2\kappa+\frac{hV\rho}{2}) & (\frac{hV\rho}{2}-\kappa) \\ 0 & \vdots & \vdots & 0 & \kappa - \frac{2h\kappa_{v}}{\kappa}(\frac{hv\rho}{2} - \kappa) & -2\kappa \\ \end{array} \right) \left( \begin{array}{c} T_{1} \\ T_{2} \\ \vdots \\ T_{n} \\ \end{array} \right) = \left( \begin{array}{c} Q(0) + \kappa T_{0} \\ Q(h) \\ Q(2h) \\ \vdots \\ Q((n-1)h) \\ 2\frac{\kappa_{v}}{\kappa_{v}}T_{out} \\ \end{array} \right) \]

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• Performance question: Inverting an array of pointers in-place vs array of values

  - by Anders

  The background for asking this question is that I am solving a linearized equation system (Ax=b), where A is a matrix (typically of dimension less than 100x100) and x and b are vectors. I am using a direct method, meaning that I first invert A, then find the solution by x=A^(-1)b. This step is repated in an iterative process until convergence. The way I'm doing it now, using a matrix library (MTL4): For every iteration I copy all coeffiecients of A (values) in to the matrix object, then invert. This the easiest and safest option. Using an array of pointers instead: For my particular case, the coefficients of A happen to be updated between each iteration. These coefficients are stored in different variables (some are arrays, some are not). Would there be a potential for performance gain if I set up A as an array containing pointers to these coefficient variables, then inverting A in-place? The nice thing about the last option is that once I have set up the pointers in A before the first iteration, I would not need to copy any values between successive iterations. The values which are pointed to in A would automatically be updated between iterations. So the performance question boils down to this, as I see it: - The matrix inversion process takes roughly the same amount of time, assuming de-referencing of pointers is non-expensive. - The array of pointers does not need the extra memory for matrix A containing values. - The array of pointers option does not have to copy all NxN values of A between each iteration. - The values that are pointed to the array of pointers option are generally NOT ordered in memory. Hopefully, all values lie relatively close in memory, but *A[0][1] is generally not next to *A[0][0] etc. Any comments to this? Will the last remark affect performance negatively, thus weighing up for the positive performance effects?

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• Thread-safe initialization of function-local static const objects

  - by sbi

  This question made me question a practice I had been following for years. For thread-safe initialization of function-local static const objects I protect the actual construction of the object, but not the initialization of the function-local reference referring to it. Something like this: namspace { const some_type& create_const_thingy() { lock my_lock(some_mutex); static const some_type the_const_thingy; return the_const_thingy; } } void use_const_thingy() { static const some_type& the_const_thingy = create_const_thingy(); // use the_const_thingy } The idea is that locking takes time, and if the reference is overwritten by several threads, it won't matter. I'd be interested if this is safe enough in practice? safe according to The Rules? (I know, the current standard doesn't even know what "concurrency" is, but what about trampling over an already initialized reference? And do other standards, like POSIX, have something to say that's relevant to this?) For the inquiring minds: Many such function-local static const objects I used are maps which are initialized from const arrays upon first use and used for lookup. For example, I have a few XML parsers where tag name strings are mapped to enum values, so I could later switch over the tags enum values.

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• Really simple JSON serialization in .NET

  - by Evgeny

  I have some simple .NET objects I'd like to serialize to JSON and back again. The set of objects to be serialized is quite small and I control the implementation, so I don't need a generic solution that will work for everything. Since my assembly will be distributed as a library I'd really like to avoid a dependency on some third-party DLL: I just want to give users one assembly that they can reference. I've read the other questions I could find on converting to and from JSON in .NET. The recommended solution of JSON.NET does work, of course, but it requires distributing an extra DLL. I don't need any of the fancy features of JSON.NET. I just need to handle a simple object (or even dictionary) that contains strings, integers, DateTimes and arrays of strings and bytes. On deserializing I'm happy to get back a dictionary - it doesn't need to create the object again. Is there some really simple code out there that I could compile into my assembly to do this simple job? I've also tried System.Web.Script.Serialization.JavaScriptSerializer, but where it falls down is the byte array: I want to base64-encode it and even registering a converter doesn't let me easily accomplish that due to the way that API works (it doesn't pass in the name of the field).

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• Beyond core java

  - by Paul

  Coming to the end of the first year of my CS degree, we've done some Java but just the core stuff; manipulating strings and arrays, inheritance, implementing logic etc. I visit this website daily and I see so much stuff that is beyond me; using frameworks; managing databases etc. It makes me feel like I've just learned the syntax of Java, and there is so much more to do with it. My question is though, how do I get there? I don't think I'm advanced enough to join an open source project, which seems to be suggested often (though I'd love to) and I've looked at other similar questions on here (like this one) but even then I don't think that'd work for me.. firstly, could somebody try give me some commonly used frameworks etc and how and what they are used for? Where would be a good place to start? How did you get started in using the things you do? Or perhaps you think I'm going down the wrong route. Should I learn another language, and just wait until the moment occurs where it's clear what I should be doing? I'm only in the first year of my degree, so far we've lightly covered Haskell and Java, and I've done a little HTML and CSS in my free time. I know that next year we cover python, so perhaps I should just wait till then and see if I prefer that? I feel like I also risk learning something in depth and then never using it... I suppose I'm also asking for personal experiences; was there a point where you felt you'd exceeded the basic grasp of a language (does not necessarily have to be Java related) and reach a more "advanced" level? I guess I'll put subjective tag on this, but really I just want to know how to get beyond the basic understanding of a language.

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• Why do System.IO.Log SequenceNumbers have variable length?

  - by Doug McClean

  I'm trying to use the System.IO.Log features to build a recoverable transaction system. I understand it to be implemented on top of the Common Log File System. The usual ARIES approach to write-ahead logging involves persisting log record sequence numbers in places other than the log (for example, in the header of the database page modified by the logged action). Interestingly, the documentation for CLFS says that such sequence numbers are always 64-bit integers. Confusingly, however, the .Net wrapper around those SequenceNumbers can be constructed from a byte[] but not from a UInt64. It's value can also be read as a byte[], but not as a UInt64. Inspecting the implementation of SequenceNumber.GetBytes() reveals that it can in fact return arrays of either 8 or 16 bytes. This raises a few questions: Why do the .Net sequence numbers differ in size from the CLFS sequence numbers? Why are the .Net sequence numbers variable in length? Why would you need 128 bits to represent such a sequence number? It seems like you would truncate the log well before using up a 64-bit address space (16 exbibytes, or around 10^19 bytes, more if you address longer words)? If log sequence numbers are going to be represented as 128 bit integers, why not provide a way to serialize/deserialize them as pairs of UInt64s instead of rather-pointlessly incurring heap allocations for short-lived new byte[]s every time you need to write/read one? Alternatively, why bother making SequenceNumber a value type at all? It seems an odd tradeoff to double the storage overhead of log sequence numbers just so you can have an untruncated log longer than a million terabytes, so I feel like I'm missing something here, or maybe several things. I'd much appreciate it if someone in the know could set me straight.

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• Returning searched results in an array in Java without ArrayList

  - by Crystal

  I started down this path of implementing a simple search in an array for a hw assignment without knowing we could use ArrayList. I realized it had some bugs in it and figured I'd still try to know what my bug is before using ArrayList. I basically have a class where I can add, remove, or search from an array. public class AcmeLoanManager { public void addLoan(Loan h) { int loanId = h.getLoanId(); loanArray[loanId - 1] = h; } public Loan[] getAllLoans() { return loanArray; } public Loan[] findLoans(Person p) { //Loan[] searchedLoanArray = new Loan[10]; // create new array to hold searched values searchedLoanArray = this.getAllLoans(); // fill new array with all values // Looks through only valid array values, and if Person p does not match using Person.equals() // sets that value to null. for (int i = 0; i < searchedLoanArray.length; i++) { if (searchedLoanArray[i] != null) { if (!(searchedLoanArray[i].getClient().equals(p))) { searchedLoanArray[i] = null; } } } return searchedLoanArray; } public void removeLoan(int loanId) { loanArray[loanId - 1] = null; } private Loan[] loanArray = new Loan[10]; private Loan[] searchedLoanArray = new Loan[10]; // separate array to hold values returned from search } When testing this, I thought it worked, but I think I am overwriting my member variable after I do a search. I initially thought that I could create a new Loan[] in the method and return that, but that didn't seem to work. Then I thought I could have two arrays. One that would not change, and the other just for the searched values. But I think I am not understanding something, like shallow vs deep copying???....

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