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  • Why do Scala maps have poor performance relative to Java?

    - by Mike Hanafey
    I am working on a Scala app that consumes large amounts of CPU time, so performance matters. The prototype of the system was written in Python, and performance was unacceptable. The application does a lot with inserting and manipulating data in maps. Rex Kerr's Thyme was used to look at the performance of updating and retrieving data from maps. Basically "n" random Ints were stored in maps, and retrieved from the maps, with the time relative to java.util.HashMap used as a reference. The full results for a range of "n" are here. Sample (n=100,000) performance relative to java, smaller is worse: Update Read Mutable 16.06% 76.51% Immutable 31.30% 20.68% I do not understand why the scala immutable map beats the scala mutable map in update performance. Using the sizeHint on the mutable map does not help (it appears to be ignored in the tested implementation, 2.10.3). Even more surprisingly the immutable read performance is worse than the mutable read performance, more significantly so with larger maps. The update performance of the scala mutable map is surprisingly bad, relative to both scala immutable and plain Java. What is the explanation?

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  • Dealing with state problems in functional programming

    - by Andrew Martin
    I've learned how to program primarily from an OOP standpoint (like most of us, I'm sure), but I've spent a lot of time trying to learn how to solve problems the functional way. I have a good grasp on how to solve calculational problems with FP, but when it comes to more complicated problems I always find myself reverting to needing mutable objects. For example, if I'm writing a particle simulator, I will want particle "objects" with a mutable position to update. How are inherently "stateful" problems typically solved using functional programming techniques?

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  • Simple XNA 2D demo: why is my F# version slower than C# version?

    - by Den
    When running this XNA application it should display a rotated rectangle that moves from top-left corner to bottom-right corner. It looks like my F# version is noticeably much slower. It seems that the Draw method skips a lot of frames. I am using VS 2012 RC, XNA 4.0, .NET 4.5, F# 3.0. I am trying to make it as functional as possible. What could be the reason for poor performance? C#: class Program { static void Main(string[] args) { using (var game = new FlockGame()) { game.Run(); } } } public class FlockGame : Game { private GraphicsDeviceManager graphics; private DrawingManager drawingManager; private Vector2 position = Vector2.Zero; public FlockGame() { graphics = new GraphicsDeviceManager(this); } protected override void Initialize() { drawingManager = new DrawingManager(graphics.GraphicsDevice); this.IsFixedTimeStep = false; } protected override void Update(GameTime gameTime) { position = new Vector2(position.X + 50.1f * (float)gameTime.ElapsedGameTime.TotalSeconds, position.Y + 50.1f * (float)gameTime.ElapsedGameTime.TotalSeconds); base.Update(gameTime); } protected override void Draw(GameTime gameTime) { //this.GraphicsDevice.Clear(Color.Lavender) drawingManager.DrawRectangle(position, new Vector2(100.0f, 100.0f), 0.7845f, Color.Red); base.Draw(gameTime); } } public class DrawingManager { private GraphicsDevice GraphicsDevice; private Effect Effect; public DrawingManager(GraphicsDevice graphicsDevice) { GraphicsDevice = graphicsDevice; this.Effect = new BasicEffect(this.GraphicsDevice) { VertexColorEnabled = true, Projection = Matrix.CreateOrthographicOffCenter(0.0f, this.GraphicsDevice.Viewport.Width, this.GraphicsDevice.Viewport.Height, 0.0f, 0.0f, 1.0f) }; } private VertexPositionColor[] GetRectangleVertices (Vector2 center, Vector2 size, float radians, Color color) { var halfSize = size/2.0f; var topLeft = -halfSize; var bottomRight = halfSize; var topRight = new Vector2(bottomRight.X, topLeft.Y); var bottomLeft = new Vector2(topLeft.X, bottomRight.Y); topLeft = Vector2.Transform(topLeft, Matrix.CreateRotationZ(radians)) + center; topRight = Vector2.Transform(topRight, Matrix.CreateRotationZ(radians)) + center; bottomRight = Vector2.Transform(bottomRight, Matrix.CreateRotationZ(radians)) + center; bottomLeft = Vector2.Transform(bottomLeft, Matrix.CreateRotationZ(radians)) + center; return new VertexPositionColor[] { new VertexPositionColor(new Vector3(topLeft, 0.0f), color), new VertexPositionColor(new Vector3(topRight, 0.0f), color), new VertexPositionColor(new Vector3(topRight, 0.0f), color), new VertexPositionColor(new Vector3(bottomRight, 0.0f), color), new VertexPositionColor(new Vector3(bottomRight, 0.0f), color), new VertexPositionColor(new Vector3(bottomLeft, 0.0f), color), new VertexPositionColor(new Vector3(bottomLeft, 0.0f), color), new VertexPositionColor(new Vector3(topLeft, 0.0f), color) }; } public void DrawRectangle(Vector2 center, Vector2 size, float radians, Color color) { var vertices = GetRectangleVertices(center, size, radians, color); foreach (var pass in this.Effect.CurrentTechnique.Passes) { pass.Apply(); this.GraphicsDevice.DrawUserPrimitives(PrimitiveType.LineList, vertices, 0, vertices.Length/2); } } } F#: namespace Flocking module FlockingProgram = open System open Flocking [<STAThread>] [<EntryPoint>] let Main _ = use g = new FlockGame() g.Run() 0 //------------------------------------------------------------------------------ namespace Flocking open System open System.Diagnostics open Microsoft.Xna.Framework open Microsoft.Xna.Framework.Graphics open Microsoft.Xna.Framework.Input type public FlockGame() as this = inherit Game() let mutable graphics = new GraphicsDeviceManager(this) let mutable drawingManager = null let mutable position = Vector2.Zero override Game.LoadContent() = drawingManager <- new Rendering.DrawingManager(graphics.GraphicsDevice) this.IsFixedTimeStep <- false override Game.Update gameTime = position <- Vector2(position.X + 50.1f * float32 gameTime.ElapsedGameTime.TotalSeconds, position.Y + 50.1f * float32 gameTime.ElapsedGameTime.TotalSeconds) base.Update gameTime override Game.Draw gameTime = //this.GraphicsDevice.Clear(Color.Lavender) Rendering.DrawRectangle(drawingManager, position, Vector2(100.0f, 100.0f), 0.7845f, Color.Red) base.Draw gameTime //------------------------------------------------------------------------------ namespace Flocking open System open System.Collections.Generic open Microsoft.Xna.Framework open Microsoft.Xna.Framework.Graphics open Microsoft.Xna.Framework.Input module Rendering = [<AllowNullLiteral>] type DrawingManager (graphicsDevice : GraphicsDevice) = member this.GraphicsDevice = graphicsDevice member this.Effect = new BasicEffect(this.GraphicsDevice, VertexColorEnabled = true, Projection = Matrix.CreateOrthographicOffCenter(0.0f, float32 this.GraphicsDevice.Viewport.Width, float32 this.GraphicsDevice.Viewport.Height, 0.0f, 0.0f, 1.0f)) let private GetRectangleVertices (center:Vector2, size:Vector2, radians:float32, color:Color) = let halfSize = size / 2.0f let mutable topLeft = -halfSize let mutable bottomRight = halfSize let mutable topRight = new Vector2(bottomRight.X, topLeft.Y) let mutable bottomLeft = new Vector2(topLeft.X, bottomRight.Y) topLeft <- Vector2.Transform(topLeft, Matrix.CreateRotationZ(radians)) + center topRight <- Vector2.Transform(topRight, Matrix.CreateRotationZ(radians)) + center bottomRight <- Vector2.Transform(bottomRight, Matrix.CreateRotationZ(radians)) + center bottomLeft <- Vector2.Transform(bottomLeft, Matrix.CreateRotationZ(radians)) + center [| new VertexPositionColor(new Vector3(topLeft, 0.0f), color) new VertexPositionColor(new Vector3(topRight, 0.0f), color) new VertexPositionColor(new Vector3(topRight, 0.0f), color) new VertexPositionColor(new Vector3(bottomRight, 0.0f), color) new VertexPositionColor(new Vector3(bottomRight, 0.0f), color) new VertexPositionColor(new Vector3(bottomLeft, 0.0f), color) new VertexPositionColor(new Vector3(bottomLeft, 0.0f), color) new VertexPositionColor(new Vector3(topLeft, 0.0f), color) |] let DrawRectangle (drawingManager:DrawingManager, center:Vector2, size:Vector2, radians:float32, color:Color) = let vertices = GetRectangleVertices(center, size, radians, color) for pass in drawingManager.Effect.CurrentTechnique.Passes do pass.Apply() drawingManager.GraphicsDevice.DrawUserPrimitives(PrimitiveType.LineList, vertices, 0, vertices.Length/2)

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  • Vectors or Java arrays for Tetris?

    - by StackedCrooked
    I'm trying to create a Tetris-like game with Clojure and I'm having some trouble deciding the data structure for the playing field. I want to define the playing field as a mutable grid. The individual blocks are also grids, but don't need to be mutable. My first attempt was to define a grid as a vector of vectors. For example an S-block looks like this: :s-block { :grids [ [ [ 0 1 1 ] [ 1 1 0 ] ] [ [ 1 0 ] [ 1 1 ] [ 0 1 ] ] ] } But that turns out to be rather tricky for simple things like iterating and painting (see the code below). For making the grid mutable my initial idea was to make each row a reference. But then I couldn't really figure out how to change the value of a specific cell in a row. One option would have been to create each individual cell a ref instead of each row. But that feels like an unclean approach. I'm considering using Java arrays now. Clojure's aget and aset functions will probably turn out to be much simpler. However before digging myself in a deeper mess I want to ask ideas/insights. How would you recommend implementing a mutable 2d grid? Feel free to share alternative approaches as well. Source code current state: Tetris.clj (rev452)

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  • Common Pitfalls in Python

    - by Anurag Uniyal
    Today I was bitten again by "Mutable default arguments" after many years. I usually don't use mutable default arguments unless needed but I think with time I forgot about that, and today in the application I added tocElements=[] in a pdf generation function's argument list and now 'Table of Content' gets longer and longer after each invocation of "generate pdf" :) My question is what other things should I add to my list of things to MUST avoid? Mutable default arguments Import modules always same way e.g. from y import x and import x are different things, they are treated as different modules. Do not use range in place of lists because range() will become an iterator anyway, the following will fail: myIndexList = [0,1,3] isListSorted = myIndexList == range(3) # will fail in 3.0 isListSorted = myIndexList == list(range(3)) # will not same thing can be mistakenly done with xrange: `myIndexList == xrange(3)`. Catching multiple exceptions try: raise KeyError("hmm bug") except KeyError,TypeError: print TypeError It prints "hmm bug", though it is not a bug, it looks like we are catching exceptions of type KeyError,TypeError but instead we are catching KeyError only as variable TypeError, use this instead: try: raise KeyError("hmm bug") except (KeyError,TypeError): print TypeError

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  • Fibonacci Numbers in Haskell

    - by boraer
    Hi everbody I need to change my F# code to Haskell code but I am so new in Haskell and I can not this My code simply read data from keyboard if data not an integer return an error message then calculate the n fibonacci number then writes to a list after that writes the list into a txt file Here is my code open System let rec fib n = match n with |0->0 |1->1 |2->1 |n->fib(n-1)+fib(n-2);; let printFibonacci list = for i=0 to (List.length list)-1 do printf "%d " (list.Item(i));; let writeToFile list = let file = System.IO.File.Create("C:\out2.txt") let mutable s ="" let writer = new System.IO.StreamWriter(file) try for i=0 to (List.length list)-1 do s <- list.Item(i).ToString() writer.Write(s+" ") finally writer.Close() file.Dispose() printfn "Writed To File" let mutable control = true let mutable num = 0 while control do try printfn "Enter a Number:" num <- Convert.ToInt32(stdin.ReadLine()) let listFibonacci = [for i in 0 .. num-1->fib(i)] printFibonacci(listFibonacci) printfn "\n%A"(listFibonacci) writeToFile(listFibonacci) control<-false with | :? System.FormatException->printfn "Number Format Exception"; Console.ReadKey true|>ignore

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  • Why enumerator structs are a really bad idea

    - by Simon Cooper
    If you've ever poked around the .NET class libraries in Reflector, I'm sure you would have noticed that the generic collection classes all have implementations of their IEnumerator as a struct rather than a class. As you will see, this design decision has some rather unfortunate side effects... As is generally known in the .NET world, mutable structs are a Very Bad Idea; and there are several other blogs around explaining this (Eric Lippert's blog post explains the problem quite well). In the BCL, the generic collection enumerators are all mutable structs, as they need to keep track of where they are in the collection. This bit me quite hard when I was coding a wrapper around a LinkedList<int>.Enumerator. It boils down to this code: sealed class EnumeratorWrapper : IEnumerator<int> { private readonly LinkedList<int>.Enumerator m_Enumerator; public EnumeratorWrapper(LinkedList<int> linkedList) { m_Enumerator = linkedList.GetEnumerator(); } public int Current { get { return m_Enumerator.Current; } } object System.Collections.IEnumerator.Current { get { return Current; } } public bool MoveNext() { return m_Enumerator.MoveNext(); } public void Reset() { ((System.Collections.IEnumerator)m_Enumerator).Reset(); } public void Dispose() { m_Enumerator.Dispose(); } } The key line here is the MoveNext method. When I initially coded this, I thought that the call to m_Enumerator.MoveNext() would alter the enumerator state in the m_Enumerator class variable and so the enumeration would proceed in an orderly fashion through the collection. However, when I ran this code it went into an infinite loop - the m_Enumerator.MoveNext() call wasn't actually changing the state in the m_Enumerator variable at all, and my code was looping forever on the first collection element. It was only after disassembling that method that I found out what was going on The MoveNext method above results in the following IL: .method public hidebysig newslot virtual final instance bool MoveNext() cil managed { .maxstack 1 .locals init ( [0] bool CS$1$0000, [1] valuetype [System]System.Collections.Generic.LinkedList`1/Enumerator CS$0$0001) L_0000: nop L_0001: ldarg.0 L_0002: ldfld valuetype [System]System.Collections.Generic.LinkedList`1/Enumerator EnumeratorWrapper::m_Enumerator L_0007: stloc.1 L_0008: ldloca.s CS$0$0001 L_000a: call instance bool [System]System.Collections.Generic.LinkedList`1/Enumerator::MoveNext() L_000f: stloc.0 L_0010: br.s L_0012 L_0012: ldloc.0 L_0013: ret } Here, the important line is 0002 - m_Enumerator is accessed using the ldfld operator, which does the following: Finds the value of a field in the object whose reference is currently on the evaluation stack. So, what the MoveNext method is doing is the following: public bool MoveNext() { LinkedList<int>.Enumerator CS$0$0001 = this.m_Enumerator; bool CS$1$0000 = CS$0$0001.MoveNext(); return CS$1$0000; } The enumerator instance being modified by the call to MoveNext is the one stored in the CS$0$0001 variable on the stack, and not the one in the EnumeratorWrapper class instance. Hence why the state of m_Enumerator wasn't getting updated. Hmm, ok. Well, why is it doing this? If you have a read of Eric Lippert's blog post about this issue, you'll notice he quotes a few sections of the C# spec. In particular, 7.5.4: ...if the field is readonly and the reference occurs outside an instance constructor of the class in which the field is declared, then the result is a value, namely the value of the field I in the object referenced by E. And my m_Enumerator field is readonly! Indeed, if I remove the readonly from the class variable then the problem goes away, and the code works as expected. The IL confirms this: .method public hidebysig newslot virtual final instance bool MoveNext() cil managed { .maxstack 1 .locals init ( [0] bool CS$1$0000) L_0000: nop L_0001: ldarg.0 L_0002: ldflda valuetype [System]System.Collections.Generic.LinkedList`1/Enumerator EnumeratorWrapper::m_Enumerator L_0007: call instance bool [System]System.Collections.Generic.LinkedList`1/Enumerator::MoveNext() L_000c: stloc.0 L_000d: br.s L_000f L_000f: ldloc.0 L_0010: ret } Notice on line 0002, instead of the ldfld we had before, we've got a ldflda, which does this: Finds the address of a field in the object whose reference is currently on the evaluation stack. Instead of loading the value, we're loading the address of the m_Enumerator field. So now the call to MoveNext modifies the enumerator stored in the class rather than on the stack, and everything works as expected. Previously, I had thought enumerator structs were an odd but interesting feature of the BCL that I had used in the past to do linked list slices. However, effects like this only underline how dangerous mutable structs are, and I'm at a loss to explain why the enumerators were implemented as structs in the first place. (interestingly, the SortedList<TKey, TValue> enumerator is a struct but is private, which makes it even more odd - the only way it can be accessed is as a boxed IEnumerator!). I would love to hear people's theories as to why the enumerators are implemented in such a fashion. And bonus points if you can explain why LinkedList<int>.Enumerator.Reset is an explicit implementation but Dispose is implicit... Note to self: never ever ever code a mutable struct.

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  • F# Objects &ndash; Integrating with the other .Net Languages &ndash; Part 1

    - by MarkPearl
    In the next few blog posts I am going to explore objects in F#. Up to now, my dabbling in F# has really been a few liners and while I haven’t reached the point where F# is my language of preference – I am already seeing the benefits of the language when solving certain types of programming problems. For me I believe that the F# language will be used in a silo like architecture and that the real benefit of having F# under your belt is so that you can solve problems that F# lends itself towards and then interact with other .Net languages in doing the rest. When I was still very new to the F# language I did the following post covering how to get F# & C# to talk to each other. Today I am going to use a similar approach to demonstrate the structure of F# objects when inter-operating with other languages. Lets start with an empty F# class … type Person() = class end   Very simple, and all we really have done is declared an object that has nothing in it. We could if we wanted to make an object that takes a constructor with parameters… the code for this would look something like this… type Person =     {         Firstname : string         Lastname : string     }   What’s interesting about this syntax is when you try and interop with this object from another .Net language like C# - you would see the following…   Not only has a constructor been created that accepts two parameters, but Firstname and Lastname are also exposed on the object. Now it’s important to keep in mind that value holders in F# are immutable by default, so you would not be able to change the value of Firstname after the construction of the object – in C# terms it has been set to readonly. One could however explicitly state that the value holders were mutable, which would then allow you to change the values after the actual creation of the object. type Person = { mutable Firstname : string mutable Lastname : string }   Something that bugged me for a while was what if I wanted to have an F# object that requires values in its constructor, but does not expose them as part of the object. After bashing my head for a few moments I came up with the following syntax – which achieves this result. type Person(Firstname : string, Lastname : string) = member v.Fullname = Firstname + " " + Lastname What I haven’t figured out yet is what is the difference between the () & {} brackets when declaring an object.

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  • No sound out of headphone port on laptop

    - by Thanatos
    I cannot get sound out of the headphone port on a laptop. Headphones are plugged in, and sound comes out of the internal speakers. Windows behaves normally (sound switches to headphones when headphones are inserted). It did work in Linux at one point, but something changed, we're just not sure what. Rebooting doesn't fix. This appears to occur whether or not PulseAudio is running. Things I've tried: Rebooting. No effect. Booting into Windows. It works properly, so probably not a hardware issue. All of alsamixer. My only controls are this: "Master" Volume bar & mutable, unmuted. Controls volume. "PCM" Volume bar only. 100%. "S/PDIF" Mutable only, currently muted, has no effect. "S/PDIF" Default PCM", Mutable only, currently unmuted, has no effect. Killing PulseAudio. No effect. (It also won't stay dead! Something appears to be restarting it, and I can't tell what, but it is annoying as fuck.) alsactl init 0, no effect. sudo rm -f /var/lib/alsa/asound.state, no effect. General system info: Ubuntu 10.04 LTS Toshiba Satellite T135D-S1324 lspci says I have: 00:14.2 Audio device: ATI Technologies Inc SBx00 Azalia (Intel HDA) 01:05.1 Audio device: ATI Technologies Inc RS780 Azalia controller Some edits: Yes, the headphones are in all the way. This works in Windows: You plug headphones in, the internal speakers stop making noise, and noise comes out the head phones. Windows says I only have two sound cards: the HDMI port (which I don't care about) and the "sound card", which it claims is a "Conexant Pebble High Definition SmartAudio" In Windows, both the internal speakers and the headphone jack show up as one soundcard, which in my experience, is typical. (This is a laptop)

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  • No sound out of headphone port

    - by Thanatos
    I cannot get sound out of the headphone port. Headphones are plugged in, and sound comes out of the internal speakers. Windows behaves normally (sound switches to headphones when headphones are inserted). It did work in Linux at one point, but something changed, we're just not sure what. Rebooting doesn't fix. This appears to occur whether or not PulseAudio is running. Things I've tried: Rebooting. No effect. Booting into Windows. It works properly, so probably not a hardware issue. All of alsamixer. My only controls are this: "Master" Volume bar & mutable, unmuted. Controls volume. "PCM" Volume bar only. 100%. "S/PDIF" Mutable only, currently muted, has no effect. "S/PDIF" Default PCM", Mutable only, currently unmuted, has no effect. Killing PulseAudio. No effect. (It also won't stay dead! Something appears to be restarting it, and I can't tell what, but it is annoying as fuck.) alsactl init 0, no effect. sudo rm -f /var/lib/alsa/asound.state, no effect. General system info: Ubuntu 10.04 LTS Toshiba Satellite T135D-S1324 lspci says I have: 00:14.2 Audio device: ATI Technologies Inc SBx00 Azalia (Intel HDA) 01:05.1 Audio device: ATI Technologies Inc RS780 Azalia controller

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  • Proper way to return an array

    - by Ward
    Hey there, I never seem to get this right. I've got a method that returns a mutable array. What is the proper way to return the array and avoid potential memory leaks? If I plan to store the results locally inside another view controller, does that affect the way the array should be returned? Lastly, what if it's just an non-mutable array? Does that require a different technique? thanks, Howie

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  • Changing array values in a VBA dictionary

    - by Pawan Jain
    Hi I have a piece of code that does not seem to do what it is expected to do. VBA Arrays are mutable by all means, but it seems that when they are stored into a Dictionary as values of some keys, they are not mutable anymore. Any ideas? Sub foo() Dim mydict As New Dictionary mydict.Add "A", Array(1, 2, 3) MsgBox mydict("A")(1) ' The above shows 2, which is fine mydict("A")(1) = 34 MsgBox mydict("A")(1) ' The above also shows 2, which is not fine End Sub

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  • Request for member Name in something not a structure or Union

    - by Ashutosh
    Hey Folks, I am trying to Call the webservices in my app and while checking it in console it's showing the number of objects. (as i am using the Mutable array) and then while trying to display the name of the objects (here it's location) it's giving me an error in this line for (SDZArrayOfLocationWithLatestGrades* location in a) { NSLog(@" - %@", location.Name); // error here and a is the mutable array } PLease help me with this and Happy New Year to All!!!!!! Thanks,

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  • How do I make defensive copy of an object?

    - by kunjaan
    How do I make defensive copies of a Mutable Object which contains a mutable field in an Immutable Object? class ImmutableObject { private final MutableObject immutable_field; ImmutableObject(MutableObject y) { this.immutable_field = y; } MutableObject return_immutable_field() { return immutable_field; } } class MutableObject { public int mutable_field; } The MutableObject does not have a constructor that lets me set the field. The MutableObject's current state should be captured in the Immutable Object and never changed.

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  • Mutability design patterns in Objective C and C++

    - by Mac
    Having recently done some development for iPhone, I've come to notice an interesting design pattern used a lot in the iPhone SDK, regarding object mutability. It seems the typical approach there is to define an immutable class NSFoo, and then derive from it a mutable descendant NSMutableFoo. Generally, the NSFoo class defines data members, getters and read-only operations, and the derived NSMutableFoo adds on setters and mutating operations. Being more familiar with C++, I couldn't help but notice that this seems to be a complete opposite to what I'd do when writing the same code in C++. While you certainly could take that approach, it seems to me that a more concise approach is to create a single Foo class, mark getters and read-only operations as const functions, and also implement the mutable operations and setters in the same class. You would then end up with a mutable class, but the types Foo const*, Foo const& etc all are effectively the immutable equivalent. I guess my question is, does my take on the situation make sense? I understand why Objective-C does things differently, but are there any advantages to the two-class approach in C++ that I've missed? Or am I missing the point entirely? Not an overly serious question - more for my own curiosity than anything else.

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  • Drag and Drop in Silverlight with F# and Asynchronous Workflows

    - by knotig
    Hello everyone! I'm trying to implement drag and drop in Silverlight using F# and asynchronous workflows. I'm simply trying to drag around a rectangle on the canvas, using two loops for the the two states (waiting and dragging), an idea I got from Tomas Petricek's book "Real-world Functional Programming", but I ran into a problem: Unlike WPF or WinForms, Silverlight's MouseEventArgs do not carry information about the button state, so I can't return from the drag-loop by checking if the left mouse button is no longer pressed. I only managed to solve this by introducing a mutable flag. Would anyone have a solution for this, that does not involve mutable state? Here's the relevant code part (please excuse the sloppy dragging code, which snaps the rectangle to the mouse pointer): type MainPage() as this = inherit UserControl() do Application.LoadComponent(this, new System.Uri("/SilverlightApplication1;component/Page.xaml", System.UriKind.Relative)) let layoutRoot : Canvas = downcast this.FindName("LayoutRoot") let rectangle1 : Rectangle = downcast this.FindName("Rectangle1") let mutable isDragged = false do rectangle1.MouseLeftButtonUp.Add(fun _ -> isDragged <- false) let rec drag() = async { let! args = layoutRoot.MouseMove |> Async.AwaitEvent if (isDragged) then Canvas.SetLeft(rectangle1, args.GetPosition(layoutRoot).X) Canvas.SetTop(rectangle1, args.GetPosition(layoutRoot).Y) return! drag() else return() } let wait() = async { while true do let! args = Async.AwaitEvent rectangle1.MouseLeftButtonDown isDragged <- true do! drag() } Async.StartImmediate(wait()) () Thank you very much for your time!

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  • Python - Things one MUST avoid

    - by Anurag Uniyal
    Today I was bitten again by "Mutable default arguments" after many years. I usually don't use mutable default arguments unless needed but I think with time I forgot about that, and today in the application I added tocElements=[] in a pdf generation function's argument list and now 'Table of Content' gets longer and longer after each invocation of "generate pdf" :) My question is what other things should I add to my list of things to MUST avoid? 1 Mutable default arguments 2 import modules always same way e.g. 'from y import x' and 'import x' are totally different things actually they are treated as different modules see http://stackoverflow.com/questions/1459236/module-reimported-if-imported-from-different-path 3 Do not use range in place of lists because range() will become an iterator anyway, so things like this will fail, so wrap it by list myIndexList = [0,1,3] isListSorted = myIndexList == range(3) # will fail in 3.0 isListSorted = myIndexList == list(range(3)) # will not same thing can be mistakenly done with xrange e.g myIndexList == xrange(3). 4 Catching multiple exceptions try: raise KeyError("hmm bug") except KeyError,TypeError: print TypeError It prints "hmm bug", though it is not a bug, it looks like we are catching exceptions of type KeyError,TypeError but instead we are catching KeyError only as variable TypeError, instead use try: raise KeyError("hmm bug") except (KeyError,TypeError): print TypeError

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  • data not reloading into tableview from core data on minor update

    - by Martin KS
    I've got a basic photo album application, on the first view a list of albums is displayed with a subtitle showing how many images are in each album. I've got everything working to add albums, and add images to albums. The problem is that the image count lines are accurate whenever the app loads, but I can't get them to update during execution. The following viewdidload correctly populates all lines of the tableview when the app loads: - (void)viewDidLoad { [super viewDidLoad]; // Set the title. self.title = @"Photo albums"; // Configure the add and edit buttons. self.navigationItem.leftBarButtonItem = self.editButtonItem; addButton = [[UIBarButtonItem alloc] initWithBarButtonSystemItem:UIBarButtonSystemItemAdd target:self action:@selector(addAlbum)]; addButton.enabled = YES; self.navigationItem.rightBarButtonItem = addButton; /* Fetch existing albums. Create a fetch request; find the Album entity and assign it to the request; add a sort descriptor; then execute the fetch. */ NSFetchRequest *request = [[NSFetchRequest alloc] init]; NSEntityDescription *entity = [NSEntityDescription entityForName:@"Album" inManagedObjectContext:managedObjectContext]; [request setEntity:entity]; // Order the albums by name. NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"albumName" ascending:NO]; NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil]; [request setSortDescriptors:sortDescriptors]; [sortDescriptor release]; [sortDescriptors release]; // Execute the fetch -- create a mutable copy of the result. NSError *error = nil; NSMutableArray *mutableFetchResults = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy]; if (mutableFetchResults == nil) { // Handle the error. } LocationsAppDelegate *mainDelegate = (LocationsAppDelegate *)[[UIApplication sharedApplication] delegate]; // Set master albums array to the mutable array, then clean up. [mainDelegate setAlbumsArray:mutableFetchResults]; [mutableFetchResults release]; [request release]; } But when I run similar code inside viewdidappear, nothing happens: { /* Fetch existing albums. Create a fetch request; find the Album entity and assign it to the request; add a sort descriptor; then execute the fetch. */ NSFetchRequest *request = [[NSFetchRequest alloc] init]; NSEntityDescription *entity = [NSEntityDescription entityForName:@"Album" inManagedObjectContext:managedObjectContext]; [request setEntity:entity]; // Order the albums by creation date, most recent first. NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"albumName" ascending:NO]; NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil]; [request setSortDescriptors:sortDescriptors]; [sortDescriptor release]; [sortDescriptors release]; // Execute the fetch -- create a mutable copy of the result. NSError *error = nil; NSMutableArray *mutableFetchResults = [[managedObjectContext executeFetchRequest:request error:&error] mutableCopy]; if (mutableFetchResults == nil) { // Handle the error. } LocationsAppDelegate *mainDelegate = (LocationsAppDelegate *)[[UIApplication sharedApplication] delegate]; // Set master albums array to the mutable array, then clean up. [mainDelegate setAlbumsArray:mutableFetchResults]; [self.tableView reloadData]; [mutableFetchResults release]; [request release]; } Apologies if I've missed the answer to this question elsewhere, but what am I missing?

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  • How are objects modelled in a functional programming language?

    - by Giorgio
    In an answer to this question (written by Pete) there are some considerations about OOP versus FP. In particular, it is suggested that FP languages are not very suitable for modelling (persistent) objects that have an identity and a mutable state. I was wondering if this is true or, in other words, how one would model objects in a functional programming language. From my basic knowledge of Haskell I thought that one could use monads in some way, but I really do not know enough on this topic to come up with a clear answer. So, how are entities with an identity and a mutable persistent state normally modelled in a functional language? EDIT Here are some further details to clarify what I have in mind. Take a typical Java application in which I can (1) read a record from a database table into a Java object, (2) modify the object in different ways, (3) save the modified object to the database. How would this be implemented e.g. in Haskell? I would initially read the record into a record value (defined by a data definition), perform different transformations by applying functions to this initial value (each intermediate value is a new, modified copy of the original record) and then write the final record value to the database. Is this all there is to it? How can I ensure that at each moment in time only one copy of the record is valid / accessible? One does not want to have different immutable values representing different snapshots of the same object to be accessible at the same time.

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  • Threading extra state through a parser in Scala

    - by Travis Brown
    I'll give you the tl;dr up front I'm trying to use the state monad transformer in Scalaz 7 to thread extra state through a parser, and I'm having trouble doing anything useful without writing a lot of t m a -> t m b versions of m a -> m b methods. An example parsing problem Suppose I have a string containing nested parentheses with digits inside them: val input = "((617)((0)(32)))" I also have a stream of fresh variable names (characters, in this case): val names = Stream('a' to 'z': _*) I want to pull a name off the top of the stream and assign it to each parenthetical expression as I parse it, and then map that name to a string representing the contents of the parentheses, with the nested parenthetical expressions (if any) replaced by their names. To make this more concrete, here's what I'd want the output to look like for the example input above: val target = Map( 'a' -> "617", 'b' -> "0", 'c' -> "32", 'd' -> "bc", 'e' -> "ad" ) There may be either a string of digits or arbitrarily many sub-expressions at a given level, but these two kinds of content won't be mixed in a single parenthetical expression. To keep things simple, we'll assume that the stream of names will never contain either duplicates or digits, and that it will always contain enough names for our input. Using parser combinators with a bit of mutable state The example above is a slightly simplified version of the parsing problem in this Stack Overflow question. I answered that question with a solution that looked roughly like this: import scala.util.parsing.combinator._ class ParenParser(names: Iterator[Char]) extends RegexParsers { def paren: Parser[List[(Char, String)]] = "(" ~> contents <~ ")" ^^ { case (s, m) => (names.next -> s) :: m } def contents: Parser[(String, List[(Char, String)])] = "\\d+".r ^^ (_ -> Nil) | rep1(paren) ^^ ( ps => ps.map(_.head._1).mkString -> ps.flatten ) def parse(s: String) = parseAll(paren, s).map(_.toMap) } It's not too bad, but I'd prefer to avoid the mutable state. What I want Haskell's Parsec library makes adding user state to a parser trivially easy: import Control.Applicative ((*>), (<$>), (<*)) import Data.Map (fromList) import Text.Parsec paren = do (s, m) <- char '(' *> contents <* char ')' h : t <- getState putState t return $ (h, s) : m where contents = flip (,) [] <$> many1 digit <|> (\ps -> (map (fst . head) ps, concat ps)) <$> many1 paren main = print $ runParser (fromList <$> paren) ['a'..'z'] "example" "((617)((0)(32)))" This is a fairly straightforward translation of my Scala parser above, but without mutable state. What I've tried I'm trying to get as close to the Parsec solution as I can using Scalaz's state monad transformer, so instead of Parser[A] I'm working with StateT[Parser, Stream[Char], A]. I have a "solution" that allows me to write the following: import scala.util.parsing.combinator._ import scalaz._, Scalaz._ object ParenParser extends ExtraStateParsers[Stream[Char]] with RegexParsers { protected implicit def monadInstance = parserMonad(this) def paren: ESP[List[(Char, String)]] = (lift("(" ) ~> contents <~ lift(")")).flatMap { case (s, m) => get.flatMap( names => put(names.tail).map(_ => (names.head -> s) :: m) ) } def contents: ESP[(String, List[(Char, String)])] = lift("\\d+".r ^^ (_ -> Nil)) | rep1(paren).map( ps => ps.map(_.head._1).mkString -> ps.flatten ) def parse(s: String, names: Stream[Char]) = parseAll(paren.eval(names), s).map(_.toMap) } This works, and it's not that much less concise than either the mutable state version or the Parsec version. But my ExtraStateParsers is ugly as sin—I don't want to try your patience more than I already have, so I won't include it here (although here's a link, if you really want it). I've had to write new versions of every Parser and Parsers method I use above for my ExtraStateParsers and ESP types (rep1, ~>, <~, and |, in case you're counting). If I had needed to use other combinators, I'd have had to write new state transformer-level versions of them as well. Is there a cleaner way to do this? I'd love to see an example of a Scalaz 7's state monad transformer being used to thread state through a parser, but Scala 6 or Haskell examples would also be useful.

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  • XNA 4.0 - What happens when the window is minimized?

    - by Conrad Clark
    Hello. I'm learning F#, and decided to try making simple XNA games for windows using F# (pure enthusiasm) , and got a window with some images showing up. Here's the code: (*Methods*) member self.DrawSprites() = _spriteBatch.Begin() for i = 0 to _list.Length-1 do let spentity = _list.List.ElementAt(i) _spriteBatch.Draw(spentity.ImageTexture,new Rectangle(100,100,(int)spentity.Width,(int)spentity.Height),Color.White) _spriteBatch.End() (*Overriding*) override self.Initialize() = ChangeGraphicsProfile() _graphicsDevice <- _graphics.GraphicsDevice _list.AddSprite(0,"NagatoYuki",992.0,990.0) base.Initialize() override self.LoadContent() = _spriteBatch <- new SpriteBatch(_graphicsDevice) base.LoadContent() override self.Draw(gameTime : GameTime) = base.Draw(gameTime) _graphics.GraphicsDevice.Clear(Color.CornflowerBlue) self.DrawSprites() And the AddSprite Method: member self.AddSprite(ID : int,imageTexture : string , width : float, height : float) = let texture = content.Load<Texture2D>(imageTexture) list <- list @ [new SpriteEntity(ID,list.Length, texture,Vector2.Zero,width,height)] The _list object has a ContentManager, here's the constructor: type SpriteList(_content : ContentManager byref) = let mutable content = _content let mutable list = [] But I can't minimize the window, since when it regains its focus, i get this error: ObjectDisposedException Cannot access a disposed object. Object name: 'GraphicsDevice'. What is happening?

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