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• Simple in-place discrete fourier transform ( DFT )

  - by Adam

  I'm writing a very simple in-place DFT. I am using the formula shown here: http://en.wikipedia.org/wiki/Discrete_Fourier_transform#Definition along with Euler's formula to avoid having to use a complex number class just for this. So far I have this: private void fft(double[] data) { double[] real = new double[256]; double[] imag = new double[256]; double pi_div_128 = -1 * Math.PI / 128; for (int k = 0; k < 256; k++) { for (int n = 0; n < 256; n++) { real[k] += data[k] * Math.Cos(pi_div_128 * k * n); imag[k] += data[k] * Math.Sin(pi_div_128 * k * n); } data[k] = Math.Sqrt(real[k] * real[k] + imag[k] * imag[k]); } } But the Math.Cos and Math.Sin terms eventually go both positive and negative, so as I'm adding those terms multiplied with data[k], they cancel out and I just get some obscenely small value. I see how it is happening, but I can't make sense of how my code is perhaps mis-representing the mathematics. Any help is appreciated. FYI, I do have to write my own, I realize I can get off-the shelf FFT's.

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• F# passing an operator with arguments to a function

  - by dan

  Can you pass in an operation like "divide by 2" or "subtract 1" using just a partially applied operator, where "add 1" looks like this: List.map ((+) 1) [1..5];; //equals [2..6] // instead of having to write: List.map (fun x-> x+1) [1..5] What's happening is 1 is being applied to (+) as it's first argument, and the list item is being applied as the second argument. For addition and multiplication, this argument ordering doesn't matter. Suppose I want to subtract 1 from every element (this will probably be a common beginners mistake): List.map ((-) 1) [1..5];; //equals [0 .. -4], the opposite of what we wanted 1 is applied to the (-) as its first argument, so instead of (list_item - 1), I get (1 - list_item). I can rewrite it as adding negative one instead of subtracting positive one: List.map ((+) -1) [1..5];; List.map (fun x -> x-1) [1..5];; // this works too I'm looking for a more expressive way to write it, something like ((-) _ 1), where _ denotes a placeholder, like in the Arc language. This would cause 1 to be the second argument to -, so in List.map, it would evaluate to list_item - 1. So if you wanted to map divide by 2 to the list, you could write: List.map ((/) _ 2) [2;4;6] //not real syntax, but would equal [1;2;3] List.map (fun x -> x/2) [2;4;6] //real syntax equivalent of the above Can this be done or do I have to use (fun x -> x/2)? It seems that the closest we can get to the placeholder syntax is to use a lambda with a named argument.

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• Getting Argument Names In Ruby Reflection

  - by Joe Soul-bringer

  I would like to do some fairly heavy-duty reflection in the Ruby programming language. I would like to create a function which would return the names of the arguments of various calling functions higher up the call stack (just one higher would be enough but why stop there?). I could use Kernel.caller go to the file and parse the argument list but that would be ugly and unreliable. The function that I would like would work in the following way: module A def method1( tuti, fruity) foo end def method2(bim, bam, boom) foo end def foo print caller_args[1].join(",") #the "1" mean one step up the call stack end end A.method1 #prints "tuti,fruity" A.method2 #prints "bim, bam, boom" I would not mind using ParseTree or some similar tool for this task but looking at Parsetree, it is not obvious how to use it for this purpose. Creating a C extension like this is another possibility but it would be nice if someone had already done it for me. Edit2: I can see that I'll probably need some kind of C extension. I suppose that means my question is what combination of C extension would work most easily. I don't think caller+ParseTree would be enough by themselves. As far as why I would like to do this goes, rather than saying "automatic debugging", perhaps I should say that I would like to use this functionality to do automatic checking of the calling and return conditions of functions. Say def add x, y check_positive return x + y end Where check_positive would throw an exception if x and y weren't positive (obviously, there would be more to it than that but hopefully this gives enough motivation)

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• Fast sign in C++ float...are there any platform dependencies in this code?

  - by Patrick Niedzielski

  Searching online, I have found the following routine for calculating the sign of a float in IEEE format. This could easily be extended to a double, too. // returns 1.0f for positive floats, -1.0f for negative floats, 0.0f for zero inline float fast_sign(float f) { if (((int&)f & 0x7FFFFFFF)==0) return 0.f; // test exponent & mantissa bits: is input zero? else { float r = 1.0f; (int&)r |= ((int&)f & 0x80000000); // mask sign bit in f, set it in r if necessary return r; } } (Source: ``Fast sign for 32 bit floats'', Peter Schoffhauzer) I am weary to use this routine, though, because of the bit binary operations. I need my code to work on machines with different byte orders, but I am not sure how much of this the IEEE standard specifies, as I couldn't find the most recent version, published this year. Can someone tell me if this will work, regardless of the byte order of the machine? Thanks, Patrick

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• Regressing panel data in SAS.

  - by John

  Hey Guys, thanks to your help I succesfully managed all my databases! I am now looking at a panel data set on which I have to regress. Since I only started my Phd this semester together with the econometrics courses I am still new to many statistic applications and regression methods. I want to do a simple regression as in Y = x1 x2 x3 etc, now I already browsed through some literature and found that for panel data it's common to do a fixed effects regression. Also, my Y variable only has positive values so I was thinking in the direction of a Tobit model? I'm doing some research concerning the coverage of analysts in the financial business. My independent variable is the coverage of analysts on a certain firm, so per observation i have 1 analyst and 1 firm, together with different characteristics(market cap and betas etc) of the firm. All this data is monthly. As coverage cannot become negative (only 0) I was thinking of a Tobit model? Do you guys have any ideas what would be a good regression method? Or have some good sources (e books, written books, through university I have access to almost anything concerning my field of work) of information (cause I do have to learn these things for future research)? Thanks!

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• How to minimize total cost of shortest path tree

  - by Michael

  I have a directed acyclic graph with positive edge-weights. It has a single source and a set of targets (vertices furthest from the source). I find the shortest paths from the source to each target. Some of these paths overlap. What I want is a shortest path tree which minimizes the total sum of weights over all edges. For example, consider two of the targets. Given all edge weights equal, if they share a single shortest path for most of their length, then that is preferable to two mostly non-overlapping shortest paths (fewer edges in the tree equals lower overall cost). Another example: two paths are non-overlapping for a small part of their length, with high cost for the non-overlapping paths, but low cost for the long shared path (low combined cost). On the other hand, two paths are non-overlapping for most of their length, with low costs for the non-overlapping paths, but high cost for the short shared path (also, low combined cost). There are many combinations. I want to find solutions with the lowest overall cost, given all the shortest paths from source to target. Does this ring any bells with anyone? Can anyone point me to relevant algorithms or analogous applications? Cheers!

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• Proving that the distance values extracted in Dijkstra's algorithm is non-decreasing?

  - by Gail

  I'm reviewing my old algorithms notes and have come across this proof. It was from an assignment I had and I got it correct, but I feel that the proof certainly lacks. The question is to prove that the distance values taken from the priority queue in Dijkstra's algorithm is a non-decreasing sequence. My proof goes as follows: Proof by contradiction. Fist, assume that we pull a vertex from Q with d-value 'i'. Next time, we pull a vertex with d-value 'j'. When we pulled i, we have finalised our d-value and computed the shortest-path from the start vertex, s, to i. Since we have positive edge weights, it is impossible for our d-values to shrink as we add vertices to our path. If after pulling i from Q, we pull j with a smaller d-value, we may not have a shortest path to i, since we may be able to reach i through j. However, we have already computed the shortest path to i. We did not check a possible path. We no longer have a guaranteed path. Contradiction.

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• Would it be possible to have a UTF-8-like encoding limited to 3 bytes per character?

  - by dan04

  UTF-8 requires 4 bytes to represent characters outside the BMP. That's not bad; it's no worse than UTF-16 or UTF-32. But it's not optimal (in terms of storage space). There are 13 bytes (C0-C1 and F5-FF) that are never used. And multi-byte sequences that are not used such as the ones corresponding to "overlong" encodings. If these had been available to encode characters, then more of them could have been represented by 2-byte or 3-byte sequences (of course, at the expense of making the implementation more complex). Would it be possible to represent all 1,114,112 Unicode code points by a UTF-8-like encoding with at most 3 bytes per character? If not, what is the maximum number of characters such an encoding could represent? By "UTF-8-like", I mean, at minimum: The bytes 0x00-0x7F are reserved for ASCII characters. Byte-oriented find / index functions work correctly. You can't find a false positive by starting in the middle of a character like you can in Shift-JIS.

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• why unsigned int 0xFFFFFFFF is equal to int -1?

  - by conejoroy

  perhaps it's a very stupid question but I'm having a hard time figuring this out =) in C or C++ it is said that the maximum number a size_t (an unsigned int data type) can hold is the same as casting -1 to that data type. for example see http://stackoverflow.com/questions/1420982/invalid-value-for-sizet Why?? I'm confused.. I mean, (talking about 32 bit ints) AFAIK the most significant bit holds the sign in a signed data type (that is, bit 0x80000000 to form a negative number). then, 1 is 0x00000001.. 0x7FFFFFFFF is the greatest positive number a int data type can hold. then, AFAIK the binary representation of -1 int should be 0x80000001 (perhaps I'm wrong). why/how this binary value is converted to anything completely different (0xFFFFFFFF) when casting ints to unsigned?? or.. how is it possible to form a binary -1 out of 0xFFFFFFFF? I have no doubt that in C: ((unsigned int)-1) == 0xFFFFFFFF or ((int)0xFFFFFFFF) == -1 is equally true than 1 + 1 == 2, I'm just wondering why. thanks!

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• Alright, I'm still stuck on this homework problem. C++

  - by Josh

  Okay, the past few days I have been trying to get some input on my programs. Well I decided to scrap them for the most part and try again. So once again, I'm in need of help. For the first program I'm trying to fix, it needs to show the sum of SEVEN numbers. Well, I'm trying to change is so that I don't need the mem[##] = ####. I just want the user to be able to input the numbers and the program run from there and go through my switch loop. And have some kind of display..saying like the sum is?.. Here's my code so far. #include <iostream> #include <iomanip> #include <ios> using namespace std; int main() { const int READ = 10; const int WRITE = 11; const int LOAD = 20; const int STORE = 21; const int ADD = 30; const int SUBTRACT = 31; const int DIVIDE = 32; const int MULTIPLY = 33; const int BRANCH = 40; const int BRANCHNEG = 41; const int BRANCHZERO = 42; const int HALT = 43; int mem[100] = {0}; //Making it 100, since simpletron contains a 100 word mem. int operation; //taking the rest of these variables straight out of the book seeing as how they were italisized. int operand; int accum = 0; // the special register is starting at 0 int counter; for ( counter=0; counter < 100; counter++) mem[counter] = 0; // This is for part a, it will take in positive variables in //a sent-controlled loop and compute + print their sum. Variables from example in text. mem[0] = 1009; mem[1] = 1109; mem[2] = 2010; mem[3] = 2111; mem[4] = 2011; mem[5] = 3100; mem[6] = 2113; mem[7] = 1113; mem[8] = 4300; counter = 0; //Makes the variable counter start at 0. while(true) { operand = mem[ counter ]%100; // Finds the op codes from the limit on the mem (100) operation = mem[ counter ]/100; //using a switch loop to set up the loops for the cases switch ( operation ){ case READ: //reads a variable into a word from loc. Enter in -1 to exit cout <<"\n Input a positive variable: "; cin >> mem[ operand ]; counter++; break; case WRITE: // takes a word from location cout << "\n\nThe content at location " << operand << " is " << mem[operand]; counter++; break; case LOAD:// loads accum = mem[ operand ];counter++; break; case STORE: //stores mem[ operand ] = accum;counter++; break; case ADD: //adds accum += mem[operand];counter++; break; case SUBTRACT: // subtracts accum-= mem[ operand ];counter++; break; case DIVIDE: //divides accum /=(mem[ operand ]);counter++; break; case MULTIPLY: // multiplies accum*= mem [ operand ];counter++; break; case BRANCH: // Branches to location counter = operand; break; case BRANCHNEG: //branches if acc. is < 0 if (accum < 0) counter = operand; else counter++; break; case BRANCHZERO: //branches if acc = 0 if (accum == 0) counter = operand; else counter++; break; case HALT: // Program ends break; } } return 0; } part B int main() { const int READ = 10; const int WRITE = 11; const int LOAD = 20; const int STORE = 21; const int ADD = 30; const int SUBTRACT = 31; const int DIVIDE = 32; const int MULTIPLY = 33; const int BRANCH = 40; const int BRANCHNEG = 41; const int BRANCHZERO = 41; const int HALT = 43; int mem[100] = {0}; int operation; int operand; int accum = 0; int pos = 0; int j; mem[22] = 7; // loop 7 times mem[25] = 1; // increment by 1 mem[00] = 4306; mem[01] = 2303; mem[02] = 3402; mem[03] = 6410; mem[04] = 3412; mem[05] = 2111; mem[06] = 2002; mem[07] = 2312; mem[08] = 4210; mem[09] = 2109; mem[10] = 4001; mem[11] = 2015; mem[12] = 3212; mem[13] = 2116; mem[14] = 1101; mem[15] = 1116; mem[16] = 4300; j = 0; while ( true ) { operand = memory[ j ]%100; // Finds the op codes from the limit on the memory (100) operation = memory[ j ]/100; //using a switch loop to set up the loops for the cases switch ( operation ){ case 1: //reads a variable into a word from loc. Enter in -1 to exit cout <<"\n enter #: "; cin >> memory[ operand ]; break; case 2: // takes a word from location cout << "\n\nThe content at location " << operand << "is " << memory[operand]; break; case 3:// loads accum = memory[ operand ]; break; case 4: //stores memory[ operand ] = accum; break; case 5: //adds accum += mem[operand];; break; case 6: // subtracts accum-= memory[ operand ]; break; case 7: //divides accum /=(memory[ operand ]); break; case 8: // multiplies accum*= memory [ operand ]; break; case 9: // Branches to location j = operand; break; case 10: //branches if acc. is < 0 break; case 11: //branches if acc = 0 if (accum == 0) j = operand; break; case 12: // Program ends exit(0); break; } j++; } return 0; }

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• Validation for Alert dialog

  - by Hossam

  Hi Everyone, Im working in a game and when the player finished the level , Im asking the Player to enter his name on EditText in Alert Dialog : AlertDialog.Builder builder = new Builder(this); LayoutInflater inflater = getLayoutInflater(); // Inflate and set the layout for the dialog // Pass null as the parent view because its going in the dialog layout builder.setView(inflater.inflate(R.layout.dialog_name, null)) // Add action buttons .setPositiveButton(R.string.alertName, new DialogInterface.OnClickListener() { @Override public void onClick(DialogInterface dialog, int id) { playerText = (EditText) ((AlertDialog)dialog).findViewById(R.id.username); if(playerText.getText() != null && playerText.getText().length() != 0) Helper.currentPlayer.setName(playerText.getText().toString()); //calculate the score // save in data base PlayerDataSource playerDS = new PlayerDataSource(getApplicationContext()); playerDS.open(); Helper.currentPlayer = playerDS.createPlayer(Helper.currentPlayer.getName(), Helper.currentPlayer.getScore(), Helper.currentPlayer.levels, Helper.currentPlayer.times, LanguageHelper.div); playerDS.close(); saveStmc(); mainScene.setChildScene(choicesMenu, false, true, true); } }) .setTitle("Please Enter Your name") .setNegativeButton(R.string.cancel, new DialogInterface.OnClickListener() { public void onClick(DialogInterface dialog, int id) { } }); alert = builder.create(); When the Player enters his name and press OK (Positive button) I will check in database if the name exists or not, if the name is exist : show Toast and not dismiss the alert to write another name. How to do that?

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• Maven Mojo & SCM Plugin: Check for a valid working directory

  - by Patrick Bergner

  Hi there. I'm using maven-scm-plugin from within a Mojo and am trying to figure out how to determine if a directory D is a valid working directory of an SCM URL U (i.e. a checkout of U to D already happened). The context is that I want to do a checkout of U if D is a working set or do an update if it isn't. The plan is to check out U to D if D does not exist, update D if D is a valid working directory of U, display an error if D exists and is not a valid working directory of U. What I tried is to call ScmManager.status(), ScmManager.list() and ScmManager.changelog() and try to guess something from their results. But that didn't work. The results from status and changelog always return something positive (isSuccess() = true, getChangedFiles() = valid List, no exceptions to catch), whereas list throws an exception in any case. ScmRepository and ScmFileSet don't seem to provide suitable methods as well. An option would be to always do an update if D exists but then I cannot tell if it is a working directory of U or any SCM working directory at all. The ideal solution would be independent of the actual SCM system and a specific ScmVersion. Thanks for your help! Patrick

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• WM_NCHITTEST and secondary monitor to left of primary monitor

  - by AlanKley

  The described setup with 2nd monitor to left of primary causes WM_NCHITTEST to send negative values which is apparently not supported according to this post. I have a custom control written in win32 that is like a Group control. It has a small clickable area. No MOUSE events are coming to my control when the window containing the custom control lies on a second monitor to the left of the primary monitor. SPY++ shows WM_NCHITTEST messages but no Mouse messages. When window is moved to primary monitor or secondary monitor is positioned to right of primary (all points are positive) then everything works fine. Below is how the WM_NCHITTEST is handled in my custom control. In general I need it to return HTTRANSPARENT so as not to obscure other controls placed inside of it. Anybody have any suggestions what funky coordinate translation I need to do and what to return in response to WM_NCHITTEST to get Mouse messages translated and sent to my control in the case where it is on a 2nd monitor placed to the left of the primary monitor? case WM_NCHITTEST: { POINT Pt = {LOWORD(lP), HIWORD(lP)}; int i; ScreenToClient (hWnd, &Pt); if (PtInRect (&rClickableArea, Pt)) { return(DefWindowProc( hWnd, Msg, wP, lP )); } } lReturn = HTTRANSPARENT; break;

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• C Programming Logic Error?

  - by mbpluvr64

  The following code compiles fine, but does not allow the user to choose whether or not the program is to run again. After giving the user the answer, the program automatically terminates. I placed the main code in a "do while" loop to have the ability to convert more than one time if I wanted too. I have tried to run the program in the command line (Mac and Ubuntu machines) and within XCode with the exact same results. Any assistance would be greatly appreciated. C Beginner P.S. Compiling on MacBookPro running Snow Leopard. #include <stdio.h> #include <stdlib.h> int main(void) { char anotherIteration = 'Y'; do { const float Centimeter = 2.54f; float inches = 0.0f; float result = 0.0f; // user prompt printf("\nEnter inches: "); scanf("%f", &inches); if (inches < 0) { printf("\nTry again. Enter a positive number.\n"); break; } else { // calculate result result = inches * Centimeter; } printf("%0.2f inches is %0.2f centimeters.\n", inches, result); // flush input fflush(stdin); // user prompt printf("\nWould you like to run the program again? (Y/N): "); scanf("%c", &anotherIteration); if ((anotherIteration != 'Y') || (anotherIteration != 'N')) { printf("\nEnter a Y or a N."); break; } } while(toupper(anotherIteration == 'Y')); printf("Program terminated.\n"); return 0; }

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• Is there a website to look up common, already written functions?

  - by pinnacler

  I'm sitting here writing a function that I'm positive has been written before, somewhere on earth. It's just too common to have not been attempted, and I'm wondering why I can't just go to a website and search for a function that I can then copy and paste into my project in 2 seconds, instead of wasting my day reinventing the wheel. Sure there are certain libraries you can use, but where do you find these libraries and when they are absent, is there a site like I'm describing? Possibly a wiki of some type that contains free code that anybody can edit and improve? Edit: I can code things fine, I just don't know HOW to do them. So for example, right now, I'm trying to localize a robot/car/point in space. I KNOW there is a way to do it, just based off of range and distance. Triangulation and Trilateration. How to code that is a different story. A site that could have psuedo code, step by step how to do that would be ridiculously helpful. It would also ensure the optimal solution since everybody can edit it. I'm also writing in Matlab, which I hate because it's quirky, adding to my desire for creating a website like I describe.

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• What is reading source on an ebook reader like?

  - by Daniel Straight

  There are a few open source projects I would really like to read through the code of to understand better / improve on. The problem is, for me, reading for a long time on the computer screen is tedious. I would love to be able to read code while lounging around and on a screen that was better on my eyes. Has anyone ever tried reading code on an ebook reader? It seems like the e-ink would make it easy on the eyes, plus it would be portable, plus it wouldn't kill any trees. In other words, it sounds perfect, but before I drop a couple benjamins on an ebook reader, I'd like to hear if anyone else has tried this and how it went. It seems Scott Hanselman discussed using the Kindle to read code, but I wouldn't necessarily want a Kindle. Has anyone had a positive experience with some other device? I realize this is not directly about programming, but it relates directly to my ability to improve my skills. I think if a good portable reading device for code exists, it could help a lot of programmers.

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• Is it possible to store pointers in shared memory without using offsets?

  - by Joseph Garvin

  When using shared memory, each process may mmap the shared region into a different area of their address space. This means that when storing pointers within the shared region, you need to store them as offsets of the start of the shared region. Unfortunately, this complicates use of atomic instructions (e.g. if you're trying to write a lock free algorithm). For example, say you have a bunch of reference counted nodes in shared memory, created by a single writer. The writer periodically atomically updates a pointer 'p' to point to a valid node with positive reference count. Readers want to atomically write to 'p' because it points to the beginning of a node (a struct) whose first element is a reference count. Since p always points to a valid node, incrementing the ref count is safe, and makes it safe to dereference 'p' and access other members. However, this all only works when everything is in the same address space. If the nodes and the 'p' pointer are stored in shared memory, then clients suffer a race condition: x = read p y = x + offset Increment refcount at y During step 2, p may change and x may no longer point to a valid node. The only workaround I can think of is somehow forcing all processes to agree on where to map the shared memory, so that real pointers rather than offsets can be stored in the mmap'd region. Is there any way to do that? I see MAP_FIXED in the mmap documentation, but I don't know how I could pick an address that would be safe.

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• determining True/False.

  - by sil3nt

  Hi there, the following code #include <iostream> using namespace std; int main(){ char greeting[50] = "goodmorning everyone"; char *s1 = greeting; char *s2 = &greeting[7]; bool test = s2-s1; cout << "s1 is: " << s1 << endl; cout << "s2 is: " << s2 << endl; if (test == true ){ cout << "test is true and is: " << test << endl; } if (test == false){ cout<< "test is false and is: " << test << endl; } return 0; } outputs: s1 is: goodmorning everyone s2 is: ning everyone test is true and is: 1 here what does the line bool test = s2-s1; actually evaluate?, is it the length of the string?. If so, then seeing as s2 is a smaller than s1 it should be negative correct?, and yet the output is true?. Also if i change it to bool test = s1-s2; I still end up with the same result. So it doesnt matter whether its negative or positive the it will be true? and only false when 0?. what does the s2-s1 mean? -cheers (trying to get rid of doubts:))

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• does sfWidgetFormSelect provide a string or an int of the selected item?

  - by han

  Hey guys, I'm having an annoying problem. I'm trying to find out what fields of a form were changed, and then insert that into a table. I managed to var_dump in doUpdateObjectas shown in the following public function doUpdateObject($values) { parent::doUpdateObject($values); var_dump($this->getObject()->getModified(false)); var_dump($this->getObject()->getModified(true)); } And it seems like $this-getObject()-getModified seems to work in giving me both before and after values by setting it to either true or false. The problem that I'm facing right now is that, some how, sfWidgetFormSelect seems to be saving one of my fields as a string. before saving, that exact same field was an int. (I got this idea by var_dump both before and after). Here is what the results on both var dumps showed: array(1) {["annoying_field"]=> int(3)} array(1) {["annoying_field"]=>string(1)"3"} This seems to cause doctrine to think that this is a modification and thus gives a false positive. In my base form, I have under $this->getWidgets() 'annoying_field' => new sfWidgetFormInputText(), under $this->setValidators 'annoying_field' => new sfValidatorInteger(array('required' => false)), and lastly in my configured Form.class.php I have reconfigured the file as such: $this->widgetSchema['annoying_field'] = new sfWidgetFormSelect(array('choices' => $statuses)); statuses is an array containing values like {""a", "b", "c", "d"} and I just want the index of the status to be stored in the database. And also how can I insert the changes into another database table? let's say my Log table? Any ideas and advice as to why this is happen is appreciated, I've been trying to figure it out and browsing google for various keywords with no avail. Thanks! Edit: ok so I created another field, integer in my schema just for testing. I created an entry, saved it, and edited it. this time the same thing happened!

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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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• Reversible pseudo-random sequence generator

  - by user350651

  I would like some sort of method to create a fairly long sequence of random numbers that I can flip through backwards and forwards. Like a machine with "next" and "previous" buttons, that will give you random numbers. Something like 10-bit resolution (i.e. positive integers in a range from 0 to 1023) is enough, and a sequence of 100k numbers. It's for a simple game-type app, I don't need encryption-strength randomness or anything, but I want it to feel fairly random. I have a limited amount of memory available though, so I can't just generate a chunk of random data and go through it. I need to get the numbers in "interactive time" - I can easily spend a few ms thinking about the next number, but not comfortably much more than that. Eventually it will run on some sort of microcontroller, probably just an Arduino. I could do it with a simple linear congruential generator (LCG). Going forwards is simple, to go backwards I'd have to cache the most recent numbers and store some points at intervals so I can recreate the sequence from there. But maybe there IS some pseudo-random generator that allows you to go both forwards and forwards? It should be possible to hook up two linear feedback shift registers (LFSRs) to roll in different directions, no? Or maybe I can just get by with garbling the index number using a hash function of some sort? I'm going to try that first. Any other ideas?

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• SQL Server 2008 - db mail issue

  - by Chris

  Hello. I have two instances of SQL Server 2008. One was upgraded from SQL Server 2000 and one was a clean, new install. SQL Mail operates perfectly on both instances. DB Mail operates perfectly on the newly installed instance. On the upgraded instance, DB Mail does not send any mail. Of course, I am not positive that the fact this instance is upgraded has anything to do with the issue, but it might. The configuration of my db mail profile and account looks identical to my functioning instance. In the configuration of the 'alerts' tab in the SQL Agent properties i have tried selecting both DB Mail and SQL Mail to no avail. Both instances use the same SMTP server with the same authentication (domain with db engine account). All messages sent via sp_send_db mail and those sent via the 'test email' option are visible in the sysmail_allitems queue and remain there as 'unsent'. The send_status eventually changes to 'failed'. The only messages in the sysmail_event_log are 'mail queue stopped by login domain\myuser', 'mail queue started by login domain/myuser' and 'activiation successful.'. selecting from the externalmailqueue has the same number of rows as sysmail_allitems. i have tried bouncing the agent, the entire instance and moving the other functioning instance to the other node in the cluster. any thoughts? thx.

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• C# Vector maths questions

  - by Mark

  Im working in a screen coordinate space that is different to that of the classical X/Y coordinate space, where my Y direction goes down in the positive instead of up. Im also trying to figure out how to make a Circle on my screen always face away from the center point of the screen. If the center point of my screen is at x(200) y(300) and the point of my circle's center is at x(150) and y(380) then I would like to calculate the angle that the circle should be facing. At the moment I have this: Point centerPoint = new Point(200, 300); Point middleBottom = new Point(200, 400); Vector middleVector = new Vector(centerPoint.X - middleBottom.X, centerPoint.Y - middleBottom.Y); Vector vectorOfCircle = new Vector(centerPoint.X - 150, centerPoint.Y - 400); middleVector.Normalize(); vectorOfCircle.Normalize(); var angle = Math.Acos(Vector.CrossProduct(vectorOfCircle, middleVector)); Console.WriteLine("Angle: {0}", angle * (180/Math.PI)); Im not getting what I would expect. I would say that when I enter in x(150) and y(300) of my circle, I would expect to see the rotation of 90 deg, but Im not getting that... Im getting 180!! Any help here would be greatly appreciated. Cheers, Mark

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• Multiple Unpacking Assignment in Python when you don't know the sequence length

  - by doug

  The textbook examples of multiple unpacking assignment are something like: import numpy as NP M = NP.arange(5) a, b, c, d, e = M # so of course, a = 0, b = 1, etc. M = NP.arange(20).reshape(5, 4) # numpy 5x4 array a, b, c, d, e = M # here, a = M[0,:], b = M[1,:], etc. (ie, a single row of M is assigned each to a through e) (My Q is not numpy specfic; indeed, i would prefer a pure python solution.) W/r/t the piece of code i'm looking at now, i see two complications on that straightforward scenario: i usually won't know the shape of M; and i want to unpack a certain number of items (definitely less than all items) and i want to put the remainder into a single container so back to the 5x4 array above, what i would very much like to be able to do is, for instance, assign the first three rows of M to a, b, and c respectively (exactly as above) and the rest of the rows (i have no idea how many there will be, just some positive integer) to a single container, all_the_rest = []. I'm not sure if i have explained this clearly; in any event, if i get feedback i'll promptly edit my Question.

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• Issue in setting alarm time in AlarmManager Class

  - by Anshuman

  I have used the following code in setting alarm time in AlarmManager class. Now Suppose my device current date 9-july-2012 11:31:00, Now suppose i set set a alarm at 9-july-2012 11:45:00, then it works fine and pop-up an alarm at that time. But if i set an alarm at 10-aug-2012 11:40:00, then as soon as exit the app the alarm pop-up, which is wrong because i set an alarm at month of august, So why this happen, is anything wrong in my code. if anyone knows help me to solve this out. Code For Setting Alarm time in AlarmManager class Intent myIntent = new Intent(context, AlarmService.class); PendingIntent pendingIntent = PendingIntent.getService(context, i, myIntent, i); AlarmManager alarmManager = (AlarmManager)context.getSystemService(AlarmService.ALARM_SERVICE); Calendar calendar = Calendar.getInstance(); calendar.setTimeInMillis(System.currentTimeMillis()); calendar.add(Calendar.MILLISECOND, (int) dateDifferenceFromSystemTime(NoteManager.getSingletonObject().getAlarmTime(i))); alarmManager.set(AlarmManager.RTC_WAKEUP, calendar.getTimeInMillis(), pendingIntent); public static long dateDifferenceFromSystemTime(Date date) { long difference = 0; try { Calendar c = Calendar.getInstance(); difference = date.getTime() - c.getTimeInMillis(); if (difference < 0) { // if difference is -1 - means alarm time is of previous time then current // then firstly change it to +positive and subtract form 86400000 to get exact new time to play alarm // 86400000-Total no of milliseconds of 24hr Day difference = difference * -1; difference = 86400000 - difference; } } catch (Exception e) { e.printStackTrace(); } return difference; } Service class which pop-up alarm when matches time public class AlarmService extends IntentService { public void onCreate() { super.onCreate(); } public AlarmService() { super("MyAlarmService"); } @Override public int onStartCommand(Intent intent, int flags, int startId) { super.onStartCommand(intent, startId, startId); return START_STICKY; } @Override protected void onHandleIntent(Intent intent) { startActivity(new Intent(this,AlarmDialogActivity.class).setFlags(Intent.FLAG_ACTIVITY_NEW_TASK)); } }

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