int - Page 182 - Developer IT

Better data-structure design

- by Tempname

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Difference in behaviour( gcc and MSVC++ )

- by Prasoon Saurav

Consider the following code. #include <stdio.h> #include <vector> #include <iostream> struct XYZ { int X,Y,Z; }; std::vector<XYZ> A; int rec(int idx) { int i = A.size(); A.push_back(XYZ()); if (idx >= 5) return i; A[i].X = rec(idx+1); return i; } int main(){ A.clear(); rec(0); puts("FINISH!"); } I couldn't figure out the reason why the code gives segmentation fault on Linux(IDE used: Code::Blocks) whereas on Windows(IDE used : MSVC++) it doesn't. When I used valgrind just to check what actually the problem was, I got this output. I got Invalid write of size 4 at four different places. Then why didn't the code crash when I used MSVC++? Am I missing something?

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sqlite3 date operations when joining two tables in a view?

- by duncan

In short, how to add minutes to a datetime from an integer located in another table, in one select statement, by joining them? I have a table P(int id, ..., int minutes) and a table S(int id, int p_id, datetime start) I want to generate a view that gives me PS(S.id, P.id, S.start + P.minutes) by joining S.p_id=P.id The problem is, if I was generating the query from the application, I can do stuff like: select datetime('2010-04-21 14:00', '+20 minutes'); 2010-04-21 14:20:00 By creating the string '+20 minutes' in the application and then passing it to sqlite. However I can't find a way to create this string in the select itself: select p.*,datetime(s.start_at, formatstring('+%s minutes', p.minutes)) from p,s where s.p_id=p.id; Because sqlite as far the documentation tells, does not provide any string format function, nor can I see any alternative way of expressing the date modifiers.

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std::map insert segmentation fault

- by Jakub Czaplicki

Why does this code stop with segmentation fault : class MapFile { public: /* ... */ std::map <unsigned int, unsigned int> inToOut; }; bool MapFile::LoadMapFile( const wxString& fileName ) { /* ... */ inToOut.insert( std::make_pair(input,output) ); } but this one works fine : class MapFile { public: /* ... */ }; bool MapFile::LoadMapFile( const wxString& fileName ) { /* ... */ std::map <unsigned int, unsigned int> inToOut; inToOut.insert( std::make_pair(input,output) ); } ?

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Drag N Drop utilizing simple cursor

- by Cameron

I'm using CommonsGuy's drag n drop example and I am basically trying to integrate it with the Android notepad example. Drag N Drop Out of the 2 different drag n drop examples i've seen they have all used a static string array where as i'm getting a list from a database and using simple cursor adapter. So my question is how to get the results from simple cursor adapter into a string array, but still have it return the row id when the list item is clicked so I can pass it to the new activity that edits the note. Here is my code: Cursor notesCursor = mDbHelper.fetchAllNotes(); startManagingCursor(notesCursor); // Create an array to specify the fields we want to display in the list (only NAME) String[] from = new String[]{WeightsDatabase.KEY_NAME}; // and an array of the fields we want to bind those fields to (in this case just text1) int[] to = new int[]{R.id.weightrows}; // Now create a simple cursor adapter and set it to display SimpleCursorAdapter notes = new SimpleCursorAdapter(this, R.layout.weights_row, notesCursor, from, to); setListAdapter(notes); And here is the code i'm trying to work that into. public class TouchListViewDemo extends ListActivity { private static String[] items={"lorem", "ipsum", "dolor", "sit", "amet", "consectetuer", "adipiscing", "elit", "morbi", "vel", "ligula", "vitae", "arcu", "aliquet", "mollis", "etiam", "vel", "erat", "placerat", "ante", "porttitor", "sodales", "pellentesque", "augue", "purus"}; private IconicAdapter adapter=null; private ArrayList<String> array=new ArrayList<String>(Arrays.asList(items)); @Override public void onCreate(Bundle icicle) { super.onCreate(icicle); setContentView(R.layout.main); adapter=new IconicAdapter(); setListAdapter(adapter); TouchListView tlv=(TouchListView)getListView(); tlv.setDropListener(onDrop); tlv.setRemoveListener(onRemove); } private TouchListView.DropListener onDrop=new TouchListView.DropListener() { @Override public void drop(int from, int to) { String item=adapter.getItem(from); adapter.remove(item); adapter.insert(item, to); } }; private TouchListView.RemoveListener onRemove=new TouchListView.RemoveListener() { @Override public void remove(int which) { adapter.remove(adapter.getItem(which)); } }; class IconicAdapter extends ArrayAdapter<String> { IconicAdapter() { super(TouchListViewDemo.this, R.layout.row2, array); } public View getView(int position, View convertView, ViewGroup parent) { View row=convertView; if (row==null) { LayoutInflater inflater=getLayoutInflater(); row=inflater.inflate(R.layout.row2, parent, false); } TextView label=(TextView)row.findViewById(R.id.label); label.setText(array.get(position)); return(row); } } } I know i'm asking for a lot, but a point in the right direction would help quite a bit! Thanks

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Merge n files using a C program

- by Amal

I am writing a download Accelerator. So I download a file from the webserver into n parts. Now I want to merge the files into 1 single file. So I use the following code. And the file names are in the correct order. But the output file I am getting is different from the original download file. Can you tell me where could the error be ?C int cbd_merge_files(const char** filenames, int n, const char* final_filename) { FILE* fp = fopen(final_filename, "wb"); if (fp == NULL) return 1; char buffer[4097]; for (int i = 0; i < n; ++i) { const char* fname = filenames[i]; FILE* fp_read = fopen(fname, "rb"); if (fp_read == NULL) return 1; int n; while ((n = fread(buffer, sizeof(char), 4096, fp_read))) { int k = fwrite(buffer, sizeof(char), n, fp); if (!k) return 1; } fclose(fp_read); } fclose(fp); return 0; }

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In C++, what is the "order of precedence" for shadowed variable names?

- by Emile Cormier

In C++, what is the "order of precedence" for shadowed variable names? I can't seem to find a concise answer online. For example: #include <iostream> int shadowed = 1; struct Foo { Foo() : shadowed(2) {} void bar(int shadowed = 3) { std::cout << shadowed << std::endl; // What does this output? } int shadowed; }; int main() { Foo().bar(); } I can't think of any other scopes where a variable might conflict. Please let me know if I missed one.

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How to adjust microphone gain from C# (needs to work on XP & W7)...

- by Ed

First, note that I know there are a few questions like this already posted; however they don't seem to address the problem adequately. I have a C# application, with all the pInvoke hooks to talk to the waveXXX API, and I'm able to do capture and play back of audio with that. I'm also able to adjust speaker (WaveOut) volume with that API. The problem is that for whatever reason, that API does not allow me to adjust microphone (WaveIn) volume. So, I managed to find some mixer code that I've also pulled in and access through pInvoke and that allows me to adjust microphone volume, but only on my W7 PC. The mixer code I started with comes from here: http://social.msdn.microsoft.com/Forums/en-US/isvvba/thread/05dc2d35-1d45-4837-8e16-562ee919da85 and it works, but is written to adjust speaker volume. I added the SetMicVolume method shown here... public static void SetMicVolume(int mxid, int percentage) { bool rc; int mixer, vVolume; MIXERCONTROL volCtrl = new MIXERCONTROL(); int currentVol; mixerOpen(out mixer, mxid, 0, 0, MIXER_OBJECTF_WAVEIN); int type = MIXERCONTROL_CONTROLTYPE_VOLUME; rc = GetVolumeControl(mixer, MIXERLINE_COMPONENTTYPE_SRC_MICROPHONE, type, out volCtrl, out currentVol); if (rc == false) { mixerClose(mixer); mixerOpen(out mixer, 0, 0, 0, 0); rc = GetVolumeControl(mixer, MIXERLINE_COMPONENTTYPE_SRC_MICROPHONE, type, out volCtrl, out currentVol); if (rc == false) throw new Exception("SetMicVolume/GetVolumeControl() failed"); } vVolume = ((int)((float)(volCtrl.lMaximum - volCtrl.lMinimum) / 100.0F) * percentage); rc = SetVolumeControl(mixer, volCtrl, vVolume); if (rc == false) throw new Exception("SetMicVolume/SetVolumeControl() failed"); mixerClose(mixer); } Note the "second attempt" to call GetVolumeControl(). This is done because on XP, in the first call to GetVolumeControl (refer to site above for that code), the call to mixerGetLineControlsA() fails with XP systems returning MIXERR_INVALCONTROL. Then, with this second attempt using mixerOpen(out mixer, 0, 0, 0, 0), the code doesn't return a failure but the mic gain is unaffected. Note, as I said above, this works on W7 (the second attempt is never executed because it doesn't fail using mixerOpen(out mixer, mxid, 0, 0, MIXER_OBJECTF_WAVEIN)). I admit to not having a good grasp on the mixer API, so that's what I'm looking into now; however if anyone has a clue why this would work on W7, but not XP, I'd sure like to hear it. Meanwhile, if I figure it out before I get a response, I'll post my own answer...

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Showing an image after loading it from sql database

- by user330075

I have a problem showing the image form database in a view Details and a ImageController. Inside the view I have: img src=Url.Action("GetFile","Image", new {id= Model.id}) and in controller: public FileContentResult GetFile(int idl) { //int idl = 32; SqlDataReader rdr; byte[] fileContent = null; ........... return File(,,); } When the view is called, function GetFile it just won't work. But if I cut out the parameter int id1 and I instantiate it as a variable it does work. public FileContentResult GetFile() { int idl = 32; SqlDataReader rdr; byte[] fileContent = null; ........... return File(,,); } Why?

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How do I marshal a pointer to an array of pointers to structures?

- by Daniel Stutzbach

I have a C function with the following signature: int my_function(int n, struct player **players) players is a pointer to an array of pointers to struct player objects. n is the number of pointers in the array. The function does not modify the array nor the contents of the structures, and it does not retain any pointers after returning. I tried the following: [DllImport("mylibary.dll")] static extern int my_function(int n, [In, MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 0)] player_in []players); However, that marshals the data as a pointer to an array of structures, not a pointer to an array of pointers to structures.

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[C++] My First Go With Function Templates

- by bobber205

Thought it was pretty straight forward. But I get a "iterator not dereferencable" errro when running the below code. What's wrong? template<typename T> struct SumsTo : public std::binary_function<T, T, bool> { int myInt; SumsTo(int a) { myInt = a; } bool operator()(const T& l, const T& r) { cout << l << " + " << r; if ((l + r) == myInt) { cout << " does add to " << myInt; } else { cout << " DOES NOT add to " << myInt; } return true; } }; void main() { list<int> l1; l1.push_back(1); l1.push_back(2); l1.push_back(3); l1.push_back(4); list<int> l2; l2.push_back(9); l2.push_back(8); l2.push_back(7); l2.push_back(6); transform(l1.begin(), l1.end(), l2.begin(), l2.end(), SumsTo<int>(10) ); }

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Optional parameters for interfaces

- by bryanjonker

Using c# 4.0 -- building an interface and a class that implements the interface. I want to declare an optional parameter in the interface and have it be reflected in the class. So, I have the following: public interface IFoo { void Bar(int i, int j=0); } public class Foo { void Bar(int i, int j=0) { // do stuff } } This compiles, but it doesn't look right. The interface needs to have the optional parameters, because otherwise it doesn't reflect correctly in the interface method signature. Should I skip the optional parameter and just use a nullable type? Or will this work as intended with no side effects or consequences?

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Boost threading/mutexs, why does this work?

- by Flamewires

Code: #include <iostream> #include "stdafx.h" #include <boost/thread.hpp> #include <boost/thread/mutex.hpp> using namespace std; boost::mutex mut; double results[10]; void doubler(int x) { //boost::mutex::scoped_lock lck(mut); results[x] = x*2; } int _tmain(int argc, _TCHAR* argv[]) { boost::thread_group thds; for (int x = 10; x>0; x--) { boost::thread *Thread = new boost::thread(&doubler, x); thds.add_thread(Thread); } thds.join_all(); for (int x = 0; x<10; x++) { cout << results[x] << endl; } return 0; } Output: 0 2 4 6 8 10 12 14 16 18 Press any key to continue . . . So...my question is why does this work(as far as i can tell, i ran it about 20 times), producing the above output, even with the locking commented out? I thought the general idea was: in each thread: calculate 2*x copy results to CPU register(s) store calculation in correct part of array copy results back to main(shared) memory I would think that under all but perfect conditions this would result in some part of the results array having 0 values. Is it only copying the required double of the array to a cpu register? Or is it just too short of a calculation to get preempted before it writes the result back to ram? Thanks.

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Is scala's cake pattern possible with parametrized components?

- by Nicolas

Parametrized components work well with the cake pattern as long as you are only interested in a unique component for each typed component's, example: trait AComponent[T] { val a:A[T] class A[T](implicit mf:Manifest[T]) { println(mf) } } class App extends AComponent[Int] { val a = new A[Int]() } new App Now my application requires me to inject an A[Int] and an A[String], obviously scala's type system doesn't allow me to extends AComponent twice. What is the common practice in this situation ?

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Is it necessarily bad style to ignore the return value of a method

- by Jono

Let's say I have a C# method public void CheckXYZ(int xyz) { // do some operation with side effects } Elsewhere in the same class is another method public int GetCheckedXYZ(int xyz) { int abc; // functionally equivalent operation to CheckXYZ, // with additional side effect of assigning a value to abc return abc; // this value is calculated during the check above } Is it necessarily bad style to refactor this by removing the CheckXYZ method, and replacing all existing CheckXYZ() calls with GetCheckedXYZ(), ignoring the return value? The returned type isn't IDisposable in this case. Does it come down to discretion?

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How can I detect endianness on a system where all primitive integer sizes are the same?

- by Joe Wreschnig

(This question came out of explaining the details of CHAR_BIT, sizeof, and endianness to someone yesterday. It's entirely hypothetical.) Let's say I'm on a platform where CHAR_BIT is 32, so sizeof(char) == sizeof(short) == sizeof(int) == sizeof(long). I believe this is still a standards-conformant environment. The usual way to detect endianness at runtime (because there is no reliable way to do it at compile time) is to make a union { int i, char c[sizeof(int)] } x; x.i = 1 and see whether x.c[0] or x.c[sizeof(int)-1] got set. But that doesn't work on this platform, as I end up with a char[1]. Is there a way to detect whether such a platform is big-endian or little-endian, at runtime? Obviously it doesn't matter inside this hypothetical system, but one can imagine it is writing to a file, or some kind of memory-mapped area, which another machine reads and reconstructs it according to its (saner) memory model.

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I can't set main Class variable in onCreate Method

- by natrollus

Main class has two variables that want to reach another class: public class MyClassA extends Activity { int i = 1; Button b1; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); this.i = 31; this.b1 = (Button) findViewById(R.id.btn1); ~~ } } Second class want to call variables in mainClass object: public class MyclassB implements OnClickListener{ MyClassA mainClass = new MyClassA(); Button btn = mainClass.b1; int n = mainClass.i; public void OnClick(View arg0){ Log.v("btn:",btn); Log.v("int:",n); } //btn returns null; //int returns 1; But onCreate method not set variables.. Why not set main class variables like this.i=31 ?

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MySQL Removing Some Foreign keys

- by Drew

I have a table whose primary key is used in several other tables and has several foreign keys to other tables. CREATE TABLE location ( locationID INT NOT NULL AUTO_INCREMENT PRIMARY KEY ... ) ENGINE = InnoDB; CREATE TABLE assignment ( assignmentID INT NOT NULL AUTO_INCREMENT PRIMARY KEY, locationID INT NOT NULL, FOREIGN KEY locationIDX (locationID) REFERENCES location (locationID) ... ) ENGINE = InnoDB; CREATE TABLE assignmentStuff ( ... assignmentID INT NOT NULL, FOREIGN KEY assignmentIDX (assignmentID) REFERENCES assignment (assignmentID) ) ENGINE = InnoDB; The problem is that when I'm trying to drop one of the foreign key columns (ie locationIDX) it gives me an "ERROR 1025 (HY000): Error on rename" error. How can I drop the column in the assignment table above without getting this error?

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Final enum in Thread's run() method

- by portoalet

Hi, Why is the Elvis elvis definition has to be final to be used inside the Thread run() method? Elvis elvis = Elvis.INSTANCE; // ----> should be final Elvis elvis = Elvis.INSTANCE elvis.sing(4); Thread t1 = new Thread( new Runnable() { @Override public void run() { elvis.sing(6); // --------> elvis has to be final to compile } } ); public enum Elvis { INSTANCE(2); Elvis() { this.x = new AtomicInteger(0); } Elvis(int x){ this.x = new AtomicInteger(x); } private AtomicInteger x = new AtomicInteger(0); public int getX() { return x.get(); } public void setX(int x) {this.x = new AtomicInteger(x);} public void sing(int x) { this.x = new AtomicInteger(x); System.out.println("Elvis singing.." + x); } }

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how to write binary copy of structure array to file

- by cerr

I would like to write a binary image of a structure array to a binary file. I have tried this so far: #include <stdio.h> #include <string.h> #define NUM 256 const char *fname="binary.bin"; typedef struct foo_s { int intA; int intB; char string[20]; }foo_t; void main (void) { foo_t bar[NUM]; bar[0].intA = 10; bar[0].intB = 999; strcpy(bar[0].string,"Hello World!"); Save(bar); printf("%s written succesfully!\n",fname); } int Save(foo_t* pData) { FILE *pFile; int ptr = 0; int itr = 0; pFile = fopen(fname, "w"); if (pFile == NULL) { printf("couldn't open %s\n", fname); return; } for (itr = 0; itr<NUM; itr++) { for (ptr=0; ptr<sizeof(foo_t); ptr++) { fputc((unsigned char)*((&pData[itr])+ptr), pFile); } fclose(pFile); } } but the compiler is saying aggregate value used where an integer was expected fputc((unsigned char)*((&pData[itr])+ptr), pFile); and I don't quite understand why, what am I doing wrong? Thanks!

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What is the difference between Inversion of Control and Dependency injection in C++?

- by rlbond

I've been reading recently about DI and IoC in C++. I am a little confused (even after reading related questions here on SO) and was hoping for some clarification. It seems to me that being familiar with the STL and Boost leads to use of dependency injection quite a bit. For example, let's say I made a function that found the mean of a range of numbers: template <typename Iter> double mean(Iter first, Iter last) { double sum = 0; size_t number = 0; while (first != last) { sum += *(first++); ++number; } return sum/number; }; Is this dependency injection? Inversion of control? Neither? Let's look at another example. We have a class: class Dice { public: typedef boost::mt19937 Engine; Dice(int num_dice, Engine& rng) : n_(num_dice), eng_(rng) {} int roll() { int sum = 0; for (int i = 0; i < num_dice; ++i) sum += boost::uniform_int<>(1,6)(eng_); return sum; } private: Engine& eng_; int n_; }; This seems like dependency injection. But is it inversion of control? Also, if I'm missing something, can someone help me out?

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Database Structure for CakePHP Models

- by Michael T. Smith

We're building a data tracking web app using CakePHP, and I'm having some issues getting the database structure right. We have Companies that haveMany Sites. Sites haveMany DataSamples. Tags haveAndBelongToMany Sites. That is all set up fine. The problem is "ranking" the sites within tags. We need to store it in the database as an archive. I created a Rank model that is setup like this: rank ( id (int), sample_id (int), tag_id (int), site_id (int), rank (int), total_rows) ) So, the question is, how do I create the associations for tag, site and sample to rank? I originally set them as haveMany. But the returned structures don't get me where I'd like to be. It looks like: [Site] => Array ( [Sample] = Array(), [Tag] = Array() ) When I'm really looking for: [Site] => Array ( [Tag] = Array ( [Sample] => Array ( [Rank] => Array ( ...data... ) ) ) ) I think that I may not be structuring the database properly; so if I need to update please let me know. Otherwise, how do I write a find query that gets me where I need to be? Thanks! Thoughts? Need more details? Just ask!

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typedef declaration syntax

- by mt_serg

Some days ago I looked at boost sources and found interesting typedef. There is a code from "boost\detail\none_t.hpp": namespace boost { namespace detail { struct none_helper{}; typedef int none_helper::*none_t ; } // namespace detail } // namespace boost I didn't see syntax like that earlier and can't explain the sense of that. This typedef introduces name "none_t" as pointer to int in boost::detail namespace. What the syntax is? And what difference between "typedef int none_helper::*none_t" and for example "typedef int *none_t" ?

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How to wrap a C function whose parameters are pointer to structs, so that it can be called from Lua?

- by pierr

I have the follwing C function. How should I wrap it so it can be called from a Lua script? typedef struct tagT{ int a ; int b ; } type_t; int lib_a_f_4(type_t *t) { return t->a * t->b ; } I know how to wrapr it if the function parameter type were int or char *. Should I use table type for a C structure? EDIT: I am using SWIG for the wraping , according to this doc, It seems that I should automatically have this funtion new_type_t(2,3) , but it is not the case. If you wrap a C structure, it is also mapped to a Lua userdata. By adding a metatable to the userdata, this provides a very natural interface. For example, struct Point{ int x,y; }; is used as follows: p=example.new_Point() p.x=3 p.y=5 print(p.x,p.y) 3 5 Similar access is provided for unions and the data members of C++ classes. C structures are created using a function new_Point(), but for C++ classes are created using just the name Point().

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Slow query. Wrong database structure?

- by Tin

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