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• Reference Paramters in C++: VERY basic example please.

  - by Sagistic

  I am trying to understand how to use reference parameters. There are several examples in my text, however they are too complicated for me to understand why and how to use them. Could anyone give me the MOST basic example of how/why to use one, and perhaps the difference with or without it (what would happen if you didn't attach the '&'). for example, if I've created a function: int doSomething(int& a, int& b), what would be the consequences of not putting in that '&.' I understand that reference variables are used in order to change a formal-reference, which then allows a two-way exchange of parameters. However, that is the extent of my knowledge, and a more concrete example would be of much help. Thank you.

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• C pointer initialization and dereferencing, what's wrong here?

  - by randombits

  This should be super simple, but I'm not sure why the compiler is complaining here. #include <stdio.h> #include <stdlib.h> int main(int argc, char *argv[]) { int *n = 5; printf ("n: %d", *n); exit(0); } Getting the following complaints: foo.c: In function ‘main’: foo.c:6: warning: initialization makes pointer from integer without a cast I just want to print the value that the pointer n references. I'm dereferencing it in the printf() statement and I get a segmentation fault. Compiling this with gcc -o foo foo.c.

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• What does this recursive function do?

  - by null

  What does this function do? And how do you evaluate it? How to trace it? How to understand recursive method? I just can't understand the recursion, and the exam date is coming soon, and I know in order to understand recursion, I must first understand recursion. But I just can't write a slightly complex recursive method, can anyone help me out using the simplest English words. public class BalancedStrings { public static void printBalanced(String prefix, int a, int b) { if (a > 0) printBalanced(prefix + "a", a - 1, b); if (b > 0) printBalanced(prefix + "b", a, b - 1); if (a == 0 && b == 0) System.out.println(prefix); } public static void printBalanced(int n) { if (n % 2 == 0) { printBalanced("", n / 2, n / 2); } } public static void main(String[] args) { printBalanced(4); } }

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• only return random number when it is unique

  - by phil

  My brain is melting today and i cannot think how to do this simple bit of code. numberList is a string of numbers seperated by commas like '2, 34, 10' etc.. when i request a random number i need to check if the string has the number, if it does i want to keep requesting a random number until the random number is definitely not in the string. i cant think what kind of loop i would do to get this to work: Random r = new Random(); public int RandomPos(int max) { int i; do { i = r.Next(max) + 1; } while (!numberList.Contains(i.ToString())); return i; }

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• Correct Exceptions in C++

  - by Dr.Ackula

  I am just learning how to handle errors in my C++ code. I wrote this example that looks for a text file called some file, and if its not found will throw an exception. #include <iostream> #include <fstream> using namespace std; int main() { int array[90]; try { ifstream file; file.open("somefile.txt"); if(!file.good()) throw 56; } catch(int e) { cout<<"Error number "<<e<<endl; } return 0; } Now I have two questions. First I would like to know if I am using Exceptions correctly. Second, (assuming the first is true) what is the benefit to using them vs an If else statement?

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• Printing escape character

  - by danutenshu

  When I am given "d""\"/""b", I need to print out the statement character for character. (d, b, a slash, a backslash, and 5 quotes) in C++. The only errors that show now are the lines if(i.at(j)="\\") and else if(i.at(j)="\""). Also, how should the outside double apostrophes be excluded? #include <iostream> #include <cstdlib> using namespace std; int main (int argc, const char* argv[] ) { string i= argv[1]; for (int j=0; j>=sizeof(i)-1; j++) { if(i.at(j)="\\") { cout << "\\"; } else if(i.at(j)="\"") { cout << "\""; } else { cout << i.at(j); } } return 0; }

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• What is the accepted way to replace java.util.Date(year,month,day)

  - by dagw

  I'm trying to do something really simple, but starting to realize that dates in Java are a bit of minefield. All I want is to get passed groups of three ints ( a year, a month and a date) create some Date objects, do some simple test on them (along the lines of as date A before date B and after January 1 1990), convert them to java.sql.Date objects and pass them off to the database via JDBC. All very simple and works fine using the java.util.Date(int year,int month,int day) constructor. Of course that constructor is depreciated, and I'd like to avoid using depreciated calls in new code I'm writing. However all the other options to solve this simple problem seem stupidly complicated. Is there really no simple way to do what I want without using depreciated constructors? I know the standard answer to all Java date related questions is "use joda time", but I really don't want to start pulling in third party libraries for such a seemingly trivial problem.

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• How to introduce multi-column constraint with JPA annotations?

  - by plouh

  I am trying to introduce a multi-key constraint on a JPA-mapped entity: public class InventoryItem { @Id private Long id; @Version private Long version; @ManyToOne @JoinColumn("productId") private Product product; @Column(nullable=false); private long serial; } Basically (product, serial) pair should be unique, but I only found a way to say that serial should be unique. This obviously isn't a good idea since different products might have same serial numbers. Is there a way to generate this constraint via JPA or am I forced to manually create it to DB?

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• (C++) If statement not effective

  - by Galileo

  void spriteput(int x,int y, int stype) { char sprite1[5]="OOOO"; char sprite2[5]="OOOO"; char sprite3[5]="OOOO"; char sprite4[5]="OOOO"; if (stype == 1) { char sprite1[5] = " OO "; char sprite2[5] = "OOOO"; char sprite3[5] = "OOOO"; char sprite4[5] = " OO "; mvprintw(2,y,"%s \n",sprite1); } mvprintw(x+1,y,"%s \n",sprite2); mvprintw(x+2,y,"%s \n",sprite3); mvprintw(x+3,y,"%s \n",sprite4); } If I'm correct that block of code should print out on a NCURSES screen OO OOOO OOOO OO Instead however, it prints out the default text (the first char statements). Can anyone tell me why this is? The "printw" statement inside the If-block prints out the proper text, so it's being assigned correctly. Thank you in advance.

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• Partition loop understanding

  - by user1795732

  Why the loop body of the partition method never throws an ArrayIndexOutOfBounds Exception? public static int partition( int[] a, low, high ) { int k = low, m = low; /* loop invariant: * low <= k <= m <= high and * all elements in a[low..k-1] are RED (i.e., < pivot) and * all elements in a[k..m-1] are BLUE (i.e., >= pivot) */ while (m != high) { if (a[m] >= pivot) // a[m] is BLUE { } else { // a[m] is RED swap(a,k,m); k = k+1; } m = m+1; } return k; }

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• Linq TakeWhile depending on sum (or aggregate) of elements

  - by martinweser

  I have a list of elements and want to takeWhile the sum (or any aggregation of the elements) satisfy a certain condition. The following code does the job, but i am pretty sure this is not an unusual problem for which a proper pattern should exist. var list = new List<int> { 1, 2, 3, 4, 5, 6, 7 }; int tmp = 0; var listWithSum = from x in list let sum = tmp+=x select new {x, sum}; int MAX = 10; var result = from x in listWithSum where x.sum < MAX select x.x; Does somebody know how to solve the task in nicer way, probably combining TakeWhile and Aggregate into one query? Thx

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• C# overloading with generics: bug or feature?

  - by TN

  Let's have a following simplified example: void Foo<T>(IEnumerable<T> collection, params T[] items) { // ... } void Foo<C, T>(C collection, T item) where C : ICollection<T> { // ... } void Main() { Foo((IEnumerable<int>)new[] { 1 }, 2); } Compiler says: The type 'System.Collections.Generic.IEnumerable' cannot be used as type parameter 'C' in the generic type or method 'UserQuery.Foo(C, T)'. There is no implicit reference conversion from 'System.Collections.Generic.IEnumerable' to 'System.Collections.Generic.ICollection'. If I change Main to: void Main() { Foo<int>((IEnumerable<int>)new[] { 1 }, 2); } It will work ok. Why compiler does not choose the right overload?

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• Using JLists and ListModels

  - by devoured elysium

  I have defined a DirectoryListModel class that extends the AbstractListModel class from the Java Api. Internally, I have a list of File objects. I have defined the getElementAt(int index) method as: @Override public Object getElementAt(int index) { return directoryElements.get(index) } The problem is that when I try to run my JList with my DirectoryListModel, it is going to show up the full paths of files instead of just the filenames. I could change this code to: @Override public Object getElementAt(int index) { return directoryElements.get(index).getName(); } and it'd work wonders, but the problem is that in the onclick event I'll want to have the File objects so I can do some checking with them (check if they're directories, etc). If I make getElementAt() return a String, I'm losing that possibility, thus I'd like to konw if there is a way I can format my File objects before the JList shows them in my window. Thanks

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• Heap property of this array

  - by davit-datuashvili

  From programming pearls, it is known that array[1...n] has heap property if for all 2<=i<=n x[i/2]<=x[i]. Here is my code: import java.math.*; public class Heap { public static void main(String[]args){ int x[]=new int[]{12,20,15,29,23,17,22,35,40,26,51,19}; int i=1; while (i<x.length) { if (x[Math.round(i/2)]<=x[i]) System.out.println("heap"); i++; } System.out.println("not heap"); } } Here I used Math.round because 4/2 and 5/2 is same and =2. When I compile this code it shows me at last line that it is not heap. Maybe because the index starts from 1 and we don't pay attention to index 0, yes?

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• Strange GCC 'expected primary expression...' error

  - by uj2

  template <typename T> struct A { template <int I> void f(); }; template <typename T> void F(A<T> &a) { a.f<0>(); // expected primary-expression before ‘)’ token } int main() { A<int> a; a.f<0>(); // This one is ok. } What it's all about?

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• C++ template overloading - wrong function called

  - by DeadMG

  template<typename T> T* Push(T* ptr); template<typename T> T* Push(T& ref); template<typename T, typename T1> T* Push(T1&& ref); I have int i = 0; Push<int>(i); But the compiler calls it ambiguous. How is that ambiguous? The second function is clearly the preferred match since it's more specialized. Especially since the T1&& won't bind to an lvalue unless I explicitly forward/move it. Sorry - i is an int. Otherwise, the question would make no sense, and I thought people would infer it since it's normally the loop iterator.

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• C++ return type overload hack

  - by aaa

  I was bored and came up with such hack (pseudocode): 1 struct proxy { 2 operator int(); // int function 3 operator double(); // double function 4 proxy(arguments); 5 arguments &arguments_; 6 }; 7 8 proxy function(arguments &args) { 9 return proxy(args); 10 } 11 int v = function(...); 12 double u = function(...); is it evil to use in real code?

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• Python to C# Conversion of RGBA values not working

  - by clangers

  I'm currently converting some python code to C#, and I'm having an issue with the RGBA C# libraries # Original Python Code: d = math.sqrt( (x - size/2.0)**2 + (y - size/2.0)**2 ) rgbVal = int(200*d/md + 50) rgba = (0,0,0, 255 - rgbVal) img.putpixel((x,y), rgba) // My C# Code double d = Math.Sqrt(Math.Pow((x - DotSize / 2.0), 2) + Math.Pow((y - DotSize / 2.0), 2)); int rgbVal = (int) (200 * d / md + 50); Color color = Color.FromArgb(255 - rgbVal, 0, 0, 0); // ** ERROR ** img.SetPixel(x,y, color); At both instances of the code d is equal to 106 and md is equal to 53. However the resulting rgbVal value is 450. This would obviously mean that 255 - 450 is -195, which causes an error to be thrown as each individual value must be between 0 and 255. Anyone have any idea how I can fix this. Please note that the data is the same when running both the python and C# versions.

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• Unassigned value error asp.net C#

  - by slash shogdhe

  if variable is not assigned,then it takes the default value at run time. for example int A1; if i will check the value of A1 at runtime it will be 0; then why at compile time it throws a error of unassigned value; why clr dont use to alot the default value at runtime; int A1; int B1 = A1+10; it shud be 11 as the default value of A1 is 0; there project property where i can check for "assignn defalut values for unassigned variable"; can any one tell me where i can find it;

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• a newbie gcc compiler and c language question

  - by dydx

  Hi, when I'm trying to compile my c program it gives me this error warning: integer constant is too large for 'long' type which refers to these lines int barcode, a, b, c; scanf("%d", &barcode); a = barcode / 1000000000000; b = barcode / 100000000000 % 10; c = barcode / 10000000000 % 10; and the rest is fine. I know I'm not supposed to use int for such a large number, any suggestions on what I should use? if I replace int with double what should the '%d' part be replaced with then?

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• Do I have to hash twice in C#?

  - by Joe H

  I have code like the following: class MyClass { string Name; int NewInfo; } List<MyClass> newInfo = .... // initialize list with some values Dictionary<string, int> myDict = .... // initialize dictionary with some values foreach(var item in newInfo) { if(myDict.ContainsKey(item.Name)) // 'A' I hash the first time here myDict[item.Name] += item.NewInfo // 'B' I hash the second (and third?) time here else myDict.Add(item.Name, item.NewInfo); } Is there any way to avoid doing two lookups in the Dictionary -- the first time to see if contains an entry, and the second time to update the value? There may even be two hash lookups on line 'B' -- one to get the int value and another to update it.

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• In a C++ template, is it allowed to return an object with spesific type parameters?

  - by nieldw

  When I've got a template with certain type parameters, is it allowed for a function to return an object of this same template, but with different types? In other words, is the following allowed? template<class edgeDecor, class vertexDecor, bool dir> Graph<edgeDecor,int,dir> Graph<edgeDecor,vertexDecor,dir>::Dijkstra(vertex s, bool print = false) const { /* Construct new Graph with apropriate decorators */ Graph<edgeDecor,int,dir> span = new Graph<edgeDecor,int,dir>(); /* ... */ return span; }; If this is not allowed, how can I accomplish the same kind of thing?

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• Abort early in a fold

  - by Heptic

  What's the best way to terminate a fold early? As a simplified example, imagine I want to sum up the numbers in an Iterable, but if I encounter something I'm not expecting (say an odd number) I might want to terminate. This is a first approximation def sumEvenNumbers(nums: Iterable[Int]): Option[Int] = { nums.foldLeft (Some(0): Option[Int]) { case (None, _) => None case (Some(s), n) if n % 2 == 0 => Some(s + n) case (Some(_), _) => None } } However, this solution is pretty ugly (as in, if I did a .foreach and a return -- it'd be much cleaner and clearer) and worst of all, it traverses the entire iterable even if it encounters a non-even number. So what would be the best way to write a fold like this, that terminates early? Should I just go and write this recursively, or is there a more accepted way?

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• partition from programming pearls

  - by davit-datuashvili

  hi suppose i have following array int a[]=new int[]{55,41,59,26,53,58,97,93}; i want to partition it around 55 so new array will be such } 41,26,53,55,59,58,93,93}; i have done such kinds of problems myself but this is from programming pearls and here code is like this we have some array[a..b] and given value t we write code following way int m=a-1; for i=[a..b] if ( array[i]<t) swap (++m;i); where swap function exchange two element in array at indexes ++m and i, i have run this program and it showed me mistake Exception java.lang.NullPointerException can anybody help me?

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• Unreachable code detected

  - by habbo95

  Hi all, I'm getting a "Unreachable code detected" message in Visual Studio at the point con.close() in my code below. Can you spot what I've done wrong? Thanks private int chek1(String insert) { OleDbConnection con = new OleDbConnection("provider=microsoft.jet.oledb.4.0;data source=d:\\sdb.mdb"); OleDbCommand com = new OleDbCommand("select count(*) from sn where sn='" + insert + "\'", con); con.Open(); int po = (int)com.ExecuteScalar(); if (po > 0) return 1; else return 0; con.Close(); }

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