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• Iterator to last element in std::list

  - by Dave18

  #include <list> using std::list; int main() { list <int> n; n.push_back(1); n.push_back(2); n.push_back(3); list <int>::iterator iter = n.begin(); std::advance(iter, n.size() - 1); //iter is set to last element } is there any other way to have an iter to the last element in list?

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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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• semicolon in C++?

  - by SysAdmin

  Here is the question Is "missing semicolon" error really required? why not treat it as a warning? Why I am asking this stupid question? When I compile this code int f = 1 int h=2; the compiler intelligently tells me that where I am missing it. but to me its like - "If you know it, just treat it as if its there and go ahead. (Later I can fix the warning) int sdf = 1,df=2; sdf=1 df =2 even for this code it behaves the same. i.e even if multiple statements (without ;) are in the same line, the compiler knows. So, why not just remove this requirement? why not behave like python,vb etc

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• Why my mysql DISTINCT doesn't work ?

  - by belaz

  Hello, Why the two query below return duplicate member_id and not the third ? i need the second query to work with distinct. Anytime i run a GROUP BY, this query is incredibly slow and the resultset doesn't return the same value as distinct (the value is wrong). SELECT member_id, id FROM ( SELECT * FROM table1 ORDER BY created_at desc ) as u LIMIT 5 +-----------+--------+ | member_id | id | +-----------+--------+ | 11333 | 313095 | | 141831 | 313094 | | 141831 | 313093 | | 12013 | 313092 | | 60821 | 313091 | +-----------+--------+ SELECT distinct member_id, id FROM ( SELECT * FROM table1 ORDER BY created_at desc ) as u LIMIT 5 +-----------+--------+ | member_id | id | +-----------+--------+ | 11333 | 313095 | | 141831 | 313094 | | 141831 | 313093 | | 12013 | 313092 | | 60821 | 313091 | +-----------+--------+ SELECT distinct member_id FROM ( SELECT * FROM table1 ORDER BY created_at desc ) as u LIMIT 5 +-----------+ | member_id | +-----------+ | 11333 | | 141831 | | 12013 | | 60821 | | 64980 | +-----------+ my table sample CREATE TABLE `table1` ( `id` int(11) NOT NULL AUTO_INCREMENT, `member_id` int(11) NOT NULL, `s_type_id` int(11) NOT NULL, `created_at` datetime DEFAULT NULL, PRIMARY KEY (`id`), KEY `s_FI_1` (`member_id`), KEY `s_FI_2` (`s_type_id`) ) ENGINE=InnoDB AUTO_INCREMENT=313096 DEFAULT CHARSET=utf8;

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• Problem with Initializing Consts

  - by UdiM

  This code, when compiled in xlC 8.0 (on AIX 6.1), produces the wrong result. It should print 12345, but instead prints 804399880. Removing the const in front of result makes the code work correctly. Where is the bug? #include <stdio.h> #include <stdlib.h> #include <string> long int foo(std::string input) { return strtol(input.c_str(), NULL, 0); } void bar() { const long int result = foo("12345"); printf("%u\n", result); } int main() { bar(); return 0; } Compilation command: /usr/vacpp/bin/xlC example.cpp -g

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• C++ standard: dereferencing NULL pointer to get a reference?

  - by shoosh

  I'm wondering about what the C++ standard says about code like this: int* ptr = NULL; int& ref = *ptr; int* ptr2 = &ref; In practice the result is that ptr2 is NULL but I'm wondering, is this just an implementation detail or is this well defined in the standard? Under different circumstances a dereferencing of a NULL pointer should result in a crash but here I'm dereferencing it to get a reference which is implemented by the compiler as a pointer so there's really no actual dereferencing of NULL.

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• c++ - what is faster ?

  - by VaioIsBorn

  If we have the following 2 snippets of code in c++ that do the same task: int a, b=somenumber; while(b > 0) { a = b % 3; b /= 3; } or int b=somenumber; while(b > 0) { int a=b%3; b /= 3; } I don't know much about computer architecture/c++ design, but i think that the first code is faster because it declares the integer a at the beginning and just uses it in the while-loop, and in the second code the integer a is being declared everytime the while-loop starts over. Can some one help me with this, am i correct or what and why ?

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• variable scope in statement blocks

  - by fearofawhackplanet

  for (int i = 0; i < 10; i++) { Foo(); } int i = 10; // error, 'i' already exists ---------------------------------------- for (int i = 0; i < 10; i++) { Foo(); } i = 10; // error, 'i' doesn't exist By my understanding of scope, the first example should be fine. The fact neither of them are allowed seems even more odd. Surely 'i' is either in scope or not. Is there something non-obvious about scope I don't understand which means the compiler genuinely can't resolve this? Or is just a case of nanny-state compilerism?

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• Are Triggers Based On Queries Atomic?

  - by David

  I have a table that has a Sequence number. This sequence number will change and referencing the auto number will not work. I fear that the values of the trigger will collide. If two transactions read at the same time. I have ran simulated tests on 3 connections @ ~1 million records each and no collisions. CREATE TABLE `aut` ( `au_id` int(10) NOT NULL AUTO_INCREMENT, `au_control` int(10) DEFAULT NULL, `au_name` varchar(50) DEFAULT NULL, `did` int(10) DEFAULT NULL, PRIMARY KEY (`au_id`), KEY `Did` (`did`) ) ENGINE=InnoDB AUTO_INCREMENT=1 DEFAULT CHARSET=latin1 TRIGGER `binc_control` BEFORE INSERT ON `aut` FOR EACH ROW BEGIN SET NEW.AU_CONTROL = (SELECT COUNT(*)+1 FROM aut WHERE did = NEW.did); END;

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• Break the limit of threading, segmentation fault

  - by user353573

  use pthread_create to create limited number of threads running concurrently Successfully compile and run However, after adding function pointer array to run the function, Segmentation fault Where is wrong? workserver number: 0 Segmentation fault void* workserver(void arg) { int status; while(true) { printf("workserver number: %d\n", (int)arg); ( job_queue[(int)arg])(); sleep(3); status = pthread_mutex_lock(&data.mutex); if(status != 0) printf("%d lock mutex", status); data.value = 1; status = pthread_cond_signal(&data.cond); if(status != 0) printf("%d signal condition", status); status = pthread_mutex_unlock(&data.mutex); if(status != 0) printf("%d unlock mutex", status); } }

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• SQL Server 15MM rows, simple COUNT query. 15+ seconds?

  - by john

  We took over a website from another company after a client decided to switch. We have a table that grows by about 25k records a day, and is currently at 15MM records. The table looks something like: id (PK, int, not null) member_id (int, not null) another_id (int, not null) date (datetime, not null) SELECT COUNT(id) FROM tbl can take up to 15 seconds. A simple inner join on 'another_id' takes over 30 seconds. I can't imagine why this is taking so long. Any advice? SQL Server 2005 Express

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• Using popen() to invoke a shell command?

  - by Anvar

  When running the following code through xcode I get inconsistent behavior. Sometimes it prints the git version correctly, other times it doesn't print anything. The return code from the shell command is always 0 though. Any ideas on why this might be? What am I doing wrong? #define BUFFER_SIZE 256 int main (int argc, const char * argv[]) { FILE *fpipe; char *command="/opt/local/bin/git --version"; char line[BUFFER_SIZE]; if ( !(fpipe = (FILE*)popen(command, "r")) ) { // If fpipe is NULL perror("Problems with pipe"); exit(1); } while ( fgets( line, sizeof(char) * BUFFER_SIZE, fpipe)) { // Inconsistent (happens sometimes) printf("READING LINE"); printf("%s", line); } int status = pclose(fpipe); if (status != 0) { // Never happens printf("Strange error code: %d", status); } return 0; }

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• What's the difference between => , ()=>, and Unit=>

  - by Malvolio

  I'm trying to represent a function that takes no arguments and returns no value (I'm simulating the setTimeout function in JavaScript, if you must know.) case class Scheduled(time : Int, callback : => Unit) doesn't compile, saying " `val' parameters may not be call-by-name" case class Scheduled(time : Int, callback : () => Unit) compiles, but has to be invoked strangely, instead of Scheduled(40, { println("x") } ) I have to do this Scheduled(40, { () => println("x") } ) What also works is class Scheduled(time : Int, callback : Unit => Unit) but is invoked in an even-less-sensible way Scheduled(40, { x : Unit => println("x") } ) (What would a variable of type Unit be?) What I want of course is a constructor that can be invoke the way I would invoke it if it were an ordinary function: Scheduled(40, println("x") ) Give baby his bottle!

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• In a C++ template, is it allowed to return an object with specific 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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• NSString inheritance

  - by Stef

  Hi, I'm doing an useless thing for my first step in Obj-C @interface String : NSString { int m_isnull; } - (id) init; - (int) isNull; @end @implementation String - (id) init { self = [super init]; m_isnull=1; return self; } - (int) isNull { return m_isnull; } @end test : String *a; a=@"ok"; Works fine, but just 2 little questions 1) When I'm compiling I have this warning warning: incompatible Objective-C types assigning 'struct NSString *', expected 'struct String *' I don't know how to avoid it !? 2) a=@"ok" is a fastest way to initialize a string, but when I'm debugging, I don't stop by at my init constructor why ?

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• Writing array to text file

  - by user3661876

  I have an array, but it won't write each index to the text file. Instead it is only writing the last index and the other indexes are not appearing. Can anyone help me to get the entire array printed to the text file? static void solve(int k) { if (k == N) // We placed N-1 queens (0 included), problem solved! { // Solution found! using (StreamWriter sw = new StreamWriter("names.txt")) { Console.Write("Solution: "); for (int i = 0; i < N; i++) { Console.Write(position[i] + " "); foreach (int s in position[i].ToString()) { string list = position[i].ToString(); sw.Write(list + " "); } } Console.WriteLine(); sum += 1; } }

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• Getting the fractional part of a float without using modf()

  - by knight666

  Hi, I'm developing for a platform without a math library, so I need to build my own tools. My current way of getting the fraction is to convert the float to fixed point (multiply with (float)0xFFFF, cast to int), get only the lower part (mask with 0xFFFF) and convert it back to a float again. However, the imprecision is killing me. I'm using my Frac() and InvFrac() functions to draw an anti-aliased line. Using modf I get a perfectly smooth line. With my own method pixels start jumping around due to precision loss. This is my code: const float fp_amount = (float)(0xFFFF); const float fp_amount_inv = 1.f / fp_amount; inline float Frac(float a_X) { return ((int)(a_X * fp_amount) & 0xFFFF) * fp_amount_inv; } inline float Frac(float a_X) { return (0xFFFF - (int)(a_X * fp_amount) & 0xFFFF) * fp_amount_inv; } Thanks in advance!

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• array of objects of a class

  - by anurag18294

  #include class test{ int a; char b; public: test() { cout<<"\n\nDefault constructor being called"; } test(int i,char j) { a=i; b=j; cout<<"\n\nConstructor with arguments called"; } }; int main() { test tarray[5]; test newobj(31,'z'); }; In the above code snippet can we intialize values to tarray[5].

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• Missing MethodInfo for overloaded function with different return type

  - by Charvak

  I have a class defined as follows interface ITest { List<T> Find<T>(int i); } class Test: ITest { public T List<T> Find<T>(int i) { return default(T); } List<T> ITest.Find<T>(int i) { return null; } } When I use typeof(Test).GetMethods() (both with and without appropriate BindingFlags) I do not get the MethodInfo for ITest.Find function. What is the best way of getting the MethodInfo for the missing method? Thanks

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• Python - multi-line array

  - by Ockonal

  Hi guys, in c++ I can wrote: int someArray[8][8]; for (int i=0; i < 7; i++) for (int j=0; j < 7; j++) someArray[i][j] = 0; And how can I initialize multi-line arrays in python? I tried: array = [[],[]] for i in xrange(8): for j in xrange(8): array[i][j] = 0

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• Writing different structs to a file in C++? [on hold]

  - by user58053

  I need a way to write structures of three different kinds to a binary file, which later has to be searched. (As in, for example, struct A has two fields, an int and a char; struct B has int and a long; I need to output all structures whose int equals the one given from keyboard). I understand how to write structs of the same kind to a file and how to search them, but here I am just lost, best thing I came up with is declaring a struct containing all possibly needed fields and leaving the ones I don't need empty, but it really feels wrong, there HAS to be a better way to do that. I've read about binary files and could not find anything relevant, most examples and tutorials deal with writing one data type. Could anyone point me in the right direction? EDIT: I am looking for what @Jerry_coffin called database mode, and will probably use one of the existing database systems for that, best way to go, probably. Thank you everybody for the suggestions

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• Read text and print each (byte) character in separate line

  - by user2967663

  preforming this code to read file and print each character \ (byte) in separate line works well with ASCII void preprocess_file (FILE *fp) { int cc; for (;;) { cc = getc (fp); if (cc == EOF) break; printf ("%c\n", cc); } } int main(int argc, char *argv []) { preprocess_file (stdin); exit (0); } but when i use it with UTF-8 encoded text it shows unredable character such as ï » ? ? § ? „ ? … ? ¤ ? ´ ? and advice ? Thanks

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• string reverse without new array

  - by Codeguru

  hi can anybody tell me the error in this? #include<stdio.h> int main() { char a[]="abcdefgh"; int i=0; int n=strlen(a); char *first; char *second; char *c; *first=a[0]; *second=a[7]; for(i=0;i<=n/2;i++) { *c=*first; *first=*second; *second=*c; first++; second--; } for(i=0;i<=7;i++) { printf("%c",a[i]); } }

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• Is a Critical Section around an integer getter and setter redundant?

  - by Tim Gradwell

  Do critical sections inside trivial int accessors actually do anything useful? int GetFoo() { CriticalSection(crit_id); return foo; } void SetFoo(int value) { CriticalSection(crit_id); foo = value; } Is it possible for two threads to be attempting to read and write foo simultaneously? I'd have thought 'no' unless integers are written byte-at-a-time, in which case I can see the use. But I'd have though modern cpus would read/write integers in a single atomic action...

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• ArrayIndexOutOfBound exception even though I check for array length!

  - by xtracto

  I have the following code in some app: int lowRange=50; int[] ageRangeIndividual = {6, 10, 18, 25, 45, 65, 90}; int index=0; for (; index<ageRangeIndividual.length-1 && ageRangeIndividual[index]<=lowRange;index++); I am getting an "Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 7" in the for line! even though I explicitly specify to break the cycle if index < last indexable item in the array! This does not happen always, but after some time of running said program (lowRange varies each time the function is called) What am I not seeing?

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