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• C++: defining maximum/minimum limits for a class

  - by Luis

  Basically what the title says... I have created a class that models time slots in a variable-granularity daily schedule (where for example the first time slot is 30 minutes, but the second time slot can be 40 minutes); the first available slot starts at (a value comparable to) 1. What I want to do now is to define somehow the maximum and minimum allowable values that this class takes and I have two practical questions in order to do so: 1.- does it make sense to define absolute minimum and maximum in such a way for a custom class? Or better, does it suffice that a value always compares as lower-than any other possible value of the type, given the class's defined relational operators, to be defined the min? (and analogusly for the max) 2.- assuming the previous question has an answer modeled after "yes" (or "yes but ..."), how do I define such max/min? I know that there is std::numeric_limits<> but from what I read it is intended for "numeric types". Do I interpret that as meaning "represented as a number" or can I make a broader assumption like "represented with numbers" or "having a correspondence to integers"? After all, it would make sense to define the minimum and maximum for a date class, and maybe for a dictionary class, but numeric_limits may not be intended for those uses (I don't have much experience with it). Plus, numeric_limits has a lot of extra members and information that I don't know what to make with. If I don't use numeric_limits, what other well-known / widely-used mechanism does C++ offer to indicate the available range of values for a class?

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• What is the rationale to not allow overloading of C++ conversions operator with non-member function

  - by Vicente Botet Escriba

  C++0x has added explicit conversion operators, but they must always be defined as members of the Source class. The same applies to the assignment operator, it must be defined on the Target class. When the Source and Target classes of the needed conversion are independent of each other, neither the Source can define a conversion operator, neither the Target can define a constructor from a Source. Usually we get it by defining a specific function such as Target ConvertToTarget(Source& v); If C++0x allowed to overload conversion operator by non member functions we could for example define the conversion implicitly or explicitly between unrelated types. template < typename To, typename From > operator To(const From& val); For example we could specialize the conversion from chrono::time_point to posix_time::ptime as follows template < class Clock, class Duration> operator boost::posix_time::ptime( const boost::chrono::time_point<Clock, Duration>& from) { using namespace boost; typedef chrono::time_point<Clock, Duration> time_point_t; typedef chrono::nanoseconds duration_t; typedef duration_t::rep rep_t; rep_t d = chrono::duration_cast<duration_t>( from.time_since_epoch()).count(); rep_t sec = d/1000000000; rep_t nsec = d%1000000000; return posix_time::from_time_t(0)+ posix_time::seconds(static_cast<long>(sec))+ posix_time::nanoseconds(nsec); } And use the conversion as any other conversion. For a more complete description of the problem, see here or on my Boost.Conversion library.. So the question is: What is the rationale to non allow overloading of C++ conversions operator with non-member functions?

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• Fastest way to clamp a real (fixed/floating point) value?

  - by Niklas

  Hi, Is there a more efficient way to clamp real numbers than using if statements or ternary operators? I want to do this both for doubles and for a 32-bit fixpoint implementation (16.16). I'm not asking for code that can handle both cases; they will be handled in separate functions. Obviously, I can do something like: double clampedA; double a = calculate(); clampedA = a > MY_MAX ? MY_MAX : a; clampedA = a < MY_MIN ? MY_MIN : a; or double a = calculate(); double clampedA = a; if(clampedA > MY_MAX) clampedA = MY_MAX; else if(clampedA < MY_MIN) clampedA = MY_MIN; The fixpoint version would use functions/macros for comparisons. This is done in a performance-critical part of the code, so I'm looking for an as efficient way to do it as possible (which I suspect would involve bit-manipulation) EDIT: It has to be standard/portable C, platform-specific functionality is not of any interest here. Also, MY_MIN and MY_MAX are the same type as the value I want clamped (doubles in the examples above).

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• how to append a string to next line in perl

  - by tprayush

  hi all , i have a requirement like this.. this just a sample script... $ cat test.sh #!/bin/bash perl -e ' open(IN,"addrss"); open(out,">>addrss"); @newval; while (<IN>) { @col_val=split(/:/); if ($.==1) { for($i=0;$i<=$#col_val;$i++) { print("Enter value for $col_val[$i] : "); chop($newval[$i]=<STDIN>); } $str=join(":"); $_="$str" print OUT; } else { exit 0; } } close(IN); close(OUT); ' when i run this scipt... $ ./test.sh Enter value for NAME : abc Enter value for ADDRESS : asff35 Enter value for STATE : XYZ Enter value for CITY : EIDHFF Enter value for CONTACT : 234656758 $ cat addrss NAME:ADDRESS:STATE:CITY:CONTACT abc:asff35:XYZ:EIDHFF:234656758 when ran it second time $ cat addrss NAME:ADDRESS:STATE:CITY:CONTACT abc:asff35:XYZ:EIDHFF:234656758ioret:56fgdh:ghdgh:afdfg:987643221 ## it is appended in the same line... i want it to be added to the next line..... NOTE: i want to do this by explitly using the filehandles in perl....and not with redirection operators in shell. please help me!!!

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• MySQL : delete from table that is used in the where clause

  - by Eric

  I am writing a small script to synchronize 2 MySQL tables ( t1 to be 'mirrored' to t2 ) In a step I would like to delete rows inside t2 that has been delete in t1 with the same id. I tried this query : delete from t2 where t2.id in ( select t2.id left join t1 on (t1.id=t2.id) where t1.id is null ) But Mysql forbid me to use t2 in the same time in the delete and in the select (sound logical by the way) Of course, I can split the query into 2 queries : first select IDs, then delete rows with these IDs. My question : do you have a cleaner way to delete row from t2 that does not exist anymore in t1 ? with one query only ?

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• How to support comparisons for QVariant objects containing a custom type?

  - by Tyler McHenry

  According to the Qt documentation, QVariant::operator== does not work as one might expect if the variant contains a custom type: bool QVariant::operator== ( const QVariant & v ) const Compares this QVariant with v and returns true if they are equal; otherwise returns false. In the case of custom types, their equalness operators are not called. Instead the values' addresses are compared. How are you supposed to get this to behave meaningfully for your custom types? In my case, I'm storing an enumerated value in a QVariant, e.g. In a header: enum MyEnum { Foo, Bar }; Q_DECLARE_METATYPE(MyEnum); Somewhere in a function: QVariant var1 = QVariant::fromValue<MyEnum>(Foo); QVariant var2 = QVariant::fromValue<MyEnum>(Foo); assert(var1 == var2); // Fails! What do I need to do differently in order for this assertion to be true? I understand why it's not working -- each variant is storing a separate copy of the enumerated value, so they have different addresses. I want to know how I can change my approach to storing these values in variants so that either this is not an issue, or so that they do both reference the same underlying variable. It don't think it's possible for me to get around needing equality comparisons to work. The context is that I am using this enumeration as the UserData in items in a QComboBox and I want to be able to use QComboBox::findData to locate the item index corresponding to a particular enumerated value.

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• C++, create an instance from a static method

  - by Manux

  Hello, let's say I want my users to use only one class, say SpecialData. Now, this data class would have many methods, and depending on the type of data, the methods do different things, internally, but return externally similar results. Therefore my wanting to have one "public" class and other "private", child classes that would change the behavior of methods, etc... It would be amazingly more simple for some types of data that need to be built to do something like this: SpecialData& sm = SpecialData::new_supermatrix(); and new_supermatrix() would return a SuperMatrix instance, which inherits from most behaviors of SpecialData. my header: static SpecialData& new_supermatrix(); my cpp: SpecialData& SpecialData::new_supermatrix()(){ return SuperMatrix(MATRIX_DEFAULT_MAGNITUDE,1000,1239,FLOAT32,etc...); } The problem is, I get this error, which is probably logical due to the circumstances: invalid initialization of non-const reference of type ‘SpecialData&’ from a temporary of type ‘SpecialData’ So, any ideas?

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• LINQ .Cast() extension method fails but (type)object works.

  - by Ben Robinson

  To convert between some LINQ to SQL objects and DTOs we have created explicit cast operators on the DTOs. That way we can do the following: DTOType MyDTO = (LinqToSQLType)MyLinq2SQLObj; This works well. However when you try to cast using the LINQ .Cast() extension method it trows an invalid cast exception saying cannot cast type Linq2SQLType to type DTOType. i.e. the below does not work List<DTO.Name> Names = dbContact.tNames.Cast<DTO.Name>() .ToList(); But the below works fine: DAL.tName MyDalName = new DAL.tName(); DTO.Name MyDTOName = (DTO.Name)MyDalName; and the below also works fine List<DTO.Name> Names = dbContact.tNames.Select(name => (DTO.Name)name) .ToList(); Why does the .Cast() extension method throw an invalid cast exception? I have used the .Cast() extension method in this way many times in the past and when you are casting something like a base type to a derived type it works fine, but falls over when the object has an explicit cast operator.

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• Is this overly clever or unsafe?

  - by Liberalkid

  I was working on some code recently and decided to work on my operator overloading in c++, because I've never really implemented it before. So I overloaded the comparison operators for my matrix class using a compare function that returned 0 if LHS was less than RHS, 1 if LHS was greater than RHS and 2 if they were equal. Then I exploited the properties of logical not in c++ on integers, to get all of my compares in one line: inline bool Matrix::operator<(Matrix &RHS){ return ! (compare(*this,RHS)); } inline bool Matrix::operator>(Matrix &RHS){ return ! (compare((*this),RHS)-1); } inline bool Matrix::operator>=(Matrix &RHS){ return compare((*this),RHS); } inline bool Matrix::operator<=(Matrix &RHS){ return compare((*this),RHS)-1; } inline bool Matrix::operator!=(Matrix &RHS){ return compare((*this),RHS)-2; } inline bool Matrix::operator==(Matrix &RHS){ return !(compare((*this),RHS)-2); } Obviously I should be passing RHS as a const, I'm just probably not going to use this matrix class again and I didn't feel like writing another function that wasn't a reference to get the array index values solely for the comparator operation.

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• SQL Server JOIN with optional NULL values

  - by Paul McLoughlin

  Imagine that we have two tables as follows: Trades ( TradeRef INT NOT NULL, TradeStatus INT NOT NULL, Broker INT NOT NULL, Country VARCHAR(3) NOT NULL ) CTMBroker ( Broker INT NOT NULL, Country VARCHAR(3) NULL ) (These have been simplified for the purpose of this example). Now, if we wish to join these two tables on the Broker column, and if a country exists in the CTMBroker table on the Country, we have the following two choices: SELECT T.TradeRef,T.TradeStatus FROM Trades AS T JOIN CTMBroker AS B ON B.Broker=T.Broker AND ISNULL(B.Country, T.Country) = T.Country or SELECT T.TradeRef,T.TradeStatus FROM Trades AS T JOIN CTMBroker AS B ON B.Broker=T.Broker AND (B.COUNTRY=T.Country OR B.Country IS NULL) These are both logically equivalent, however in this specific circumstance for our database (SQL Server 2008, SP1) two different execution plans are produced for these two queries with the second version significantly outperforming the first version in terms of both time and logical reads. My question really is as follows: as a general rule would (2) be preferred to (1), or does this just happen to be exploiting some particular idiosyncracy of the optimiser in 2008 SP1 (that could therefore change with future versions of SQL Server).

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• Find java classes implementing an interface

  - by Linor

  Some time ago, I came across a piece of code, that used some piece of standard java functionality to locate the classes that implemented a given interface. I know the the functions were hidden in some non logical place, but they could be used for other classes as the package name implied. Back then I did not need it, so I forgot about it, but now I do, and I can't seem to find the functions again. Does anyone know where to find these functions? edit: I'm not looking for any IDE functions or anything, but rather something that can be executed within the java application.

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• Linq-to-XML explicit casting in a generic method

  - by vlad

  I've looked for a similar question, but the only one that was close didn't help me in the end. I have an XML file that looks like this: <Fields> <Field name="abc" value="2011-01-01" /> <Field name="xyz" value="" /> <Field name="tuv" value="123.456" /> </Fields> I'm trying to use Linq-to-XML to get the values from these fields. The values can be of type Decimal, DateTime, String and Int32. I was able to get the fields one by one using a relatively simple query. For example, I'm getting the 'value' from the field with the name 'abc' using the following: private DateTime GetValueFromAttribute(IEnumerable<XElement> fields, String attName) { return (from field in fields where field.Attribute("name").Value == "abc" select (DateTime)field.Attribute("value")).FirstOrDefault() } this is placed in a separate function that simply returns this value, and everything works fine (since I know that there is only one element with the name attribute set to 'abc'). however, since I have to do this for decimals and integers and dates, I was wondering if I can make a generic function that works in all cases. this is where I got stuck. here's what I have so far: private T GetValueFromAttribute<T>(IEnumerable<XElement> fields, String attName) { return (from field in fields where field.Attribute("name").Value == attName select (T)field.Attribute("value").Value).FirstOrDefault(); } this doesn't compile because it doesn't know how to convert from String to T. I tried boxing and unboxing (i.e. select (T) (Object) field.Attribute("value").Value but that throws a runtime Specified cast is not valid exception as it's trying to convert the String to a DateTime, for instance. Is this possible in a generic function? can I put a constraint on the generic function to make it work? or do I have to have separate functions to take advantage of Linq-to-XML's explicit cast operators?

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• WM_SYSCOMMAND oddities

  - by Andreas Rejbrand

  An application recieves the WM_SYSCOMMAND message when the user selects a menu item command on the system menu, and so wParam can be SC_CLOSE, SC_CONTEXTHELP, SC_MAXIMIZE, SC_MINIMIZE, SC_RESTORE etc. That's logical. But one can also send the WM_SYSCOMMAND message to send commands to the Windows Shell. For instance, one can display the start menu (SC_TASKLIST), activate the screen saver (SC_SCREENSAVE), and turn off the monitor (SC_MONITORPOWER). This does not make sense, does it? What does this have to do with the application's system menu? This is more of a "system command", i.e. more of a completely other interpretation of the name "WM_SYSCOMMAND" of the message. It's like the message is used to send command requests to the system. Why is this message used for two seemingly entirely different things, and what thing does the name "SYSCOMMAND" refer to (command on the system menu, or command of the operating system)?

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• F# How to tokenise user input: separating numbers, units, words?

  - by David White

  I am fairly new to F#, but have spent the last few weeks reading reference materials. I wish to process a user-supplied input string, identifying and separating the constituent elements. For example, for this input: XYZ Hotel: 6 nights at 220EUR / night plus 17.5% tax the output should resemble something like a list of tuples: [ ("XYZ", Word); ("Hotel:", Word); ("6", Number); ("nights", Word); ("at", Operator); ("220", Number); ("EUR", CurrencyCode); ("/", Operator); ("night", Word); ("plus", Operator); ("17.5", Number); ("%", PerCent); ("tax", Word) ] Since I'm dealing with user input, it could be anything. Thus, expecting users to comply with a grammar is out of the question. I want to identify the numbers (could be integers, floats, negative...), the units of measure (optional, but could include SI or Imperial physical units, currency codes, counts such as "night/s" in my example), mathematical operators (as math symbols or as words including "at" "per", "of", "discount", etc), and all other words. I have the impression that I should use active pattern matching -- is that correct? -- but I'm not exactly sure how to start. Any pointers to appropriate reference material or similar examples would be great.

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• If a Matlab function returns a variable number of values, how can I get all of them as a cellarray,

  - by kaleidomedallion

  I am writing a function to remove some values from a cellarray, like so: function left = remove(cells, item); left = cells{cellfun(@(i) ~isequal(item, i), cells)}; But when I run this, left has only the first value, as the call to cells{} with a logical array returns all of the matching cells as separate values. How do I group these separate return values into a single cellarray? Also, perhaps there is already a way to remove a given item from a cellarray? I could not find it in the documentation.

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• Ruby on Rails: one-to-one mapping. Just semantics or really a different structure.

  - by Sam

  So I'm creating a plugin for Ruby on Rails to make implemented addresses including country, state, city, and zip_code for countries that can follow that paradigm a lot easier but that's beside the point expect for how the address model is associated. So starting with my address model. class Address < ActiveRecord::Base has_one :country has_one :state has_one :city has_one :zip_code end What's the difference between saying belongs_to and has_one Seems to be the same thing because both only require one model to declare ownership and foreign_key And it also seems that both are logical to say. an address belongs to an account and an account has one address Is this only semantics or is there are real difference

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• Haskel dot (.) and dollar ($) composition: correct use.

  - by Robert Massaioli

  I have been reading Real World Haskell and I am nearing the end but a matter of style has been niggling at me to do with the (.) and ($) operators. When you write a function that is a composition of other functions you write it like: f = g . h But when you apply something to the end of those functions I write it like this: k = a $ b $ c $ value But the book would write it like this: k = a . b . c $ value Now to me they look functionally equivalent, they do the exact same thing in my eyes. However, the more I look, the more I see people writing their functions in the manner that the book does: compose with (.) first and then only at the end use ($) to append a value to evaluate the lot (nobody does it with many dollar compositions). Is there a reason for using the books way that is much better than using all ($) symbols? Or is there some best practice here that I am not getting? Or is it superfluous and I shouldn't be worrying about it at all? Thanks.

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• Rails: getting logic to run at end of request, regardless of filter chain aborts?

  - by JSW

  Is there a reliable mechanism discussed in rails documentation for calling a function at the end of the request, regardless of filter chain aborts? It's not after filters, because after filters don't get called if any prior filter redirected or rendered. For context, I'm trying to put some structured profiling/reporting information into the app log at the end of every request. This information is collected throughought the request lifetime via instance variables wrapped in custom controller accessors, and dumped at the end in a JSON blob for use by a post-processing script. My end goal is to generate reports about my application's logical query distribution (things that depend on controller logic, not just request URIs and parameters), performance profile (time spent in specific DB queries or blocked on webservices), failure rates (including invalid incoming requests that get rejected by before_filter validation rules), and a slew of other things that cannot really be parsed from the basic information in the application and apache logs. At a higher level, is there a different "rails way" that solves my app profiling goal?

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• Use multiple css files or a single file organised by comments

  - by David

  Hi, what is regarded as the best approach to organising css. At the moment I am using a single link in the head of my xhtml documents as follows: <link rel="stylesheet" type="text/css" href="style/imports.css" /> In this file im importing several different css files i.e. reset.css, structure.css, skin.css I know there is an overhead in doing this as each requires an extra trip to the server but it makes things much more logical and organised in my opinion. Does anyone have an opinion on how best to organise their css. - Would it be better to put all these seperate css funcions into one single file? Also, is it best practice to minify css.

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• Haskell function composition (.) and function application ($) idioms: correct use.

  - by Robert Massaioli

  I have been reading Real World Haskell and I am nearing the end but a matter of style has been niggling at me to do with the (.) and ($) operators. When you write a function that is a composition of other functions you write it like: f = g . h But when you apply something to the end of those functions I write it like this: k = a $ b $ c $ value But the book would write it like this: k = a . b . c $ value Now to me they look functionally equivalent, they do the exact same thing in my eyes. However, the more I look, the more I see people writing their functions in the manner that the book does: compose with (.) first and then only at the end use ($) to append a value to evaluate the lot (nobody does it with many dollar compositions). Is there a reason for using the books way that is much better than using all ($) symbols? Or is there some best practice here that I am not getting? Or is it superfluous and I shouldn't be worrying about it at all? Thanks.

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• Function chaining depending on boolean result

  - by Markive

  This is just an efficiency question really.. I'm interested to know if there is a more efficient or logical way that people use to handle this sort of scenario. In my asp.net application I am running a script to generate a new project my code at the top level looks like this: Dim ok As Boolean = True ok = createFolderStructure() If ok Then ok = createMDB() If ok Then ok = createProjectConfig() If ok Then ok = updateCompanyConfig() I create a boolean and each function returns a boolean result, the next function in this chain will only run if the previous one was successful. I do this because an asp.net application will continue to run through the page life cycle unless there is an unhandled exception and I don't want my whole application to be screwed up if something in the chain goes wrong (there is a lot of copying and deleting of files etc.. in this example). I was just wondering how other people handle this scenario? the vb.net single line if statement is quite succinct but I'm wondering if there is a better way?

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• Data Usage Checker Tools

  - by Lucifer

  Hey All, I am about to begin a project for a new client, and am worried about a few things concerning data usage on their internet plan. We're in an area where most of the major networks don't cover the area, and the ones that do, have very expensive plans, with very low data allowance per month. I need to develop an app, but part of the problem lies with checking database values every 30 seconds. It's pretty important that this check is happening every 30 seconds, as the database is actually updated all day everyday, approx. every 5seconds (apparently). Each row in the database consists of about a page full of text if you were to paste it into MS Word. So, are there any logical ways of minimizing data usage in my case, and also how am I able to see exactly how much data is used just to establish a connection to the database? Are there any tools for this kind of info? Thanks :)

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• Enumerating combinations in a distributed manner

  - by Reyzooti

  I have a problem where I must analyse 500C5 combinations (255244687600) of something. Distributing it over a 10 node cluster where each cluster processes roughly 10^6 combinations per second means the job will be complete in about 7hours. The problem I have is distributing the 255244687600 combinations over the 10 nodes. I'd like to present each node with 25524468760, however the algorithms I'm using can only produce the combinations sequentially, I'd like to be able to pass the set of elements and a range of combination indicies eg: [0-10^7) or [10^7,2.0 10^7) etc and have the nodes themselves figure out the combinations. The algorithms I'm using at the moment are from the following: http://home.roadrunner.com/~hinnant/combinations.html A logical question I've considered using a master node, that enumerates each of the combinations and sends work to each of the nodes, however the overhead incurred in iterating the combinations from a single node and communicating back and forth work is enormous, and will subsequently lead to the master node becoming the bottleneck. Are there any good combination iterating algorithms geared up for efficient/optimal distributed enumeration?

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• How would I compare two Lists(Of <CustomClass>) in VB?

  - by Kumba

  I'm working on implementing the equality operator = for a custom class of mine. The class has one property, Value, which is itself a List(Of OtherClass), where OtherClass is yet another custom class in my project. I've already implemented the IComparer, IComparable, IEqualityComparer, and IEquatable interfaces, the operators =, <>, bool and not, and overriden Equals and GetHashCode for OtherClass. This should give me all the tools I need to compare these objects, and various tests comparing two singular instances of these objects so far checks out. However, I'm not sure how to approach this when they are in a List. I don't care about the list order. Given: Dim x As New List(Of OtherClass) From {New OtherClass("foo"), New OtherClass("bar"), New OtherClass("baz")} Dim y As New List(Of OtherClass) From {New OtherClass("baz"), New OtherClass("foo"), New OtherClass("bar")} Then (x = y).ToString should print out True. I need to compare the same (not distinct) set of objects in this list. The list shouldn't support dupes of OtherClass, but I'll have to figure out how to add that in later as an exception. Not interested in using LINQ. It looks nice, but in the few examples I've played with, adds a performance overhead in that bugs me. Loops are ugly, but they are fast :) A straight code answer is fine, but I'd like to understand the logic needed for such a comparison as well. I'm probably going to have to implement said logic more than a few times down the road.

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• What's the right way to handle "One, Both, or None" logic?

  - by Stephen

  I have a logic situation that is best described as two "Teams" trying to win a task. The outcome of this task could be a single winner, a tie (draw), or no winner (stalemate). Currently, I'm using a nested if/else statement like so: // using PHP, but the concept seems language agnostic. if ($team_a->win()) { if ($team_b->win()) { // this is a draw } else { // team_a is the winner } } else { if ($team_b->win()) { // team_b is the winner } else { // This is a stalemate, no winner. } } This seems rather spaghetti-like and repetitive. Is there a more logical, DRY pattern I could use?

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