S3 can only dispatch on it&#39;s first argument, whereas S4 can dispatch on multiple arguments. If you want to be able to write methods for function `foo` that should do different things if given an object of class `&quot;bar&quot;` or given objects of class `&quot;bar&quot;` **and** `&quot;foobar&quot;`, or given objects of class `&quot;barfoo&quot;` and `&quot;foobar&quot;`, then S4 provides a far better way to handle such complexities.

S3 is very simple and easy to implement, but isn&#39;t really a formal object oriented system. That simplicity comes at the cost of enforcing that objects belonging to a class have the correct components/slots etc. With S3 I can do things like `class(obj) &lt;- &quot;lm&quot;` and method dispatch will use methods for the `&quot;lm&quot;` class when passed `obj`, but there is no guarantee that `obj` really is an object of class `&quot;lm&quot;`.

S3 is easy to implement, document and requires less extra knowledge on the part of the programmer.

Which to use can only be something you can decide. Doug Bates (2003) has said, for example, that for new projects he would use S4 over S3. I haven&#39;t yet had a need to use anything other than S3 methods.

So I would sit down and think about the sorts of classes and methods you want to operate on those classes. Think about the functionality you want, and that will probably point towards one system or the other.

Douglas Bates. [Converting packages to S4][1]. R News, 3(1):6-8, June 2003 

  [1]: http://cran.r-project.org/doc/Rnews/Rnews_2003-1.pdf

What about [Reference Classes](https://stackoverflow.com/questions/5137199)? it&#39;s not S3 and some consider it syntactical sugar on top of S4, but it&#39;s _a lot of_ sugar.

For a short example, check [this answers](https://stackoverflow.com/questions/4959018/which-oo-programming-style-in-r-will-result-readable-to-a-python-programmer/4964468#4964468) to a question I asked here a few months ago.

I still have to find out how to write Rd files for such classes.

I have _users_, _videos_, _topics_, _criterias_ and _ratings_

- A _video_ has a _topic_
- A _topic_ has _criterias_
- A _user_ can create a _video_ for a given _topic_
- A _user_ can rate a _video_ on each _criterias_ given for the concerned _topic_.

You can see my original post https://stackoverflow.com/questions/6388259/django-rating-model-example-detailview-template to get details on the model used

I have extended a `DetailView` template based on the _video_ model to put the list of _ratings_ for the selected _video_ for a given _user_ as extra context.

    class VideoFileDetailView(DetailView):
      model = VideoFile

      def get_context_data(self, **kwargs):
        context = super(VideoFileDetailView, self).get_context_data(**kwargs)
        context[&#39;rates&#39;] = VideoRate.objects.filter(video=self.object, user=self.request.user)
        return context

In the template pointed by the `DetailView`, I&#39;d like to list the _criterias_ of the _video_, and for each _criteria_ display the current **rating value** form the _user_.


    &lt;div id=&quot;rating&quot;&gt;
      &lt;ul&gt;
    {% for crit in videofile.topic.crits.all %}
        &lt;li&gt;
      {% for rate in rates %}
        {% if rate.crit.id == crit.id %}
          {{ rate.rate }}
        {% endif %}
      {% endfor %}
          &lt;div class=&quot;rateit&quot;
            data-rateit-value=&quot;{# The rating value #}&quot;
            data-rateit-ispreset=&quot;true&quot;
            crit-id=&quot;{{ crit.id }}&quot;&gt;&lt;/div&gt;
          {{ crit }}
        &lt;/li&gt;
    {% endfor %}
      &lt;/ul&gt;
    &lt;/div&gt;

([rateit][1] is a jquery plugin that I use to draw pretty stars rating controls)

Actually I get my rating values here within the 2nd `for` but I&#39;m sure there is a better way to do that. In fact, I&#39;m still not sure about my [model][2] correctness.

Finally I&#39;d like to replace `{# The rating value #}` by the rating value from rate for the current crit (in the loop). How can I do that ?

  [1]: http://www.radioactivethinking.com/rateit/example/example.htm
  [2]: https://stackoverflow.com/questions/6388259/django-rating-model-example-detailview-template

Django - Simple custom template tag example

I want to get the current mouse position of the window, and assign it to 2 variables `x` and `y` (co-ordinates relative to the window, not to the screen as a whole).

I&#39;m using Win32 and C++.

And a quick bonus question: how would you go about hiding the cursor/unhiding it?


Get current cursor position

I want to create a class in R, should I use S3 or S4 class?

I read a lot of different things about them, is there one superior to the other one?

class in R: S3 vs S4

<p>I want to create a class in R, should I use S3 or S4 class?</p>
<p>I read a lot of different things about them, is there one superior to the other one?</p>


I am learning Java recently, and I came across the notion of `package-private` classes, which is the default if we don&#39;t specify anything. But then I realized:

1. I seldom see the use of package-private class. Is there a reason for this, e.g., it has serious drawbacks, it is redundant, or simply I am not reading enough? Are there strong arguments for/against its usage?

2. If it is really not useful in most cases, why would it be the default?

3. In what situation should we use package-private in the real world? I.e., when would it become irreplaceable?

In other words, what are the major pros and cons of the default package-private modifier?

Pros and cons of package private classes in Java?

I have&#39;t coded in c++ for some time and I got stuck when I tried to compile this simple snippet:

    
    class A
    {
      public:
        void f() {}
    };
    
    int main()
    {
      {
        A a;
        a.f(); // works fine
      }

      {
        A *a = new A();
        a.f(); // this doesn&#39;t
      }
    }



Expression must have class type

I have a class and an interface, and when I try to instantiate the interface, I get an error:

&gt; Cannot create an instance of the abstract class or interface

My code is below:


    namespace MyNamespace
    {
        public interface IUser
        {
            int Property1 { get; set; }
            string Property2 { get; set; }
            string Property3 { get; set; }
            void GetUser();
        }
    
        public class User : IUser
        {
            public int Property1 { get; set; }
            public string Property2 { get; set; }
            public string Property3 { get; set; }
            
            public void GetUser()
            {
               //some logic here...... 
            }
    
        }
    }

When I try to instantiate `IUser user = new IUser();` I get an error:

&gt; Cannot create an instance of the abstract class or interface


What am I doing wrong here?








Why do I get an error instantiating an interface?

Is there a work around on how to create a constructor for static class?

I need some data to be loaded when the class is initialized but I need one and only one object.

C# static class constructor

I&#39;m writing a Python program for fun but got stuck trying to import a function from a class in another file. Here is my code:


    #jurassic park mainframe

    from random import randint
    from sys import exit
    from comm_system import Comm_system #the file i want to import from



    class Jpark_mainframe(object):
	    def mainframe_home(self):
		    print &quot;=====Welcome to the Jurassic Park Mainframe=====&quot;
		    print &quot;==========Security Administration===============&quot;
		    print &quot;===========Communications Systems===============&quot;
		    print &quot;===============System Settings==================&quot;
		    print &quot;===================Quit=========================&quot;
		
		    prompt = raw_input(&quot;What would you like to do? &quot;)
	
		    while prompt != &quot;Quit&quot;:
		
			    if prompt == &quot;Security Administration&quot;:
				    print &quot;Please enter the 5-digit passcode:&quot;
				    security_passcode = &quot;%d%d%d%d%d&quot; % (2, 0, 1, 2, randint(1, 2))
				    security_guess = raw_input(&quot;: &quot;)
				    security_guesses = 0
			
				    while security_guess != security_passcode and security_guesses &lt; 7:
					    print &quot;Incorrect. Please enter the security passcode.&quot;
					    security_guesses += 1
					    security_guess = raw_input(&quot;: &quot;)
				
					    if security_guess == security_passcode:
						    print &quot;=========Security Administration=======&quot;
						    print &quot;Area 1 Fences: Off&quot;
						    print &quot;Area 2 Fences: On&quot;
						    print &quot;Area 3 Fences: Off&quot;
						    print &quot;Velociraptor Compound: Off&quot;
						    print &quot;Lobby Security System: Off&quot;
						    print &quot;Entrance Facility System: Off&quot;
						    print &quot;To enable all systems, enter &#39;On&#39;&quot;
	
				
						    enable_security = raw_input(&quot;: &quot;)
				
						    if enable_security == &quot;On&quot;:
							    print &quot;Systems Online.&quot;
						
						
			    if prompt == &quot;System Settings&quot;:
				    print &quot;You do not have access to system settings.&quot;
				    exit(0)
									
						
			    if prompt == &quot;Communications Systems&quot;:
				    print &quot;===========Communications Systems===========&quot;
				    print &quot;error: &#39;comm_link&#39; missing in directories&quot;
				    exit(0)
				return Comm_system.run #this is where I want to return the 
                                                       #the other file

    the_game = jpark_mainframe()
    the_game.mainframe_home()

I want to return a function called `run()` from a class in another file. When I import the file, it first runs the class with `run()` in it, then proceeds to run the original code. Why does this happen? 

Here is the code from comm_system:

    #communication systems


    from sys import exit

    class Comm_system(object):
	def run(self):
	
		comm_directory = [&quot;net_link&quot;, &quot;tsfa_run&quot;, &quot;j_link&quot;]
		print &quot;When the system rebooted, some files necessary for&quot;
		print &quot;communicating with the mainland got lost in the directory.&quot;
		print &quot;The files were poorly labeled as a result of sloppy&quot;
		print &quot;programming on the staff&#39;s part. You must locate the&quot;
		print &quot;the file and contact the rescue team before the dinosaurs&quot;
		print &quot;surround the visitor&#39;s center. You were also notified the&quot;
		print &quot;generators were shorting out, and the mainframe will lose&quot;
		print &quot;power at any moment. Which directory will you search in?&quot;
		print &quot;you don&#39;t have much time! Option 1: cd /comm_sys/file&quot;
		print &quot;Option 2: cd /comm_sys/dis&quot;
		print &quot;Option 3: cd /comm_sys/comm&quot;
		
		dir_choice = raw_input(&quot;jpark_edwin$ &quot;)
		
		if dir_choice == &quot;/comm_sys/file&quot; or dir_choice == &quot;/comm_sys/dis&quot;:
			print &quot;misc.txt&quot; 
			print &quot;You couldn&#39;t locate the file!&quot;
			print &quot;The system lost power and your computer shut down on you!&quot;
			print &quot;You will not be able to reach the mainland until the system&quot;
			print &quot;comes back online, and it will be too late by then.&quot;
			return &#39;death&#39;
			
		if dir_choice == &quot;/comm_sys/comm&quot;:
			comm_directory.append(&quot;comm_link&quot;)
			print comm_directory
			print &quot;You found the right file and activated it!&quot;
			print &quot;Just in time too, because the computers shut down on you.&quot;
			print &quot;The phonelines are radios are still online.&quot;
			print &quot;You and the other survivors quickly call the mainlane&quot;
			print &quot;and help is on the way. You all run to the roof and wait&quot;
			print &quot;until the helocopter picks you up. You win!&quot;
    a_game = Comm_system()
    a_game.run()

Importing a function from a class in another file?

Let&#39;s say we have a statement that produces `integer(0)`, e.g.

     a &lt;- which(1:3 == 5)

What is the safest way of catching this?

How to catch integer(0)?

I&#39;d like to have some labels stacked on top of a `geom_bar` graph. Here&#39;s an example:

    df &lt;- data.frame(x=factor(c(TRUE,TRUE,TRUE,TRUE,TRUE,FALSE,FALSE,FALSE)))
    ggplot(df) + geom_bar(aes(x,fill=x)) + opts(axis.text.x=theme_blank(),axis.ticks=theme_blank(),axis.title.x=theme_blank(),legend.title=theme_blank(),axis.title.y=theme_blank())

![][1]

Now 
    &gt; table(df$x)
    
    FALSE  TRUE 
        3     5 

I&#39;d like to have the 3 and 5 on top of the two bars. Even better if I could have the percent values as well. E.g. `3 (37.5%)` and `5 (62.5%)`. Like so:
[![][2]][2]  
&lt;sub&gt;(source: [skitch.com](https://img.skitch.com/20110623-xhbdwqj9bicqsf8nurg9dkuqsi.png))&lt;/sub&gt;  


Is this possible? If so, how?


  [1]: https://i.stack.imgur.com/GjmF8.png
 [2]: https://i.stack.imgur.com/9GNEa.png

How to put labels over geom_bar in R with ggplot2

I&#39;ve created a utility R script, util.R, which I want to use from other scripts in my project.
What is the proper way to ensure that the function this script defines are available to function in my other scripts?

I&#39;m looking for something similar to the `require` function, that loads a package only if it has not been loaded yet. I don&#39;t want to call `source(&quot;util.R&quot;)` because that will load the script every time it is called.

I know that I will get some answers telling me to create a package, as in https://stackoverflow.com/q/2284446/761313 :)
But I&#39;m not creating something that will be used elsewhere, it is just a standalone project.

How to include (source) R script in other scripts

I am writing a function to plot data. I would like to specify a nice round number for the y-axis `max` that is greater than the max of the dataset.

Specifically, I would like a function `foo` that performs the following:

    foo(4) == 5
    foo(6.1) == 10 #maybe 7 would be better
    foo(30.1) == 40
    foo(100.1) == 110 

I have gotten as far as

    foo &lt;- function(x) ceiling(max(x)/10)*10

for rounding to the nearest 10, but this does not work for arbitrary rounding intervals.

Is there a better way to do this in R?


How to round up to the nearest 10 (or 100 or X)?

I am facing problem while conecting R with internet in my office. May be this due to LAN settings. I tried the almost all possible ways I come across in the web (see below) but still in vain. 

* Method1: Invoking R using `--internet2`

* Method2: Invoking R by setting `~/Rgui.exe http_proxy=http:/999.99.99.99:8080/ http_proxy_user=ask`

* Method3: Setting `Setinternet2=TRUE`

* Method4:

        curl &lt;- getCurlHandle()
        curlSetOpt(.opts = list(proxy = &#39;999.99.99.99:8080&#39;), curl = curl)
        Res &lt;- getURL(&#39;http://www.cricinfo.com&#39;, curl = curl)

In above all methods I can able to load packages directly from CRAN also able to download files using download.file command

But using `getURL(RCurl)`, `readHTMLTable(XML)`, `htmlTreeParse(XML)` commands I am unable to extract web data. I am getting `~&lt;HEAD&gt;\n&lt;TITLE&gt;Access Denied&lt;/TITLE&gt;\n&lt;/HEAD&gt;~` error.

How to set LAN proxy settings for XML package in R?


Proxy setting for R

I&#39;ve seen some discussions in SO and other places regarding how this should be or will be done in future versions of Roxygen2. However, I am stuck. How should I go about documenting a S4 generic, as well as its methods, using Roxygen2? A working example for a brand new generic/methods, as well as an example for extending base S4 generic would be incredibly useful. I do not want to have to make separate (mostly) redundant documentation for each S4 method of the same generic. 

Due dilligence:
I have tracked down a useful example for the &quot;extract&quot; method. However, it appears to be outdated and incomplete for my question. It uses @slot tag in the class documentation, which is not (no longer?) supported. It only shows an extension of a core S4 method &quot;[&quot;, rather than a complete Roxygen example including the documentation of the S4 generic. 

https://stackoverflow.com/questions/4396768/how-to-properly-document-s4-and-methods-using-roxygen

If I fully document a new S4 generic with title, description `@param @return @name @aliases 
@docType @rdname`, and then document the S4 method with just the corresponding `@name @aliases 
@docType @rdname`, I get the following `R CMD check` warning:

    * checking for missing documentation entries ... WARNING
    Undocumented S4 methods:
    &lt;&lt; long list of apparently undocumented methods. E.g. generic &#39;plot&#39; and siglist &#39;myClass1,ANY&#39; &gt;&gt;
    All user-level objects in a package (including S4 classes and methods)
    should have documentation entries.

It looked at first as though none of my S4 methods documented in this fashion with roxygen2 had actually worked. However, so far, I&#39;ve noticed that my extensions of the core method &quot;show&quot; do not have an associated error, even though they were documented in exactly the same way as the rest. Here is an example of the complete roxygen documentation I included above one of the show methods, that did NOT generate the missing-documentation error:

    #&#39; @name show
    #&#39; @aliases show,myClass2-method
    #&#39; @docType methods
    #&#39; @rdname show-methods

Thus, I&#39;m at a loss. As you can see, I&#39;ve included the convention for aliases for S4 methods described in [the S4 documentation section of the R package manual](http://cran.r-project.org/doc/manuals/R-exts.html#Documenting-S4-classes-and-methods), namely that methods should have an alias with the following name (without space):

    generic,signature_list-method.

Somehow, this is not completely being understood by `R CMD check`.

Finally, after building the documentation using: 

    library(&quot;devtools&quot;)
    library(&quot;roxygen2&quot;)
    document(&quot;mypkgname&quot;)
    check_doc(&quot;mypkgname&quot;)

and building the package, I get functioning documentation. Any titles from a specific method&#39;s documentation winds up in the &#39;Description&#39; field, rather awkwardly. So roxygen2 obviously did something with each method&#39;s documentation and is on the right track. However, it is not enough to avoid a large and troubling warning from 
    
    &gt; R CMD check mypkgname

I&#39;ve looked at the Rd files, but I know even less about them to quickly see what&#39;s the problem is, and I anyway want to know the roxygen2 solution so that I will not have to manipulate the Rd files directly after each documentation revision.

So this is a multipart question:

1. What is the current recommended approach for both S4 generic and S4 method documentation with roxygen2?

2. Is there a good example available somewhere that shows the complete details?

3. What might be the cause, and solution, to the &lt;R CMD check mypkgname&gt; warning that documentation for most S4 methods is missing (even though the methods with &quot;missing&quot; documentation actually have had their doc parsed by roxygen2 and the resulting Rd files are at least good enough to work in the package after `R CMD build mypkgname`) ?







How to properly document S4 methods using roxygen2

For documenting classes with roxygen(2), specifying a title and description/details appears to be the same as for functions, methods, data, etc. However, slots and inheritance are their own sort of animal. What is the best practice -- current or planned -- for documenting S4 classes in roxygen2?

Due Diligence:

I found mention of an `@slot` tag in early descriptions of roxygen. 
[A 2008 R-forge mailing list post](http://lists.r-forge.r-project.org/pipermail/roxygen-devel/2008-November/000009.html) 
seems to indicate that this is dead, 
and there is no support for `@slot` in roxygen:

Is this true of roxygen2? The previously-mentioned post suggests a user should instead make their own itemized list with LaTeX markup. E.g. a new S4 class that extends the `&quot;character&quot;` class would be coded and documented like this:

    #&#39; The title for my S4 class that extends \code{&quot;character&quot;} class.
    #&#39;
    #&#39; Some details about this class and my plans for it in the body.
    #&#39;
    #&#39; \describe{
    #&#39;    \item{myslot1}{A logical keeping track of something.}
    #&#39;
    #&#39;    \item{myslot2}{An integer specifying something else.}
    #&#39; 
    #&#39;    \item{myslot3}{A data.frame holding some data.}
    #&#39;  }
    #&#39; @name mynewclass-class
    #&#39; @rdname mynewclass-class
    #&#39; @exportClass mynewclass
    setClass(&quot;mynewclass&quot;,
    	representation(myslot1=&quot;logical&quot;,
    		myslot2=&quot;integer&quot;,
    		myslot3=&quot;data.frame&quot;),
    	contains = &quot;character&quot;
    )

However, although this works, this `\describe` , `\item` approach for documenting the slots seems inconsistent with the rest of roxygen(2), in that there are no `@`-delimited tags and slots could go undocumented with no objection from `roxygenize()`. It also says nothing about a consistent way to document inheritance of the class being defined. I imagine dependency still generally works fine (if a particular slot requires a non-base class from another package) using the `@import` tag. 

So, to summarize, what is the current best-practice for roxygen(2) slots?

There seem to be three options to consider at the moment:

&gt;  - A -- Itemized list (as example above).
&gt;  - B -- `@slot` ... but with extra tags/implementation I missed. I was    unable to get @slot to work with roxygen / roxygen2 in versions where 
&gt; it was included as a replacement for the itemized list in the example 
&gt; above. Again, the example above does work with roxygen(2). 
&gt;  - C -- Some alternative tag for specifying slots, like `@param`, that    would accomplish the same thing.

I&#39;m borrowing/extending this question from a post I made to the `roxygen2` development page on [github](https://github.com/klutometis/roxygen/issues/34).


How to properly document S4 class slots using Roxygen2?

Actual question
---------------

How do I avoid Rd file name conflicts when 

 1. a S4 generic and its method(s) are **not** necessarily all defined in the same package (package containing (some of) the custom method(s) depends on the package containing the generic) **and**
 2. using `roxygenize()` from package [roxygen2][1] to generate the actual Rd files?

I&#39;m not sure if this is a `roxygen2` problem or a common problem when the generic and its method(s) are scattered across packages (which IMHO in general definitely is a realistic use-case scenario if you follow a modular programming style). 

**What&#39;s the recommended way to handle these situations?**

Illustration
------------

**In package `pkga`**

Suppose in package `pkga` you defined a generic method `foo` and that you&#39;ve provided the respective roxygen code that `roxygenize()` picks up to generate the Rd file:

    #&#39; Test function
    #&#39; 
    #&#39; Test function.
    #&#39; 
    #&#39; @param ... Further arguments.
    #&#39; @author Janko Thyson \email{janko.thyson@@rappster.de}
    #&#39; @example inst/examples/foo.R
    #&#39; @docType methods
    #&#39; @rdname foo-methods
    #&#39; @export
    
    setGeneric(
        name=&quot;foo&quot;,
        signature=c(&quot;x&quot;),
        def=function(
             x,  
            ...
        ) {
        standardGeneric(&quot;xFoo&quot;)       
        }
    )

When `roxygenizing()` your package, a file called `foo-methods.Rd` is created in the `man` subdirectory that serves as the reference Rd file for all methods that might be created for this generic method. So far so good. If all of the methods for this generic are also part of your package, everything&#39;s good. For example, this roxygen code would make sure that documentation is added to `foo-methods.Rd` for the `ANY`-method of `foo`:

    #&#39; @param x \code{ANY}.
    #&#39; @return \code{TRUE}.
    #&#39; @rdname foo-methods
    #&#39; @aliases foo,ANY-method
    #&#39; @export
    
    setMethod(
        f=&quot;foo&quot;, 
        signature=signature(x=&quot;ANY&quot;), 
        definition=cmpfun(function(
            x,
            ...
        ) {
        return(TRUE)
        }, options=list(suppressAll=TRUE))
    )

However, if package `pkga` provides the generic for `foo` and you decide in some other package (say `pkgb`) to add a `foo`-method for `x` being of class `character`, then `R CMD check` will tell you that there is a name clash with respect to Rd file names and/or aliases (as there already exists a Rd file `foo-methods.Rd` in `pkga`):

**In package `pkgb`**

    #&#39; @param x \code{character}.
    #&#39; @return \code{character}.
    #&#39; @rdname foo-methods
    #&#39; @aliases foo,character-method
    #&#39; @export
    
    setMethod(
        f=&quot;foo&quot;, 
        signature=signature(x=&quot;character&quot;), 
        definition=cmpfun(function(
            x,
            ...
        ) {
        return(x)
        }, options=list(suppressAll=TRUE))
    )

To be more precise, this is the error that&#39;s thrown/written to file `00install.out`

    Error : Q:/pkgb/man/foo-methods.Rd: Sections \title, and \name must exist and be unique in Rd files
    ERROR: installing Rd objects failed for package &#39;pkgb&#39;


Due dilligence
--------------

I tried to change the values for `@rdname` and `@aliases` to `foo_pkgb*` (instead of `foo*`), but `\title` and `\name` still are set to `foo` when roxygenizing and thus the error remains. Any ideas **besides manually editing the Rd files** generated by `roxygenize()`?


----------


EDIT 2012-12-01
---------------

In light of starting the bounty, the actual question might get a slightly broader flavor: 

**How can we implement some sort of an &quot;inter-package&quot; check with respect to Rd files and/or how can we consolidate S4 method help files scattered across packages into one single Rd file in order to present a single source of reference to the end-user?**


  [1]: http://cran.r-project.org/web/packages/roxygen2/index.html

Content Type	Original Author	Original Content on Stackoverflow
Question	RockScience	View Question on Stackoverflow
Solution 1 - Class	Gavin Simpson	View Answer on Stackoverflow
Solution 2 - Class	mariotomo	View Answer on Stackoverflow

class in R: S3 vs S4

Class Problem Overview

Class Solutions

Solution 1 - Class

Solution 2 - Class

Get current cursor position

Django - Simple custom template tag example

Attributions