You want the (standard) `POSIXt` type from base R that can be had in &#39;compact form&#39; as a `POSIXct` (which is essentially a double representing fractional seconds since the epoch) or as long form in `POSIXlt` (which contains sub-elements).  The cool thing is that arithmetic etc are defined on this -- see `help(DateTimeClasses)`

Quick example:

    R&gt; now &lt;- Sys.time()
    R&gt; now
    [1] &quot;2009-12-25 18:39:11 CST&quot;
    R&gt; as.numeric(now)
    [1] 1.262e+09
    R&gt; now + 10  # adds 10 seconds
    [1] &quot;2009-12-25 18:39:21 CST&quot;
    R&gt; as.POSIXlt(now)
    [1] &quot;2009-12-25 18:39:11 CST&quot;
    R&gt; str(as.POSIXlt(now))
     POSIXlt[1:9], format: &quot;2009-12-25 18:39:11&quot;
    R&gt; unclass(as.POSIXlt(now))
    $sec
    [1] 11.79

    $min
    [1] 39
    
    $hour
    [1] 18
    
    $mday
    [1] 25
    
    $mon
    [1] 11
    
    $year
    [1] 109
    
    $wday
    [1] 5
    
    $yday
    [1] 358
    
    $isdst
    [1] 0
    
    attr(,&quot;tzone&quot;)
    [1] &quot;America/Chicago&quot; &quot;CST&quot;             &quot;CDT&quot;            
    R&gt; 

As for reading them in, see `help(strptime)` 
    
As for difference, easy too:

    R&gt; Jan1 &lt;- strptime(&quot;2009-01-01 00:00:00&quot;, &quot;%Y-%m-%d %H:%M:%S&quot;)
    R&gt; difftime(now, Jan1, unit=&quot;week&quot;)
    Time difference of 51.25 weeks
    R&gt; 

Lastly, the [zoo package](http://cran.r-project.org/web/packages/zoo) is an extremely versatile and well-documented container for matrix with associated date/time indices.

I am writing a library, so, I want its functions to be named as clearly and cleverly as possible. Currently, I use the following principles:

 1. Self-explanatory names: a
    function getName() will tell the
    developer what it returns as well as
    setAddress(), isMale(), etc.
 2. Short: a function name must be as
    short as possible so that it&#39;s
    simple to type as well as easy to
    remember. A function
    getNumberOfPagesInTheBook() is not
    good, something like
    getBookPageCount() is better.
 3. Use of prefixes: I always use
    prefixes in the functions such as
    getName(), setName(), hasHair(),
    isBlond(), etc.

I&#39;m interested in knowing if there&#39;s something I&#39;m missing. Also, can you think of some other prefixes other than is, has, get and set?

Function naming conventions

In every book and example always they show only binary classification (two classes) and new vector can belong to any one class.

Here the problem is I have 4 classes(c1, c2, c3, c4). I&#39;ve training data for 4 classes.

For new vector the output should be like

&gt; **C1 80%** (the winner)
&gt; 
&gt; c2 10%
&gt; 
&gt; c3 6%
&gt; 
&gt; c4 4%

How to do this? I&#39;m planning to use libsvm (because it most popular). I don&#39;t know much about it. If any of you guys used it previously please tell me specific commands I&#39;m supposed to use.

How to do multi class classification using Support Vector Machines (SVM)

I have multiple lists of measurements. In each list have the timestramp formated as a string (&quot;2009-12-24 21:00:07.0&quot;) and I know that each measurement in the list is separated by 5 seconds.
I want to combine all data into a huge data.frame in R. Afterwards I want to be able to easily access the time difference of two measurements so I probably should convert data into something different than characters.

Which format should I use to store the times? Is there some time format in some package that I should use?

Dealing with timestamps in R

<p>I have multiple lists of measurements. In each list have the timestramp formated as a string ("2009-12-24 21:00:07.0") and I know that each measurement in the list is separated by 5 seconds.
I want to combine all data into a huge data.frame in R. Afterwards I want to be able to easily access the time difference of two measurements so I probably should convert data into something different than characters.</p>
<p>Which format should I use to store the times? Is there some time format in some package that I should use?</p>


Is there a standard/common method/formula to calculate the number of months between two dates in R?

I am looking for something that is similar to [MathWorks months function](http://www.mathworks.com/access/helpdesk/help/toolbox/finance/months.html)




Number of months between two dates

Brief background: Many (most?) contemporary programming languages in widespread use have at least a handful of ADTs [abstract data types] in common, in particular,

 - **string** (a sequence comprised of characters)

 - **list** (an ordered collection of values), and

 - **map-based type** (an unordered array that maps keys to values)

In the R programming language, the first two are implemented as `character` and `vector`, respectively.

When I began learning R, two things were obvious almost from the start: `list` is the most important data type in R (because it is the parent class for the R `data.frame`), and second, I just couldn&#39;t understand how they worked, at least not well enough to use them correctly in my code.

For one thing, it seemed to me that R&#39;s `list` data type was a straightforward implementation of the map ADT (`dictionary` in Python, `NSMutableDictionary` in Objective C, `hash` in Perl and Ruby, `object literal` in Javascript, and so forth).

For instance, you create them just like you would a Python dictionary, by passing key-value pairs to a constructor (which in Python is `dict` not `list`):

    x = list(&quot;ev1&quot;=10, &quot;ev2&quot;=15, &quot;rv&quot;=&quot;Group 1&quot;)

And you access the items of an R List just like you would those of a Python dictionary, e.g., `x[&#39;ev1&#39;]`. Likewise, you can retrieve just the _&#39;keys&#39;_ or just the _&#39;values&#39;_ by: 

    names(x)    # fetch just the &#39;keys&#39; of an R list
    # [1] &quot;ev1&quot; &quot;ev2&quot; &quot;rv&quot;

    unlist(x)   # fetch just the &#39;values&#39; of an R list
    #   ev1       ev2        rv 
    #  &quot;10&quot;      &quot;15&quot; &quot;Group 1&quot; 

    x = list(&quot;a&quot;=6, &quot;b&quot;=9, &quot;c&quot;=3)  

    sum(unlist(x))
    # [1] 18

but R `list`s are also ***unlike*** other map-type ADTs (from among the languages I&#39;ve learned anyway). My guess is that this is a consequence of the initial spec for S, i.e., an intention to design a data/statistics DSL [domain-specific language] from the ground-up. 

*three* significant differences between R `list`s and mapping types in other languages in widespread use (e.g,. Python, Perl, JavaScript):

*first*, `list`s in R are an *ordered* collection, just like vectors, even though the values are keyed (ie, the keys can be any hashable value not just sequential integers). Nearly always, the mapping data type in other languages is *unordered*.

*second*, `list`s can be returned from functions even though you never passed in a `list` when you called the function, and *even though* the function that returned the `list` doesn&#39;t contain an (explicit) `list` constructor (Of course, you can deal with this in practice by wrapping the returned result in a call to `unlist`):

    x = strsplit(LETTERS[1:10], &quot;&quot;)     # passing in an object of type &#39;character&#39;

    class(x)                            # returns &#39;list&#39;, not a vector of length 2
    # [1] list

A *third* peculiar feature of R&#39;s `list`s: it doesn&#39;t seem that they can be members of another ADT, and if you try to do that then the primary container is coerced to a `list`. E.g.,

    x = c(0.5, 0.8, 0.23, list(0.5, 0.2, 0.9), recursive=TRUE)

    class(x)
    # [1] list

my intention here is not to criticize the language or how it is documented; likewise, I&#39;m not suggesting there is anything wrong with the `list` data structure or how it behaves. All I&#39;m after is to correct is my understanding of how they work so I can correctly use them in my code. 

Here are the sorts of things I&#39;d like to better understand:

 - What are the rules which determine when a function call will return a `list` (e.g., `strsplit` expression recited above)?

 - If I don&#39;t explicitly assign names to a `list` (e.g., `list(10,20,30,40)`) are the default names just sequential integers beginning with 1?  (I assume, but I am far from certain that the answer is yes, otherwise we wouldn&#39;t be able to coerce this type of `list` to a vector w/ a call to `unlist`.)

 - Why do these two different operators, `[]`, and `[[]]`, return the _same_ result?

    `x = list(1, 2, 3, 4)`
    
    both expressions return &quot;1&quot;:

    `x[1]`
    
    `x[[1]]`
  

 -  why do these two expressions **not** return the same result?

    `x = list(1, 2, 3, 4)`

    `x2 = list(1:4)`
    

Please don&#39;t point me to the R Documentation ([`?list`][1], [`R-intro`][2])--I have read it carefully and it does not help me answer the type of questions I recited just above.

(lastly, I recently learned of and began using an R Package (available on CRAN) called [`hash`][3] which implements *conventional* map-type behavior via an S4 class; I can certainly recommend this Package.)


  [1]: http://www.inside-r.org/r-doc/base/list
  [2]: http://cran.r-project.org/doc/manuals/r-devel/R-intro.html#Lists
  [3]: http://mran.revolutionanalytics.com/packages/info/?hash

How to Correctly Use Lists in R?

I am fairly new to R, but the more use it, the more I see how powerful it really is over SAS or SPSS.  Just one of the major benefits, as I see them, is the ability to get and analyze data from the web.  I imagine this is possible (and maybe even straightforward), but I am looking to parse JSON data that is publicly available on the web.  I am not a programmer by any stretch, so any help and instruction you can provide will be greatly appreciated.  Even if you point me to a basic working example, I probably can work through it.

Parse JSON with R

Histograms and scatterplots are great methods of visualizing data and the relationship between variables, but recently I have been wondering about what visualization techniques I am missing. What do you think is the most underused type of plot?

Answers should:

 1. Not be very commonly used in
    practice.
 2. Be understandable without a great deal
    of background discussion.
 3. Be applicable in many common situations.
 4. Include reproducible code to create
    an example (preferably in R). A linked image would be
    nice.



Most underused data visualization

If I have a vector of type character, how can I concatenate the values into string? Here&#39;s how I would do it with *paste()*:

    sdata = c(&#39;a&#39;, &#39;b&#39;, &#39;c&#39;)
    paste(sdata[1], sdata[2], sdata[3], sep =&#39;&#39;)

yielding `&quot;abc&quot;`.

But of course, that only works if I know the length of *sdata* ahead of time.



Concatenate a vector of strings/character

Is there a PHP function that returns the date and time in the same format as the MySQL function `NOW()`? 

I know how to do it using `date()`, but I am asking if there is a function only for this. 

For example, to return:

    2009-12-01 00:00:00


NOW() function in PHP

So I have two dates `YYYY-MM-DD` and `ZZZZ-NN-EE`

How can I find out how many seconds there are between them?

How do I calculate how many seconds between two dates?

Is there a way one can represent a time only value in .NET without the date? For example, indicating the opening time of a shop?

`TimeSpan` indicates a range, whereas I only want to store a time value. Using `DateTime` to indicate this would result in new `DateTime(1,1,1,8,30,0)` which is not really desirable. 

How do I represent a time only value in .NET?

Is there a way to determine how much time a method needs to execute (in milliseconds)?

How to log a method&#39;s execution time exactly in milliseconds?

I need to parse following String into a DateTime Object:&lt;br&gt;
`30/Nov/2009:16:29:30 +0100`

Is there an easy way to do this?

*PS:* I want to convert the string above as is. The colon after the year is not a typo. I also want to solve the problem with Ruby and not RoR.

convert String to DateTime

Is there a `TIMESTAMPDIFF()` equivalent for PostgreSQL?  

I know I can subtract two timestamps to get a postgresql `INTERVAL`.  I just want the difference between the two timestamps in in hours represented by an INT.  

I can do this in MySQL like this:

    TIMESTAMPDIFF(HOUR, links.created, NOW())

I just need the difference between two timestamps in hours represented as an integer.

**Solution works for me:**

    SELECT &quot;links_link&quot;.&quot;created&quot;,
    &quot;links_link&quot;.&quot;title&quot;,
    (EXTRACT(EPOCH FROM current_timestamp - &quot;links_link&quot;.&quot;created&quot;)/3600)::Integer AS &quot;age&quot; 
    FROM &quot;links_link&quot;


Timestamp Difference In Hours for PostgreSQL

I am coding an application where i need to assign random date between two fixed timestamps

how i can achieve this using php i&#39;ve searched first but only found the answer for Java not php

for example :

    $string = randomdate(1262055681,1262055681);



How to generate random date between two dates using php?

I want to be sure that if I commit something in Subversion from one machine, and I checkout on another, I will get the same value for datetime and timestamp.

Currently, I observed that the datetime of file&#39;s modification is from the moment when I did the synchronisation instead of the original value.

For me, it doesn&#39;t make any sense to know when the files were synchronised - I just want to see the last commit time.

This issue is not timezone related.

How can I keep the original file [commit] timestamp on Subversion?

Can anyone suggest an easy method to convert date and time to different timezones in php?

Content Type	Original Author	Original Content on Stackoverflow
Question	Christian	View Question on Stackoverflow
Solution 1 - R	Dirk Eddelbuettel	View Answer on Stackoverflow

Dealing with timestamps in R

R Problem Overview

R Solutions

Solution 1 - R

How to do multi class classification using Support Vector Machines (SVM)

Function naming conventions

Attributions