`POSIXct` objects are a measure of seconds from an origin, usually the UNIX epoch (1st Jan 1970). Just add the requisite number of seconds to the object:

    x &lt;- Sys.time()
    x
    [1] &quot;2012-08-12 13:33:13 BST&quot;
    x + 3*60*60 # add 3 hours
    [1] &quot;2012-08-12 16:33:13 BST&quot;

The `lubridate` package also implements this nicely with convenience functions `hours`, `minutes`, etc.

    x = Sys.time()
    library(lubridate)
    x + hours(3) # add 3 hours

James and Gregor&#39;s answers are great, but they handle daylight saving differently. Here&#39;s an elaboration of them.

    # Start with d1 set to 12AM on March 3rd, 2019 in U.S. Central time, two hours before daylight saving
    d1 &lt;- as.POSIXct(&quot;2019-03-10 00:00:00&quot;, tz = &quot;America/Chicago&quot;)
    print(d1)  # &quot;2019-03-10 CST&quot;
    
    # Daylight saving begins @ 2AM. See how a sequence of hours works. (Basically it skips the time between 2AM and 3AM)
    seq.POSIXt(from = d1, by = &quot;hour&quot;, length.out = 4)
    # &quot;2019-03-10 00:00:00 CST&quot; &quot;2019-03-10 01:00:00 CST&quot; &quot;2019-03-10 03:00:00 CDT&quot; &quot;2019-03-10 04:00:00 CDT&quot;
    
    # Now let&#39;s add 24 hours to d1 by adding 86400 seconds to it.
    d1 + 24*60*60  # &quot;2019-03-11 01:00:00 CDT&quot;
    
    # Next we add 24 hours to d1 via lubridate seconds/hours/days
    d1 + lubridate::seconds(24*60*60)  # &quot;2019-03-11 CDT&quot; (i.e. 2019-03-11 00:00:00 CDT)
    d1 + lubridate::hours(24)          # &quot;2019-03-11 CDT&quot; (i.e. 2019-03-11 00:00:00 CDT)
    d1 + lubridate::days(1)            # &quot;2019-03-11 CDT&quot; (i.e. 2019-03-11 00:00:00 CDT)

So, either answer is correct depending on what you want. Of course, if you&#39;re using UTC or some other timezone that doesn&#39;t observe daylight saving, these two methods should be the same.

I want to reload data in UIViewController when application become active or become foreground.

I know applicationDidBecomeActive is called in AppDelegate class.&lt;br&gt;
But I have to have a global variable for the UIViewController to reload its data in AppDelegate class like this code:

    in AppDelegate.m

    // global variable
    UIViewController *viewController1;
    UIViewController *viewController2;

    -(void)applicationDidBecomeActive:(UIApplication *)application
    {
        [viewController1 reloadData];
        [viewController2 reloadData];
    }


But it is inconvenient especially when I have a lot of UIViewControllers.

Can I use applicationDidBecomeActive in UIViewController instead of in AppDelegate class?&lt;br&gt;
Or are there better ways than having global variable for UIViewController?


I also need to use the following method from UIViewControllers:

    -(void)applicationWillResignActive:(UIApplication *)application
    -(void)applicationDidEnterBackground:(UIApplication *)application
    -(void)applicationWillEnterForeground:(UIApplication *)application


How can I use applicationDidBecomeActive in UIViewController?

We can use any of these (includes List, ArrayList, Dictionary, Hashtable, Stack, Queue) to hold value or hold reference to other objects as a collection.

But, my question is which one is used when?

List vs ArrayList vs Dictionary vs Hashtable vs Stack vs Queue?

I would like to add 1 hour to a POSIXct object, but it does not support &#39;+&#39;.

This command:

    as.POSIXct(&quot;2012/06/30&quot;,&quot;GMT&quot;) 
        + as.POSIXct(paste(event_hour, event_minute,0,&quot;:&quot;), ,&quot;%H:%M:$S&quot;)

returns this error:

    Error in `+.POSIXt`(as.POSIXct(&quot;2012/06/30&quot;, &quot;GMT&quot;), as.POSIXct(paste(event_hour,  :
        binary &#39;+&#39; is not defined for &quot;POSIXt&quot; objects

How can I add a few hours to a POSIXct object ?

Adding time to POSIXct object in R

I would like to add 1 hour to a POSIXct object, but it does not support '+'.
This command:
<pre><code class="hljs language-kotlin">as.POSIXct("2012/06/30","GMT") 
 + as.POSIXct(paste(event_hour, event_minute,0,":"), ,"%H:%M:$S")
</code></pre>
returns this error:
<pre><code class="hljs language-javascript">Error in `+.POSIXt`(as.POSIXct("2012/06/30", "GMT"), as.POSIXct(paste(event_hour, :
 binary '+' is not defined for "POSIXt" objects
</code></pre>
How can I add a few hours to a POSIXct object ?

I would like to remove specific characters from strings within a vector, similar to the *Find and Replace* feature in Excel. 

Here are the data I start with:

    group &lt;- data.frame(c(&quot;12357e&quot;, &quot;12575e&quot;, &quot;197e18&quot;, &quot;e18947&quot;)

I start with just the first column; I want to produce the second column by removing the `e`&#39;s:

    group     	group.no.e
    12357e		12357
    12575e		12575
    197e18		19718
    e18947		18947



Replace specific characters within strings

I have had trouble generating the following dummy-variables in R:  

I&#39;m analyzing yearly time series data (time period 1948-2009). I have two questions:

 1. How do I generate a dummy variable for observation #10, i.e. for year 1957 (value = 1 at 1957 and zero otherwise)?

 2. How do I generate a dummy variable which is zero before 1957 and takes the value 1 from 1957 and onwards to 2009?



Generate a dummy-variable

I&#39;d like to know if it is possible to change some default parameters of `ggplot2` graphics, like font size for instance, for a whole R session. The idea is to avoid setting them for each plot. 


How to change the default font size in ggplot2

I would like to create an automated knitr report that will produce histograms for each numeric field within my dataframe. My goal is to do this without having to specify the actual fields (this dataset contains over 70 and I would also like to reuse the script). 

I&#39;ve tried a few different approaches:

- saving the plot to an object, `p`, and then calling `p` after the loop
  - This only plots the final plot
- Creating an array of plots, `PLOTS &lt;- NULL`,  and appending the plots within the loop `PLOTS &lt;- append(PLOTS, p)`
  - Accessing these plots out of the loop did not work at all
- Even tried saving each to a `.png` file but would rather not have to deal with the overhead of saving and then re-accessing each file
 
I&#39;m afraid the intricacies of the plot devices are escaping me. 

## Question
How can I make the following chunk output each plot within the loop to the report? Currently, the best I can achieve is output of the final plot produced by saving it to an object and calling that object outside of the loop.

R markdown chunk using `knitr` in RStudio:

    ```{r plotNumeric, echo=TRUE, fig.height=3}
    suppressPackageStartupMessages(library(ggplot2))
    FIELDS &lt;- names(df)[sapply(df, class)==&quot;numeric&quot;]
    for (field in  FIELDS){
      qplot(df[,field], main=field)  
    }
    ```

From this point, I hope to customize the plots further. 

R knitr Markdown: Output Plots within For Loop

Is there an easier way to ensure that a data frame&#39;s rows are ordered according to a &quot;target&quot; vector as the one I implemented in the short example below?

    df &lt;- data.frame(name = letters[1:4], value = c(rep(TRUE, 2), rep(FALSE, 2)))
    
    df
    #   name value
    # 1    a  TRUE
    # 2    b  TRUE
    # 3    c FALSE
    # 4    d FALSE
    
    target &lt;- c(&quot;b&quot;, &quot;c&quot;, &quot;a&quot;, &quot;d&quot;)

This somehow seems to be a bit too &quot;complicated&quot; to get the job done:

    idx &lt;- sapply(target, function(x) {
        which(df$name == x)
    })
    df &lt;- df[idx,]
    rownames(df) &lt;- NULL
    
    df 
    #   name value
    # 1    b  TRUE
    # 2    c FALSE
    # 3    a  TRUE
    # 4    d FALSE

Order data frame rows according to vector with specific order

How can I get the records from a db where created date is today&#39;s date?

    SELECT [Title], [Firstname], [Surname], [Company_name], [Interest] 
    FROM [dbo].[EXTRANET_users] 
    WHERE DATE(Submission_date) = DATE(NOW())

This doesn&#39;t work im using sql server 2000 and submission date is a date time field 

SQL where datetime column equals today&#39;s date?

Do you know how I can convert this to a strtotime, or a similar type  of value to pass into the **DateTime** object?

The date I have:

    Mon, 12 Dec 2011 21:17:52 +0000

What I&#39;ve tried:

    $time = substr($item-&gt;pubDate, -14);
    $date = substr($item-&gt;pubDate, 0, strlen($time));
        
    $dtm = new DateTime(strtotime($time));
    $dtm-&gt;setTimezone(new DateTimeZone(ADMIN_TIMEZONE));
    $date = $dtm-&gt;format(&#39;D, M dS&#39;);
    $time = $dtm-&gt;format(&#39;g:i a&#39;);

The above is not correct. If I loop through a lot of different dates its all the same date.

PHP Timestamp into DateTime

I&#39;m using [strptime](http://docs.python.org/library/datetime.html#datetime.datetime.strptime) to convert a date string into a `datetime`. According to the linked page, formatting like this should work:

    &gt;&gt;&gt; # Using datetime.strptime()
    &gt;&gt;&gt; dt = datetime.strptime(&quot;21/11/06 16:30&quot;, &quot;%d/%m/%y %H:%M&quot;)

My code is:

    import datetime
    dtDate = datetime.strptime(sDate,&quot;%m/%d/%Y&quot;)

where `sDate = &quot;07/27/2012&quot;`. (I understand, from the same page, that `%Y` is *&quot;Year with century as a decimal number.&quot;*)

I have tried putting the actual value of sDate into the code:

    dtDate = datetime.strptime(&quot;07/27/2012&quot;,&quot;%m/%d/%Y&quot;)

but this does not work. The error I get is:

&gt; AttributeError: &#39;module&#39; object has no attribute &#39;strptime&#39;

What am I doing wrong?

Why is datetime.strptime not working in this simple example?

I am trying to convert time-stamps of the format &quot;2012-07-24T23:14:29-07:00&quot; 
to datetime objects in python using strptime method. The problem is with the time offset at the end(-07:00). Without the offset i can successfully do

    time_str = &quot;2012-07-24T23:14:29&quot;
    
    time_obj=datetime.datetime.strptime(time_str,&#39;%Y-%m-%dT%H:%M:%S&#39;)

But with the offset i tried

    time_str = &quot;2012-07-24T23:14:29-07:00&quot;
    
    time_obj=datetime.datetime.strptime(time_str,&#39;%Y-%m-%dT%H:%M:%S-%z&#39;).

But it gives a Value error saying &quot;z&quot; is a bad directive.

Any ideas for a work around?





Convert timestamps with offset to datetime obj using strptime

I am storing a `DATETIME` field in a table. Each value looks something like this:
&gt; 2012-09-09 06:57:12

I am using this syntax:
        
    date(&quot;Y-m-d H:i:s&quot;);


Now my question is, while fetching the data, how can get both date and time ***separately***, using a single MySQL query?

Date like &quot;**2012-09-09**&quot; and time like &quot;**06:57:12**&quot;. 
 

Content Type	Original Author	Original Content on Stackoverflow
Question	BlueTrin	View Question on Stackoverflow
Solution 1 - R	James	View Answer on Stackoverflow
Solution 2 - R	Gregor Thomas	View Answer on Stackoverflow
Solution 3 - R	Ben	View Answer on Stackoverflow

Adding time to POSIXct object in R

R Problem Overview

R Solutions

Solution 1 - R

Solution 2 - R

Solution 3 - R

List vs ArrayList vs Dictionary vs Hashtable vs Stack vs Queue?

How can I use applicationDidBecomeActive in UIViewController?

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