The package `data.table` can do this fairly quickly, using the `shift` function.
    
    require(data.table)
    df &lt;- data.table(group = rep(c(1, 2), each = 3), value = c(10,20,25,5,10,15))
    #setDT(df) #if df is already a data frame
    
    df[ , diff := value - shift(value), by = group]    
    #   group value diff
    #1:     1    10   NA
    #2:     1    20   10
    #3:     1    25    5
    #4:     2     5   NA
    #5:     2    10    5
    #6:     2    15    5
    setDF(df) #if you want to convert back to old data.frame syntax

---


Or using the `lag` function in `dplyr`

    df %&gt;%
        group_by(group) %&gt;%
        mutate(Diff = value - lag(value))
    #   group value  Diff
    #   &lt;int&gt; &lt;int&gt; &lt;int&gt;
    # 1     1    10    NA
    # 2     1    20    10
    # 3     1    25     5
    # 4     2     5    NA
    # 5     2    10     5
    # 6     2    15     5

-------

For alternatives pre-`data.table::shift` and pre-`dplyr::lag`, see edits.

You can use the base function `ave()` for this

    df &lt;- data.frame(group=rep(c(1,2),each=3),value=c(10,20,25,5,10,15))
    df$diff &lt;- ave(df$value, factor(df$group), FUN=function(x) c(NA,diff(x)))

which returns

      group value diff
    1     1    10   NA
    2     1    20   10
    3     1    25    5
    4     2     5   NA
    5     2    10    5
    6     2    15    5

try this with tapply

    df$diff&lt;-as.vector(unlist(tapply(df$value,df$group,FUN=function(x){ return (c(NA,diff(x)))})))

So I know in &lt;a href=&quot;http://www.mysql.com/&quot;&gt;MySQL&lt;/a&gt; it&#39;s possible to insert multiple rows in one query like so:
 
    INSERT INTO table (col1,col2) VALUES (1,2),(3,4),(5,6)

I would like to delete multiple rows in a similar way. I know it&#39;s possible to delete multiple rows based on the exact same conditions for each row, i.e.

    DELETE FROM table WHERE col1=&#39;4&#39; and col2=&#39;5&#39;
or
    
    DELETE FROM table WHERE col1 IN (1,2,3,4,5)

However, what if I wanted to delete multiple rows in one query, with each row having a set of conditions unique to itself? Something like this would be what I am looking for:

    DELETE FROM table WHERE (col1,col2) IN (1,2),(3,4),(5,6)

Does anyone know of a way to do this? Or is it not possible? 

MySQL delete multiple rows in one query conditions unique to each row

I&#39;ve looked a LOT for this, but I either I can&#39;t find the answer or I don&#39;t understand it.  A specific example will win the vote =)

* I have a function that returns an HTML string.
* I can&#39;t change the function.
* I want the html represented by the string to be inserted into the DOM.
* I&#39;m happy to use a controller, directive, service, or anything else that works (and is reasonably good practice).
* Disclaimer:  I don&#39;t understand $compile well.

Here&#39;s what I&#39;ve tried:

    // My magic HTML string function.
    function htmlString (str) {
        return &quot;&lt;h1&gt;&quot; + str + &quot;&lt;/h1&gt;&quot;;
    }

    function Ctrl ($scope, $compile) {
      $scope.htmlString = htmlString;
    }
    Ctrl.$inject = [&quot;$scope&quot;, &quot;$compile&quot;];

That didn&#39;t work.

I tried it as a directive too:

    // My magic HTML string function.
    function htmlString (str) {
        return &quot;&lt;h1&gt;&quot; + str + &quot;&lt;/h1&gt;&quot;;
    }

    angular.module(&quot;myApp.directives&quot;, [])
      .directive(&quot;htmlString&quot;, function () {
        return {
          restrict: &quot;E&quot;,
          scope: { content: &quot;@&quot; },
          template: &quot;{{ htmlStr(content) }}&quot;
        }
      });

      ... and in my HTML ...

      &lt;html-string content=&quot;foo&quot;&gt;&lt;/html-string&gt;

Help?

*Note*

I looked at these, among others, but couldn&#39;t make it work.

* https://stackoverflow.com/questions/9381926/insert-html-into-view-using-angularjs
* https://stackoverflow.com/questions/13141080/angularjs-http-html-parser
* https://stackoverflow.com/questions/14737795/angularjs-inserting-compile-d-html
* https://stackoverflow.com/questions/13656533/angularjs-doesnt-execute-in-html-inserted-with-document-write
* https://stackoverflow.com/questions/14462612/escape-html-text-in-an-angularjs-directive/14514299#14514299

AngularJS: Insert HTML from a string

This is a my `df` (data.frame):

    group value
    1     10
    1     20
    1     25
    2     5
    2     10
    2     15 
 

I need to calculate difference between values in consecutive rows by group.

So, I need a that result.

    group value diff
    1     10    NA # because there is a no previous value
    1     20    10 # value[2] - value[1]
    1     25    5  # value[3] value[2]
    2     5     NA # because group is changed
    2     10    5  # value[5] - value[4]
    2     15    5  # value[6] - value[5]
        

Although, I can handle this problem by using `ddply`, but it takes too much time. This is because I have a lot of groups in my `df`. (over 1,000,000 groups in my `df`)

Are there any other effective approaches to handle this problem?

Calculate difference between values in consecutive rows by group

This is a my <code>df</code> (data.frame):
<pre><code class="hljs language-csharp">group value
1 10
1 20
1 25
2 5
2 10
2 15 
</code></pre>
I need to calculate difference between values in consecutive rows by group.
So, I need a that result.
<pre><code class="hljs language-r">group value diff
1 10 NA # because there is a no previous value
1 20 10 # value[2] - value[1]
1 25 5 # value[3] value[2]
2 5 NA # because group is changed
2 10 5 # value[5] - value[4]
2 15 5 # value[6] - value[5]
 
</code></pre>
Although, I can handle this problem by using <code>ddply</code>, but it takes too much time. This is because I have a lot of groups in my <code>df</code>. (over 1,000,000 groups in my <code>df</code>)
Are there any other effective approaches to handle this problem?

I&#39;m having issues appending data to a list which is already in a list format. I have a program which will export results objects during a simulation loop. The data itself is stored as a list of matrices. My idea is to store those lists in a list, and then save this list of lists as an R object for later analysis, however I&#39;m having some issues achieving this correctly. I&#39;ll show what I&#39;ve done with small abstract example just using values instead of the matrix data from my simulation:

Say I&#39;ve run the simulation loop for 3 times.
During the iterations, the results lists need to be collected into the one list of lists that I will save as an R object:

List to contain the other lists and be saved: 
`outlist1 &lt;- list()`

First iteration:
`resultsa &lt;- list(1,2,3,4,5)`

`outlist &lt;- append(outlist1,resultsa)`

Second Iteration:
`resultsb &lt;- list(6,7,8,9,10)`

`outlist &lt;- append(outlist1,b)`

Third Iteration:
`resultsc &lt;- list(11,12,13,14,15)`

`outlist &lt;- list(outlist2,c)`


However, this solution does not work with growing a list containing lists this way, the desired result is:

    &gt;outlist
    [[1]]
    [[1]][[1]]
    [1] 1
    
    [[1]][[2]]
    [1] 2
    
    [[1]][[3]]
    [1] 3
    
    [[1]][[4]]
    [1] 4
    
    [[1]][[5]]
    [1] 5
    
    
    [[2]]
    [[2]][[1]]
    [1] 6
    
    [[2]][[2]]
    [1] 7
    
    [[2]][[3]]
    [1] 8
    
    [[2]][[4]]
    [1] 9
    
    [[2]][[5]]
    [1] 10
    
    
    [[3]]
    [[3]][[1]]
    [1] 11
    
    [[3]][[2]]
    [1] 12
    
    [[3]][[3]]
    [1] 13
    
    [[3]][[4]]
    [1] 14
    
    [[3]][[5]]
    [1] 15

However, instead what I get is:

    &gt; outlist3
    [[1]]
    [[1]][[1]]
    [[1]][[1]][[1]]
    [1] 1
    
    [[1]][[1]][[2]]
    [1] 2
    
    [[1]][[1]][[3]]
    [1] 3
    
    [[1]][[1]][[4]]
    [1] 4
    
    [[1]][[1]][[5]]
    [1] 5
    
    
    [[1]][[2]]
    [[1]][[2]][[1]]
    [1] 6
    
    [[1]][[2]][[2]]
    [1] 7
    
    [[1]][[2]][[3]]
    [1] 8
    
    [[1]][[2]][[4]]
    [1] 9
    
    [[1]][[2]][[5]]
    [1] 10

How do I grow a list, such that the resulting list formatted is like the desired result? If I do further analysis on these list I need to be able to easily access the elements.



Appending a list to a list of lists in R

I am fairly new to R and I have the following queries :

I am trying to generate a plot in R which has multiple lines (data series). Each of these lines is a category and I want it to have a unique color. 

Currently my code is setup in this way :

First, I am creating an empty plot :

    plot(1,type=&#39;n&#39;,xlim=c(1,10),ylim=c(0,max_y),xlab=&#39;ID&#39;, ylab=&#39;Frequency&#39;)

Then for each of my category, I am plotting lines in this empty plot using a &quot;for&quot; loop like so :

    for (category in categories){
    lines(data.frame.for.this.category, type=&#39;o&#39;, col=sample(rainbow(10)), lwd=2)
    }

There are 8 categories here, and so there are 8 lines produced in the plot. As you can see, I am trying to sample a color from the rainbows() function to generate a color for each line. 

However, when the plot is generated, I find that there are multiple lines which have the same color. For instance, 3 of those 8 lines have green color. 

How do I make each of these 8 lines have a unique color ? 

Also, how do I reflect this uniqueness in the legend of the plot ? I was trying to lookup the `legend()` function, however it was not clear which parameter I should use to reflect this unique color for each category ? 

Any help or suggestions would be much appreciated.

Plot multiple lines (data series) each with unique color in R

I get this error:

&gt; Error: unexpected &#39;else&#39; in &quot; else&quot;

From this `if, else` statement:

    if (dsnt&lt;0.05) {
         wilcox.test(distance[result==&#39;nt&#39;],distance[result==&#39;t&#39;],alternative=c(&quot;two.sided&quot;),paired=TRUE) }
	else {
          if (dst&lt;0.05) {
    wilcox.test(distance[result==&#39;nt&#39;],distance[result==&#39;t&#39;],alternative=c(&quot;two.sided&quot;),paired=TRUE) }
       else {
             t.test(distance[result==&#39;nt&#39;],distance[result==&#39;t&#39;],alternative=c(&quot;two.sided&quot;),paired=TRUE)       } }

What is wrong with this?

Unexpected &#39;else&#39; in &quot;else&quot; error

When reading the following [helpfile](http://stat.ethz.ch/R-manual/R-patched/library/base/html/colnames.html) it should be possible to add a prefix to the column names :

    colnames(x, do.NULL = TRUE, prefix = &quot;col&quot;)

The following doesn&#39;t work for me. What am I doing wrong here?

    m2 &lt;- cbind(1,1:4)
    colnames(m2, do.NULL = FALSE)
    colnames(m2) &lt;- c(&quot;x&quot;,&quot;Y&quot;)
    colnames(m2) &lt;- colnames(m2, prefix = &quot;Sub_&quot;)
    colnames(m2)

Add a prefix to column names

While trying to modify theme settings this simple code gives the following error:

    library(ggplot2)
    theme_nogrid &lt;- theme_set(theme_update(  
                      plot.margin=unit(c(.25, .25, .25, .25), &quot;in&quot;),))
  
    Error in do.call(theme, list(...)) : could not find function &quot;unit&quot;

`R` gives me this error for any element that uses &#39;unit&#39;. Any other settings that do not call &#39;unit&#39; work fine. I am running R v.2.15.2 (64-bit Windows). 

I extensively searched online about this problem and found nothing. 
I appreciate any suggestions to the problem.


Content Type	Original Author	Original Content on Stackoverflow
Question	kmangyo	View Question on Stackoverflow
Solution 1 - R	Blue Magister	View Answer on Stackoverflow
Solution 2 - R	MrFlick	View Answer on Stackoverflow
Solution 3 - R	Aditya Sihag	View Answer on Stackoverflow

Calculate difference between values in consecutive rows by group

R Problem Overview

R Solutions

Solution 1 - R

Solution 2 - R

Solution 3 - R

AngularJS: Insert HTML from a string

MySQL delete multiple rows in one query conditions unique to each row

Attributions