Calculate difference between values in consecutive rows by group

Alicia

Hi There,

 

I have a data table with 2 columns like the below:

group value
<SPAN class="hljs-number">1</SPAN>     <SPAN class="hljs-number">10</SPAN>
<SPAN class="hljs-number">1</SPAN>     <SPAN class="hljs-number">20</SPAN>
<SPAN class="hljs-number">1</SPAN>     <SPAN class="hljs-number">25</SPAN>
<SPAN class="hljs-number">2</SPAN>     <SPAN class="hljs-number">5</SPAN>
<SPAN class="hljs-number">2</SPAN>     <SPAN class="hljs-number">10</SPAN>
<SPAN class="hljs-number">2</SPAN>     <SPAN class="hljs-number">15</SPAN> 

I want to create a new column, "diff", that calculates the differences between consecutive rows, grouped by "group", so I would have something like this:

 

group value diff
<SPAN class="hljs-number">1</SPAN>     <SPAN class="hljs-number">10</SPAN>    . <SPAN class="hljs-comment"># because there is a no previous value</SPAN>
<SPAN class="hljs-number">1</SPAN>     <SPAN class="hljs-number">20</SPAN>    <SPAN class="hljs-number">10</SPAN> <SPAN class="hljs-comment"># value[2] - value[1]</SPAN>
<SPAN class="hljs-number">1</SPAN>     <SPAN class="hljs-number">25</SPAN>    <SPAN class="hljs-number">5</SPAN>  <SPAN class="hljs-comment"># value[3] value[2]</SPAN>
<SPAN class="hljs-number">2</SPAN>     <SPAN class="hljs-number">5</SPAN>     . <SPAN class="hljs-comment"># because group is changed</SPAN>
<SPAN class="hljs-number">2</SPAN>     <SPAN class="hljs-number">10</SPAN>    <SPAN class="hljs-number">5</SPAN>  <SPAN class="hljs-comment"># value[5] - value[4]</SPAN>
<SPAN class="hljs-number">2</SPAN>     <SPAN class="hljs-number">15</SPAN>    <SPAN class="hljs-number">5</SPAN>  <SPAN class="hljs-comment"># value[6] - value[5]</SPAN>

</CODE><CODE class="hljs language-r">

Please can you help me with the jsl for the column formula?

 

Many thanks,

 

Alicia

 

ih

If the table is always sorted by group like your example you could do this with an if statement:

 

If( Lag( :group ) == :group,
	:value - Lag( :value ),
	.
)