[R] First Derivative of Data Matrix

[David Winsemius]

delta(index) is identically 1, so taking first differences is all that  
is needed. If the dtatframe's name is df then:

df$dacflong_dx <- c(NA, diff(acflong))     # the slash would not be a  
legal character in a variable name unless you jumped through some  
hoops that appear entirely without value

If you want to get rid of the first line of df then

df[-1]

-- 
David Winsemius


On Apr 12, 2009, at 11:55 AM, thaumaturgy wrote:

>
> I am really new to R and ran across a need to take a data matrix and
> calculate an approximation of the first derivative of the data.  I  
> am more
> than happy to do an "Excel" kind of calculation (deltaY/deltaX) for  
> each
> pair of rows down the matrix, but I don't know how to get R to do  
> that kind
> of calculation.  I'd like to store it as a 3rd column in the matrix  
> as well.
>
> My data looks like this:
>     acflong
> 1  1.0000000
> 2  0.9875858
> 3  0.9871751
> 4  0.9867585
> 5  0.9863358
> 6  0.9859070
> 7  0.9854721
> 8  0.9850316
> 9  0.9817161
> 10 0.9812650
>
> and I'd like to generate a table like this:
>
>     acflong          dacflong/dx
> 1  1.0000000
> 2  0.9875858        -0.01241          #delta(acflong)/delta(index)
> 3  0.9871751        -0.00041
> 4  0.9867585        -0.00042
> 5  0.9863358        -0.00042
> 6  0.9859070        -0.00043
> 7  0.9854721        -0.00043
> 8  0.9850316        -0.00044
> 9  0.9817161        -0.00033
> 10 0.9812650       -0.00045
>
> Is there a way to do this in R and how do I eliminate the first line  
> of the
> data?
>
> Thanks,
> -Chris
> -- 
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>
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT