[R] Calculate the area under a curve

[David Winsemius]

On Aug 24, 2015, at 7:23 AM, Jeff Newmiller wrote:

> Read ?predict, paying particular attention to the newdata argument.

I agree that reading help pages is needed but need both the ?predict page and the ?integrate page. I think proximate cause of the error is that `integrate` is not getting an integrand that is a function of "x" so that it can receive a sequence of x-values and return a sequence of y-values. The interpreter is complaining because it's not getting a proper function.

I suppose one could do that with predict, but you need to build that function to accept a single value as the newdata 'x'-argument. One would, of course, need to have the model name in the integrand function matching the name of the lm-object, which at the moment is also a further error waiting to be uncovered.

Decided to avoid creating extraneous objects and assume that the data is in the form of a dataframe named 'inp':

 fitP <- lm( SOC~poly(Depth,3), data=inp )   # It's better to use correct column names with a data-argument
 integrand <-function(x) predict(fitP, newdata= list(Depth=x))  # single value of x passed to correct name of column, see ?predict
 integrate(integrand, lower = 25, upper = 80)
# 7.43651 with absolute error < 8.3e-14


> ---------------------------------------------------------------------------
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> Sent from my phone. Please excuse my brevity.
> 
> On August 24, 2015 2:10:11 AM PDT, CarstenH <cahoff at gmx.de> wrote:
>> Hi all
>> 
>> I need to calculate the area under a curve (integral) for the following
>> data
>> pairs:
>> 
>> Depth SOC	
>> 22.5	0.143
>> 28.5 	0.165	
>> 34.5	0.131	
>> 37.5	0.134	
>> 40.5	0.138	
>> 43.5 	0.107	
>> 46.5	0.132
>> 49.5 	0.175 
>> 52.5	0.087	
>> 55.5 	0.117	
>> 58.5	0.126	
>> 61.5 	0.13	
>> 64.5	0.122	
>> 67.5 	0.161	
>> 71.5	0.144	
>> 76.5 	0.146	
>> 82.5	0.156	
>> 94.5	0.132	
>> 
>> (Table name is P)
>> 
>> After reading the data set I assiged the collumns by:
>> 
>> /x <- (P$Depth)
>> y <- (P$SOC)
>> /
>> 
>> and decided to make a ploynominal function (3rd order):
>> 
>> /fitP <- lm( y~poly(x,3,raw=TRUE) )/
>> 
>> At the next step I failed. I can plot point and function but am not
>> able to
>> integrate the curve between e.g. depths 20 and 80.
>> 
>> If I try:
>> /
>> integrand <-function(fitP1)
>> predict(y)
>> integrate(integrand, lower = 25, upper = 80)/
>> 
>> the "Conosle" opend with the message: "Source unavailable or out of
>> sync"
>> and
>> /
>> function(fitP1)
>> predict(y)
>> /
>> )
>> 
>> 
>> Would be great if somebody could help!
>> 
>> Thanks
>> 
>> Carsten
>> 
>> 
>> 
>> --
>> View this message in context:
>> http://r.789695.n4.nabble.com/Calculate-the-area-under-a-curve-tp4711418.html
>> Sent from the R help mailing list archive at Nabble.com.
>> 
>> ______________________________________________
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> 
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> https://stat.ethz.ch/mailman/listinfo/r-help
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> and provide commented, minimal, self-contained, reproducible code.

David Winsemius
Alameda, CA, USA