[R] Substituting inside expression

[Ivan Calandra]

Hi,

If I follow you correctly, you could write a function:

foo <- function(a,b,gamma){
  c <- sqrt(a^2 + b^2 -2*a*b*cos(gamma))
  s <- (a+b+c)/2
  A <- sqrt(s*(s-a)*(s-b)*(s-c))
  return(A)
}

I hope I didn't make mistakes, but it can still help you, I guess.
Ivan


Le 2/25/2011 10:11, zbynek.janoska at gmail.com a écrit :
> I am having following problem:
> I´m constructing model for calculation of area of triangle.
> I know sides a, b, and gamma angle.
> I wish to calculate the area using heron´s formula:
> S<- sqrt(s*(s-a)*(s-b)*(s-c))
> where
> s<- (a+b+c)/2
> and c is calculated using law of cosines:
> c<- sqrt(a^2 + b^2 -2*a*b*cos(gamma))
>
> since i am calculating a regression model, i need derivation of this
> expression for area S.
> something like (D(expression.S,c("a","b")))
>
> To write it all into a single expression, it is too complicated, so i would
> like to use some kind of substitution. however, if i try:
>
> s.e<- substitute(expression((a+b+c)/2), list(c =
> expression(sqrt(a^2+b^2-2*a*b*cos(gamma))))),
> I get
>> s.e
> expression((a + b + expression(sqrt(a^2 + b^2 - 2 * a * b * cos(gamma))))/2)
>
> which is not what I wanted
>
> Can someone point me to the right direction?
>

-- 
Ivan CALANDRA
PhD Student
University of Hamburg
Biozentrum Grindel und Zoologisches Museum
Abt. Säugetiere
Martin-Luther-King-Platz 3
D-20146 Hamburg, GERMANY
+49(0)40 42838 6231
ivan.calandra at uni-hamburg.de

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