Benjamin Müller wrote:
HI,
As I'm trying to compute Taylor series, I'm having problems in adding and
multiplying unevaluated expressions. I searched for a solution but found none.
my Taylor function works fine for evaluating functions as you can see here:
rTaylorVal=function(exp,x0,dx,n) {
ls=list(x=x0)
newexp=eval(exp,ls)
exp0=exp
for (i in 1:n){
exp0=D(exp0,"x")
newexp=newexp+eval(exp0,ls)/factorial(i)*dx^i
}
return(newexp)
}
Where exp is an expression like exp=expression(x^2*sin(x)), x0 is the
startvalue, dx the difference between startvalue and searched value and n is
the length of the series.
So I tried to remove dx as a value, to get a Taylor series expression, but it
doesn't work as simple multiplication (*) and accumulation (+) is not good for
expressions.
That's my point so far, now my question:
Is it actually possible to add and/or multiply expressions, and how?
Well, although R as a numerical language is not designed for these kind
of things, you can do it, e.g. using a function
opExpr <- function(expr1, expr2, op = "+"){
expr <- expression(a + b)
expr[[1]][[1]] <- as.name(op)
expr[[1]][[2]] <- expr1[[1]]
expr[[1]][[3]] <- expr2[[1]]
expr
}
that can construct such expressions as in:
opExpr(expression(x * sin(x)), expression(x^2 * sin(x)))
opExpr(expression(x * sin(x)), expression(x^2 * sin(x)), "*")
Best wishes,
Uwe Ligges
Thank you so far.
Benjamin Müller
Geographer (B.Sc.)
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