-------- Original-Nachricht --------
Betreff: Re: [R] Adding and Multiplying two Unevaluated Expressions
Datum: Tue, 01 Dec 2009 23:49:39 +0100
Von: Benjamin Müller <ben_mueller...@web.de>
An: Rolf Turner <r.tur...@auckland.ac.nz>
Referenzen: <20091201144125.316...@gmx.net>
<8e40e49f-e8fc-4fbd-8cc5-93789ffb0...@auckland.ac.nz>
This works fine for your example, but doesn't work as simple if there's
more than these expressions.
In my example this would be in line 11:
newexp=as.expression(substitute(a+con1/con2*b^con3,list(a=newexp[[1]],b=expression(dx)[[1]],con1=eval(exp0,ls),con2=factorial(i),con3=1*i)))
this works just fine and you can still evaluate this. thank you very much!
If there's an easier way, let me know.
Greets,
Ben Müller
Rolf Turner schrieb:
On 2/12/2009, at 3:41 AM, 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?
This may well be a case of the blind leading the partially sighted,
but for what it's worth my answer is ``Yes, but it's a kludge.''
One needs to use substitute, it seems to me.
E.g.:
e1 <- expression((x+y)^2)
e2 <- expression(1/(x^2 + y^2))
e3 <- as.expression(substitute(a+b,list(a=e1[[1]],b=e2[[1]])))
e3
D(e3,"x")
D(e3,"y")
Older and wiser heads may provide you with better counsel.
cheers,
Rolf Turner
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