On Nov 8, 2010, at 4:30 AM, Nick Sabbe wrote:
Whenever you use a recursion (that cannot be expressed otherwise), you
always need a (for) loop.
Not necessarily true ... assuming "a" is of length "n":
a[2:n] <- a[1:(n-1))]*b + cc[1:(n-1)]
# might work if b and n were numeric vectors of length 1 and cc had
length >= n. (Never use "c" as a vector name.)
# it won't work if there are no values for the nth element at the
beginning and you are building up a element by element.
And you always need to use operations that appropriate to the object
type. So if "a" really is a list, this will always fail since
arithmetic does not work on list elements. If on the other hand, the
OP were incorrect in calling this a list and "a" were a numeric
vector, there might be a chance of success if the rules of indexing
were adhered to. The devil is in the details and the OP has not
supplied enough code to tell what might happen.
--
David.
Apply and the like do not allow to use the intermediary results
(i.e. a[i-1]
to calculate a[i]).
So: no, it cannot be avoided in your case, I guess.
Nick Sabbe
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-----Original Message-----
From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org
] On
Behalf Of PLucas
Sent: maandag 8 november 2010 10:26
To: r-help@r-project.org
Subject: [R] How to eliminate this for loop ?
Hi, I would like to create a list recursively and eliminate my for
loop :
a<-c()
a[1] <- 1; # initial value
for(i in 2:N) {
a[i]<-a[i-1]*b - c[i-1] # b is a value, c is another vector
}
Is it possible ?
Thanks
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David Winsemius, MD
West Hartford, CT
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