On Mon, 1 Feb 2010, Hao Cen wrote:
Hi,
I wonder how to write a function that remembers its state across its
calls. For example, I would like to compute the average of the pass three
values the function has seen
f(1) # NA
f(2) # NA
f(3) # 2
f(4) # 3
This would require f to keep track of the values it has seen. In other
languages like c++ or java it is easy to do by having a member variable. I
am not sure how to do similar things in R because the variables declared
in functions are gone after the function exits.
Not quite true. See R-intro Section 10.7. Note the open.account example.
g <- function(x=rep(NA,3)){ function(y) {x <<- c(x[2:3],y[1]);mean(x)}}
f <- g()
f(1)
[1] NA
f(2)
[1] NA
f(3)
[1] 2
f(4)
[1] 3
f(5)
[1] 4
HTH,
Chuck
It is possible do a quick-dirty function by declaring a variable outside
of f to keep track of what the function has seen. But then the logic of
keeping tracking that variable is not encapsulated in f. It would be messy
if I have lots of such functions.
Thanks for any advice in advance.
Jeff
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Charles C. Berry (858) 534-2098
Dept of Family/Preventive Medicine
E mailto:cbe...@tajo.ucsd.edu UC San Diego
http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901
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