[R] Questions about lexical scope

[Laurent Gautier]

As I was pointed out by Mr.R. himself(*), the call to
'cat' is done *before* the call to the function 'rule'
(hence the variable "count" should be zero... and
is zero...)

I should probably consider using bigger fonts (or bigger glasses)...


L.


(*): thanks Robert



On Sat, Aug 10, 2002 at 01:32:44PM +0200, Laurent Gautier wrote:
> Dear Joseph,
> 
> 
> Did you get any answer about your problem ?
> 
> I have been toying with your examples and get a bit scared of what
> is happening with lexical scoping...
> 
> 
> I edited your function bv.integrate to have some informations about
> what was going on.
> 
> bv.integrate <- function(f, a, b, n=100, rule=midpoint) {
>   assign("count", 0, envir=.GlobalEnv)
>   g <- function(y) {
>     assign("count", get("count", envir=.GlobalEnv)+1, envir=.GlobalEnv
>     fx <- function(x) f(x, y)
>     rule(fx, a[2], b[2], n)
>   }
>   cat("# times in g:", get("count", envir=.GlobalEnv), "\n")
>   rule(g, a[1], b[1])
> }
> 
> my R sesssion gave the following:
> 
> > bv.integrate(function(x, y) sin(x + y), c(0,0), c(1,1), rule=trapezoidal)
> # times in g: 0 
> [1] 0.007080675
> > print(count)
> [1] 2
> 
> > bv.integrate(function(x, y) sin(x + y), c(0,0), c(1,1), rule=midpoint)
> # times in g: 0 
> [1] 0.773651
> > print(count)
> [1] 100
> 
> 
> For some reason the function 'g' defined in bv.integrate is applied only
> 2 times when the rule is trapezoidal and 100 times when the rule is
> midpoint (?!)... and the variable 'count' I defined in '.GlobalEnv' has
> different values (depending on whether I am in '.GlobalEnv' or in
> 'bv.integrate'....
> 
> 
> I must have missed something obvious somewhere... but where ?
> 
> 
> 
> 
> L.
> 
> 
> 
> 
> 
> -- 
> --------------------------------------------------------------
> Laurent Gautier			CBS, Building 208, DTU
> PhD. Student			DK-2800 Lyngby,Denmark	
> tel: +45 45 25 24 89		http://www.cbs.dtu.dk/laurent
> 
> 
> 
> 
> On Wed, Aug 07, 2002 at 12:28:28AM +0900, Lu Chi-Hsien Joseph wrote:
> > Dear R-users,
> > 
> > The numerical integration example given in Gentleman and Ihaka (2000), 
> > "Lexical Scope and Statistical Computing," JCGS, 9, 491-508,
> > is very interesting and helpful in understanding how lexical scope 
> > is about.
> > However, I got some questions that I just can't figure out.
> > First all, allow me to copy the two functions given by the authors:
> > 
> > midpoint <- function(f, a, b, n=100) {
> >     h <- (b - a)/n
> >     (b - a)*mean(sapply(seq(a + h/2, b - h/2, length=n), f))
> > }
> > 
> > bv.integrate <- function(f, a, b, n=100, rule=midpoint) {
> >     g <- function(y) {
> >         fx <- function(x) f(x, y)
> >         rule(fx, a[2], b[2], n)
> >     }
> >     rule(g, a[1], b[1])
> > }
> > 
> > I modified the function name from "integrate" to "bv.integrate".
> > To integrate f(x,y)=sin(x + y) over the unit square,
> > it works:
> > > bv.integrate(function(x, y) sin(x + y), c(0,0), c(1,1))
> > [1] 0.773651
> > (compare with .7736445 calculated from 2sin(1)*(1 - cos(1)))
> > 
> > Then I write a function using trapezoidal rule as follows:
> > 
> > trapezoidal <- function(f, a, b, n=100) {
> >     h <- (b - a)/n
> >     i <- 1:n
> >     sum(f(a + (i - 1)*h), f(a + i*h))*h/2
> > }
> > 
> > I checked with
> > > trapezoidal(sin, 0, 1)
> > [1] 0.4596939
> > > 1 - cos(1)
> > [1] 0.4596977
> > 
> > and
> > > trapezoidal(dnorm, -3, 3)
> > [1] 0.9972922
> > > diff(pnorm(c(-3, 3)))
> > [1] 0.9973002
> > 
> > Then, this is what I got by plugged in "trapezoidal" for the "rule" 
> > in bv.integrate():
> > > bv.integrate(function(x, y) sin(x + y), c(0,0), c(1,1), rule=trapezoidal)
> > [1] 0.007080675
> > 
> > Hundred time smaller! 
> > Answer can be "improved" by increasing n, but always 100 times smaller.
> > 
> > Anything wrong with the way I wrote trapezoidal()?
> > I just can't figure out any difference, in terms of input/output,
> > between midpoint() and trapezoidal().
> > 
> > Then I tried to use integrate() as the rule in bv.integrate(),
> > by defining a wrap-up function:
> > 
> > intg <- function(f, a, b, n=100) integrate(f, a, b, n)$value
> > 
> > to have the value as the only output.
> > But, I got
> > > bv.integrate(function(x, y) sin(x + y), c(0,0), c(1,1), rule=intg)
> > Error in integrate(f, a, b, n) : evaluation of function gave a result 
> > of wrong length
> > 
> > I tried to use debugger() to find out what's wrong,
> > but not succeeded.
> > There must be something I missed in understanding the lexical scope,
> > however, what's going on with the function I wrote?
> > What should be the way to correct them?
> > 
> > Certainly, I know integrate() should be the one to use.
> > These are questions from my preparation of programming exercise 
> > of using R in my statistical computation class.
> > 
> > I'll be very appreciated for anyone who might help me on these
> > questions.
> > 
> > Best regards,
> > 
> > C. Joseph Lu
> > Department of Statistics
> > National Cheng-Kung University
> > Tainan, Taiwan, ROC
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-- 
--------------------------------------------------------------
Laurent Gautier			CBS, Building 208, DTU
PhD. Student			DK-2800 Lyngby,Denmark	
tel: +45 45 25 24 89		http://www.cbs.dtu.dk/laurent
-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
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