[R] How to speed up multiple for loop over list of data frames

[Gavin Simpson]

On Wed, 2007-10-17 at 19:57 -0600, James wrote:
> On Oct 17, 2007, at 4:36 PM, James wrote:
> 
> > On Oct 17, 2007, at 10:18 AM, Waterman, DG ((David)) wrote:
> >
> >> I agree. Avoid the lines like:
> >> iv     = c( iv, min(i, j) )
> >>
> >> I had code that was sped up by 70 times after fixing the size of my
> >> output object before entering a loop.
> >
> > I'm in the process of replacing that very kind of command.  In my  
> > case, I'm trying to iterate over a non-integer sequence that  
> > doesn't begin at 1.
> >
> > x<-seq(15,25,0.10)
> >
> > So when I'm iterating over that sequence in my for loop, I don't  
> > have nice, easy integers that I can also use for the assignment to  
> > my vector.  Is there a way to know where I am in the for loops  
> > progress through the vector x, without having to create a separate  
> > variable that I increment each time the loop executes?  Something  
> > along the lines of this:
> >
> > y<-numeric(length(x))
> > for(i in x) {
> >      y[i] <- GBSGreeks(Selection = 'delta', TypeFlag="c", S=i,  
> > X=20, Time=1/12, r=.05, b=.05, sigma=0.4)
> > }
> >
> > But that obviously doesn't work.  The vector x is length=101.  My  
> > vector assignment only works on the 11 integers from 15 to 25.
> >
> > Is there a clever way to fix this without the use of a separate  
> > variable to track the loops progress through the vector x and for  
> > assignment to the equal size y vector?
> >
> > thanks
> >
> > James
> 
> I guess in answer to my own question I found that on page 46 of  "An  
> Introduction to R" it describes this usage:
> 
>  > for (i in 1:length(yc)) {
> plot(xc[[i]], yc[[i]]);
> abline(lsfit(xc[[i]], yc[[i]]))
> }
> 
> So in my case that turns into:
> 
> y<-numeric(length(x))
> for(i in 1:length(x)) {
>       y[i]<-GBSGreeks(Selection = 'delta', TypeFlag="c", S=x[i],  
> X=20, Time=.0000001, r=.05, b=.05, sigma=0.4)
> }

I think a more recommended approach if you are canning this in a
function etc, is to seq(along = y) instead of 1:length(x) in the
for(...)

for(i in seq(along = y)) {
	## other stuff here
}

The reason being, I guess, is what happens when length(x) is 0 or
returns something strange.

All the best,

G

> 
> Sorry for the noise.
> 
> James
> 
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