[R] Clever R syntax for extracting a subset of observations
Ajay Shah
ajayshah at mayin.org
Thu Apr 8 17:54:30 CEST 2004
I know that if:
x = seq(1,10)
d = c(7,3,2)
and if I say
y = x[d]
then I get the vector y as (7,3,2). Very clever! This idea is used
intensively with the boot library.
Now consider the following code (which works):
---------------------------------------------------------------------------
library(boot)
sdratio <- function(D, d) {
return(sd(D$x[d])/sd(D$y[d]))
}
x = runif(100)
y = 2*runif(100)
D = data.frame(x, y)
b = boot(D, sdratio, R=1000)
cat("Standard deviation of sdratio = ", sd(b$t[,1]), "\n")
---------------------------------------------------------------------------
Now it would be so elegant to say:
sdratio <- function(D, d) {
E = D[d]
return(sd(E$x)/sd(E$y))
}
But this doesn't work since if D is a data frame, you can't say
D[d]. Let me show you:
> x = runif(100)
> y = runif(100)
> D = data.frame(x, y)
> d = c(7,3,2)
> E = D[d]
Error in "[.data.frame"(D, d) : undefined columns selected
Any suggestions on how one can do such pretty things as D[d] where D
is a data frame?
--
Ajay Shah Consultant
ajayshah at mayin.org Department of Economic Affairs
http://www.mayin.org/ajayshah Ministry of Finance, New Delhi
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