R help - Apr 2004 - Clever R syntax for extracting a subset of observations

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

Hi,
----- Original Message ----- 
From: "Ajay Shah" <ajayshah at
mayin.org>
>
> 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?
So I take it you want to select the 7th, 3rd, and 2nd observation (i.e.
rows) from D?

A data frame has two dimensions, so you need to specify the row and columns.
Maybe try:
  E = D[d,]

HTH,

Kevin Wang

sdratio <- function(D, d) with( D[d,], sd(x)/sd(y) )



Ajay Shah <ajayshah <at> mayin.org> writes:

: 
: 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?
: