accessing elements located after $ symbol

Dear Lorenzo,

This is the trade off that comes with convenience.  The `$` operator
passes its argument directly as I understand it.  This is what lets
you pass unquoted names that not variables.  The way around it is to
use the `[` extraction operator.  Look at these examples:

test[interest]
#or
test[, interest]
# but
test[first]
test[,first]

Notice that for `[`, the name of the column _must_ be quoted, or be an
object itself.  Typing:

test$interest

is trying to look up the 'interest' column, which does not exist, and
is equivalent to:  test[,"interest"] which is clearly not what you
want.

HTH,

Josh

P.S. I am sure there are others who could provide a more detailed
description.  `$` is primitive and I am only used to R code, so I have
never actually looked through its source.

On Mon, Oct 4, 2010 at 10:45 PM, Lorenzo Cattarino

Hi R-users



I am having troubles accessing elements after the $ symbol. Reproducible
example:

test <- data.frame (first=1:10, second=11:20, third=21:30)

test$first #this works fine

but when I try

interest <- "first"

test$interest # does not seem to work

Could you tell me why that happens and show how to do instead?



Thanks so much

Lorenzo






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Joshua Wiley
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University of California, Los Angeles
http://www.joshuawiley.com/

Joshua Wiley <jwiley.psych <at> gmail.com> writes:

  [snip]

Look at these examples:

test[interest]
#or
test[, interest]
# but
test[first]
test[,first]

Notice that for `[`, the name of the column _must_ be quoted, or be an
object itself.  

or test[[first]] ; you probably do not want test[first] , which returns
a list of length 1 containing the column of interest, rather than the
column itself ...