RE: [R] evaluating and walking in names

[Ted Harding]

On 06-Aug-03 Cezar Augusto de Freitas Anselmo wrote:
 Hi, all.
 Suppose I have an object with names (like a data.frame) and
 I want to walk in a loop with your names. How can I do this?
 The idea is like this:
 
 my.data-data.frame(matrix(runif(6),ncol=2))
 names(my.data)
 [1] X1 X2
 for(i in names(my.data)){
   my.variable - cat(paste(my.data$, i, \n, sep=))
   print(mean(my.variable))
 }
#it doesn't work.

Hi Cezar,

First of all, you should get rid of those \n -- they intrude:
 for(i in names(my.data)){ print(paste(my.data$, i, \n, sep=))}
[1] my.data$X1\n
[1] my.data$X2\n
 for(i in names(my.data)){ print(paste(my.data$, i, sep=))}
[1] my.data$X1
[1] my.data$X2

Second, you don't need to do the above anyway: the simplest way to
refer to the variable with the name name is simply to index the column
by name:

 for(i in names(my.data)){
  my.variable-my.data[,i];print(mean(my.variable))}
[1] 0.3302733
[1] 0.6119088

To see this, have a look at

  my.data[,X1]
  my.data[,X2]

I hope this breaks the block!

QUESTION TO EXPERTS:
While (a construction I've often used successfully):

  for(Z in c(X1,X2,X3)){
Z-eval(as.name(Z))
do.something.with(Z) }

works, going through the variables named X1, X2, X3 in
turn, when I was trying to clean up Cezar's example above I found
that if you construct the name Z by pasting so that it comes to
my.data$X1 then the object my.data$X1 is not found. Presumably
this is because the $ operator is not functional in this context,
but I can't locate an explanation of this. Can anyone elucidate?

Best wishes,
Ted.



E-Mail: (Ted Harding) [EMAIL PROTECTED]
Fax-to-email: +44 (0)870 167 1972
Date: 06-Aug-03   Time: 22:41:45
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RE: [R] evaluating and walking in names

[Thomas Lumley]

On Wed, 6 Aug 2003 [EMAIL PROTECTED] wrote:

 QUESTION TO EXPERTS:
 While (a construction I've often used successfully):

   for(Z in c(X1,X2,X3)){
 Z-eval(as.name(Z))
 do.something.with(Z) }

 works, going through the variables named X1, X2, X3 in
 turn, when I was trying to clean up Cezar's example above I found
 that if you construct the name Z by pasting so that it comes to
 my.data$X1 then the object my.data$X1 is not found. Presumably
 this is because the $ operator is not functional in this context,
 but I can't locate an explanation of this. Can anyone elucidate?

Because as.name() converts the string to a name, but you wanted an
expression (the X1 component of my.data).

In r-devel this is visually clearer
 as.name(my.data$X1)
`my.data$X1`
the backticks indicating a syntatically weird name.  You can  do eg
 `my.data$X1` - 42
 eval(as.name(my.data$X1))
[1] 42
In current R versions you would have to use
assign(mydata$X1, 42)
to assign to this variable.

To get the X1 component of my.data you would need
eval(parse(text=my.data$X1))
 my.data-data.frame(X1=1:2,X2=2:3)
 parse(text=my.data$X1)
expression(my.data$X1)
 eval(parse(text=my.data$X1))
[1] 1 2

-thomas

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