R help - Aug 2003 - evaluating and walking in names

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.

Thnaks for all,
C.

=======================================Cezar Freitas (ICQ 109128967)
IMECC - UNICAMP
Campinas, SP - Brasil

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.
> 
> Thnaks for all,
> C.
> 
> =======================================> Cezar Freitas (ICQ 109128967)
> IMECC - UNICAMP
> Campinas, SP - Brasil
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://www.stat.math.ethz.ch/mailman/listinfo/r-help
> 
> 
You want assign().

df <- data.frame(matrix(runif(20),ncol=2))

# using ii instead of i makes searches *much* easier with
# a text editor.
for(ii in names(df)) {
   assign(ii,df[[ii]])
}

ls()
[1] "X1" "X2" "df" "ii"

# might not be exactly the same on your machine due
# to floating point rounding.
all.equal(X1,df$X1)
[1] TRUE

all.equal(X2,df$X2)
[1] TRUE

Cheers

Jason
-- 
Indigo Industrial Controls Ltd.
64-21-343-545
jasont at indigoindustrial.co.nz

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.


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E-Mail: (Ted Harding) <Ted.Harding at nessie.mcc.ac.uk>
Fax-to-email: +44 (0)870 167 1972
Date: 06-Aug-03                                       Time: 22:41:45
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