R help - Mar 2011 - Passing a character argument onto a function

I am a new R user and am beginning to employ function creation in my statistical
work.  I am running into a problem when I want to pass on a character (text) to
the function as an argument.  I have a simple example below to demonstrate this
problem.  I cannot seem to find a fix in my R book or in the blog posts. 
I'm sure this has been covered before but my newbie status means I lack the
R vocabulary to even search for this problem (I've tried for a few days to
no avail).  Someone has already attempted to explain this to me.  I learn best
by seeing.  Could someone rewrite my code so that the function works.  The
function is very simple as is the data set so it should be pretty easy for an
experienced R user to correct.  The problem is that R doesn't transfer the
"blue" subgroup from the argument to the function.  I am excited with
the potential of R and look forward to your help.
 
I am a Windows user running R 2.12.2

 
CODE for TEST FUNCTION
> TEST<-function(DV,IV,group1) {
+  g1<-DV[IV=="group1"]
+  p<-mean(g1)
+ list(g1,p)
+ }

R's OUTPUT
> TEST(frequency,color,blue)
[[1]]
integer(0)
[[2]]
[1] NaN

 
The DATA FRAME
TEST<-read.table("TEST.csv", header=TRUE,
sep=",",na.strings="999")
> attach(TEST)
  color frequency
1  blue         3
2  blue         4
3  blue         3
4 green         5
5 green         2
6 green         4
7 green         5
8 green         1
 		 	   		  
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Hi:

One answer is eval(substitute(...)): [Note: it's a good idea NOT to use the
same name on different objects - R is pretty good about distinguishing TEST
the function from TEST the data frame, but you could be playing with fire if
both objects have the same class but have different contents. This is why I
renamed your function.]

Tfun <- function(DV,IV,group1) {
   g1 <- eval(substitute(DV[IV == g], list(g = group1)))
   p <- mean(g1)
   list(g1,p)
  }
> with(TEST, Tfun(frequency, color, 'blue'))
[[1]]
[1] 3 4 3

[[2]]
[1] 3.333333

HTH,
Dennis


On Fri, Mar 11, 2011 at 7:49 PM, Tyler Rinker
<tyler_rinker@hotmail.com>wrote:
>
> I am a new R user and am beginning to employ function creation in my
> statistical work.  I am running into a problem when I want to pass on a
> character (text) to the function as an argument.  I have a simple example
> below to demonstrate this problem.  I cannot seem to find a fix in my R
book
> or in the blog posts.  I'm sure this has been covered before but my
newbie
> status means I lack the R vocabulary to even search for this problem
(I've
> tried for a few days to no avail).  Someone has already attempted to
explain
> this to me.  I learn best by seeing.  Could someone rewrite my code so that
> the function works.  The function is very simple as is the data set so it
> should be pretty easy for an experienced R user to correct.  The problem is
> that R doesn't transfer the "blue" subgroup from the argument
to the
> function.  I am excited with the potential of R and look forward to your
> help.
>
> I am a Windows user running R 2.12.2
>
> CODE for TEST FUNCTION
> > TEST<-function(DV,IV,group1) {
> +  g1<-DV[IV=="group1"]
> +  p<-mean(g1)
> + list(g1,p)
> + }
>


> R's OUTPUT
> > TEST(frequency,color,blue)
> [[1]]
> integer(0)
> [[2]]
> [1] NaN
>
>
> The DATA FRAME
> TEST<-read.table("TEST.csv", header=TRUE,
sep=",",na.strings="999")
> > attach(TEST)
>
>  color frequency
> 1  blue         3
> 2  blue         4
> 3  blue         3
> 4 green         5
> 5 green         2
> 6 green         4
> 7 green         5
> 8 green         1
>
>        [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
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