R help - Nov 2009 - name of a name

I have quite a complicated problem that's hard to describe.

Suppose I have a dataframe d. I want to access the vector d$var, where
var is one of the variables in d. Just take for granted that there's a
good reason I want to do this as follows.

var<-"DeOxyA"
xx<-paste("d$",var, sep="")
mean(xx)
[1] NA
Warning message:
In mean.default(xx) : argument is not numeric or logical: returning NA
> mean(d$DeOxyA)
[1] 21.98904

How can I convert xx so I can do mean(xx), for example? Currently xx
is a character, not a variable.
as.numeric doesn't do it. I want R to see
mean(DeOxyA), not
mean("DeOxyA")

I'm stumped.

Thanks very much for any help.

Bill

Hi,

Try this,


d <- data.frame(a=1:4, b=3:6)

var <- "a"

mean(d[var])

## or, if you are not aware of
## fortune("parse")

xx <- paste("d$",var, sep="")
mean(eval(parse(text=xx)))

HTH,

baptiste




2009/11/19 William Simpson <william.a.simpson at
gmail.com>:
> I have quite a complicated problem that's hard to describe.
>
> Suppose I have a dataframe d. I want to access the vector d$var, where
> var is one of the variables in d. Just take for granted that there's a
> good reason I want to do this as follows.
>
> var<-"DeOxyA"
> xx<-paste("d$",var, sep="")
> mean(xx)
> [1] NA
> Warning message:
> In mean.default(xx) : argument is not numeric or logical: returning NA
>
>> mean(d$DeOxyA)
> [1] 21.98904
>
> How can I convert xx so I can do mean(xx), for example? Currently xx
> is a character, not a variable.
> as.numeric doesn't do it. I want R to see
> mean(DeOxyA), not
> mean("DeOxyA")
>
> I'm stumped.
>
> Thanks very much for any help.
>
> Bill
>
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